1 Bacterial Anatomy and Physio (54)

Question 1

What structure of the bacterial cell envelope is marked as 2? Refer to the image.

1 Peptidoglycan layer
2 Outer membrane
3 Mesosomes
4 Inner cell membrane
5 Capsule

Answer 1

Peptidoglycan layer

Refer to second image for identified structures.

Structure 2 is the cell wall composed of peptidoglycans: the peptidoglycan layer. The cell wall is involved in maintaining the overall shape of a bacterial cell. Most gram-positive bacteria have a relatively thick cell wall composed largely of peptidoglycan. In contrast, the peptidoglycan layer in the gram-negative bacteria is relatively thin. This structure provides the basis for the gram reaction.

Outside the peptidoglycan layer in the gram-negative envelope is a structure called the outer membrane. This membrane structure is anchored non-covalently to lipoprotein molecules (Braun’s lipoprotein), which in turn are covalently linked to the peptidoglycan. The lipopolysaccharides of the gram-negative cell envelope form part of the outer leaflet of the outer membrane structure.

The capsule is a region of material that extends beyond the outer membrane of some bacteria. Bacterial capsule contributes directly to the pathogenicity or virulence of an organism. They confer resistance to phagocytosis; they provide protection to the bacterial cell against host defenses to invasion. Encapsulated species are found among both gram-positive and gram-negative bacteria.

The inner cell membrane is a lipid bilayer that surrounds and contains the cytosol.

Mesosomes are invaginations of the plasma membrane/inner cell membrane. These structures are much more prominent in gram-positive than in gram-negative organisms.

Question 2

In the accompanying diagram of a bacterial cell envelope, identify the structure marked 5.

1 Nucleoid
2 Peptidoglycan layer
3 Outer membrane
4 Inner cell membrane
5 Mesosomes

Answer 2

Outer membrane

Arrow 5 points to the outer membrane, which is the structure found outside the peptidoglycan layer/cell wall. This structure is anchored non-covalently to lipoprotein molecules (Braun’s lipoprotein), which in turn, are covalently linked to the peptidoglycan. Like other membranes, the outer membrane is composed of proteins and phospholipids. Unlike other membranes, it contains an additional molecule, lipopolysaccharide, which displays endotoxin activity. The lipopolysaccharides of the Gram-negative cell envelope form part of the outer leaflet of the outer membrane structure, while the lipoprotein and the phospholipids form the inner leaflet of the outer membrane bi-layer. The outer membrane possesses several major outer membrane proteins; the most abundant is called porin. A capsule is often seen extending beyond this structure and contributes to bacterial virulence.

Peptidoglycan is a major component of the bacterial cell wall, particularly in gram-positive species. These complex carbohydrate polymers provide structural integrity to the cell and are a major factor in the Gram stain reaction.

The inner cell membrane surrounds and contains the bacterial cytosol.

Nucleoid refers to the bacterial DNA and associated proteins that appear within discrete areas of the cytosol. Unlike eukaryotic cell nuclei, the nucleoid is not membrane bound.

Mesosomes are invaginations of the plasma membrane/inner cell membrane. These structures are much more prominent in Gram-positive than in Gram-negative organisms.

Question 3

A Gram stain has been performed on a bacterial isolate taken from a sheep blood agar plate. Upon observation, the cells appear to be red to pink rods. The organism is presumptively identified as Citrobacter koseri. What is the most likely reason for the red color of the cells in the stain?

1 Peptidoglycan layer of the cell wall is relatively thin
2 Peptidoglycan layer of the cell wall is relatively thick
3 Organisms in this genus have a lipopolysaccharide outer membrane
4 Organisms in this genus have only a cytoplasmic membrane
5 Capsule tends to capture the safranin from the Gram stain reagent

Answer 3

Peptidoglycan layer of the cell wall is relatively thin

The Gram stain is a differential stain utilizing two dyes: crystal violet (blue) and safranin (red). Gram negative organisms stain red and Gram positive organisms stain blue. The cell wall of the organism is the determining factor that makes an organism’s Gram positive or Gram negative. Gram positive organisms have a thick peptidoglycan layered wall that tends to complex with Gram’s iodine and remains captured in the complex layer. Gram negative cells have a much thinner peptidoglycan layer that allows the crystal violet-iodine complex to be easily removed by the decolorizer. Thus, the subsequent addition of safranin stains the wall red. Bacterial membranes and capsules do not play a role in the Gram reaction.

Question 4

A successful Gram stain would use, in order

1 Crystal violet, alcohol, iodine, safranin
2 Methylene blue, iodine, alcohol, safranin
3 Crystal violet, iodine, alcohol, congo red
4 Methylene blue, iodine, saline, safranin
5 Crystal violet, iodine, alcohol, safranin

Answer 4

Crystal violet, iodine, alcohol, safranin

The Gram stain is the work horse of microbiology, but adherence to accurate technique is absolutely essential to obtaining the correct results. In fact, for some specimens that represent life-threatening illnesses, e.g. spinal fluid, the Gram stain can detect the etiologic agent directly in the specimen in time to direct empiric therapy and have a positive impact on the prognosis of the patient. The agents used in order are crystal violet (deep blue to purple), iodine (serves as a mordant), alcohol (or a mix of acetone and alcohol) which is the most critical of the steps and is the decolorizing procedure followed by the addition of safranin (red), the final stain to be added.

Question 5

If you observe an organism microscopically to be unicellular, motile, and without a membrane-bound nucleus, it will most likely be a

1 Fungus
2 Bacterium
3 Virus
4 Protozoan
5 Yeast

Answer 5

Bacterium

Most living cells are either prokaryotic or eukaryotic. Probably the most distinguishing feature that separates these two classes of organisms, especially the microscopic ones, is that prokaryotic cells do not have a membrane-bound nucleus. The DNA of bacterial cells essentially floats in the cytoplasm but is attached to the inner membrane. Fungi, yeasts, and protozoa are eukaryotic cells and have membrane-bound nuclei. Viruses are not cells in the classic sense and are classed differently.

Question 6

The medical importance of spores lies in the fact that:

1 They have keratin-like coats.
2 They contain bacterial DNA.
3 They are extremely resistant to heat and chemicals.
4 They are formed in adverse conditions.

Answer 6

They are extremely resistant to heat and chemicals.

Spore formation occurs in the medically important gram-positive rods (Bacillus and Clostridium) when nutrients such as carbon and nitrogen are depleted. The spore is remarkably resistant to heat, dehydration, radiation, and chemicals. This resistance is thought to be mediated by dipicolinic acid, a calcium ion chelator found only in spores. As a result of their resistance to heat, sterilization cannot be achieved by boiling. Autoclaving at 121°C for 30 minutes is required to ensure the inactivation of spores.

Question 7

Endotoxin is

1 The enzyme lysozyme present in human tears, mucus, and saliva.
2 The lipopolysaccharide (LPS) of the outer layer of the cell wall of gram negative bacteria.
3 Toxin released by Clostridia.
4 Always freely released from bacteria.

Answer 7

The lipopolysaccharide (LPS) of the outer layer of the cell wall of gram negative bacteria.

The LPS of the outer layer of the cell wall of gram negative bacteria is called endotoxin. It is an integral part of the cell wall and causes many features of disease, such as fever and shock. The toxic effect is caused by 1 of its 3 distinct units, lipid A. LPS is also composed of a core polysaccharide of 5 sugars and an outer polysaccharide consisting of up to 25 repeating units of 3 to 5 sugars. The outer polysaccharide is the somatic or O antigen of gram negative bacteria.

Question 8

Mycobacteria are called acid-fast bacilli because

1 They have unusual cell walls and they resist decolorization with acid alcohol after being stained with carbol-fuchsin
2 They lack endotoxins
3 They have thick walls
4 They have capsules

Answer 8

They have unusual cell walls and they resist decolorization with acid alcohol after being stained with carbol-fuchsin

Mycobacteria have a high concentration of cell wall lipids called mycolic acids. The high mycolic acid content makes these bacteria resistant to decolorization with acid alcohol after being stained with carbol-fuchsin, and hence these bacteria are said to be acid fast.

Question 9

Which of the following events is most likely the effect of bacterial conjugation?

1 A gene encoding resistance to chloramphenicol in the Escherichia coli chromosome appears in the genome of a virulent bacteriophage that has infected E.coli.
2 An encapsulated strain of Streptococcus pneumoniae acquires the gene for capsule formation from an extract of another encapsulated strain.
3 A strain of Corynebacterium diphtheriae produces a toxin encoded by a prophage.
4 A strain of Pseudomonas aeruginosa produces β-lactamase encoded by a plasmid identical to a plasmid of another gram negative organism.

Answer 9

A strain of Pseudomonas aeruginosa produces β-lactamase encoded by a plasmid identical to a plasmid of another gram negative organism.

Conjugation is the mating of two bacterial cells during which DNA is transferred from the donor to the recipient cell.

Question 10

Transduction is defined as

1 Transfer of cell DNA by means of a bacterial virus
2 Mating of two bacterial cells
3 Release of DNA by bacterial cells
4 Transfer of cell DNA by means of a plasmid

Answer 10

Transfer of cell DNA by means of a bacterial virus

Transduction is the transfer of cell DNA by means of a viral infection of a bacterial cell. During viral growth within the cell, a piece of bacterial DNA is incorporated into the virus particle and is carried into the recipient cell at the time of infection.

Question 11

Which of the following media are considered to be most useful for the cultivation of fastidious pathogens?

1 Chocolate agar
2 MacConkey agar
3 Colistin-nalidixic acid agar
4 Eosin methylene blue agar
5 Thioglycollate broth

Answer 11

Chocolate agar

Chocolate agar is a highly enriched medium composed of heated sheep blood in which the red cells have lysed due to gentle heating, spilling their nutritious contents into the basal medium. This agar is normally incubated in a carbon dioxide atmosphere to further enhance the growth of fastidious pathogens. MacConkey agar and Eosin methylene blue are designed only for the isolation of enteric, gram-negative rods. Colistin-nalidixic acid agar is an inhibitory medium that prevents the growth of gram-negative organisms and allows gram-positive organisms to grow. Thioglycollate is a broth medium designed to encourage the growth of anaerobic organisms.

Question 12

Gram negative rods are often capable of producing beta-lactamases to facilitate their survival in the presence of therapy. The beta-lactamases that are produced are often stored in the bacterial cell

1 In the teichoic acid moiety of the cell wall
2 In the protein components of the phospholipid membrane
3 In the cytoplasm of the cell
4 In the periplasmic space between the cell wall and the membrane
5 In the transposons of the cell

Answer 12

In the periplasmic space between the cell wall and the membrane

The periplasmic space serves as a storage site for beta-lactamases and some other enzymes that are produced in the cell. Therefore, any beta-lactam antibiotic attempting to penetrate the cell wall will be inactivated by the beta-lactamase. Teichoic acid is found in Gram positive organisms and is not involved in storage. Protein moieties of membranes are associated with membrane transport, not storage. The cytoplasm is the site of production of beta-lactamases by plasmids. Transposons are genetic elements that may, in fact, contribute to production of beta-lactamases but not storage.

Question 13

After studying an isolate in the laboratory, it was determined that the organism actually grows well at 4-8° C. Under these conditions, most pathogens are unable to multiply, although a few, like Yersinia enterocolitica, do. Organisms that grow under these conditions are called

1 Psychrophiles
2 Mesophiles
3 Capnophile
4 Thermophile

Answer 13

Psychrophiles

A psychrophile grows best at 4-20° C, a mesophile grows best at 20-40° C, and a thermophile grows best at 40-90° C. A capnophile refers to an organism that requires carbon dioxide for survival. Most bacteria, especially bacterial pathogens, are mesophiles. Routine laboratory incubators should be set at 35° C to accommodate these pathogens. Unfortunately, some mesophiles may survive very well at refrigerator temperatures and grow slowly. Both Staphylococcus aureus and Salmonella spp. can survive in refrigerated foods and produce toxins that are responsible for food “poisoning”.

Question 14

In a private practice office in a rural area, you must make your own broth in order to culture a specimen from a patient that would normally be sent to a local laboratory. In the process of making the medium, you discover that the portion of the label regarding pH of the medium has been torn off and the information is unavailable. Knowing the growth characteristics of most bacterial pathogens, the pH you would most likely use for your medium is

1 5.1-6.3
2 6.8-7.2
3 7.5-7.8
4 7.9-8.3
5 8.5-8.8

Answer 14

6.8-7.2

The optimum pH for the growth of most bacteria isolated from humans is 6.8-7.2. Growth is substantially reduced at pH levels above and below this level. In fact, as bacteria grow in the laboratory, one of the factors that limits their growth is the increasing acidity created by normal metabolic processes. Enteric bacteria can produce acids that lower the pH of the broth in which they are growing to pH 4 and lower.

Question 15

Chocolate agar and 5% sheep blood agar are examples of

1 Differential media
2 Selective media
3 Hypertonic media
4 Enriched media
5 One-purpose media

Answer 15

Enriched media

Chocolate agar and 5% sheep blood agar (SBA) are both made from a basal medium enriched with sheep blood. Chocolate agar is prepared by heating SBA prior to preparation of the plates. By heating (not boiling), the red cells lyse, releasing their hemoglobin-containing contents and the enrichments found within the red cells. SBA is prepared by adding the appropriate amount of sheep blood to autoclaved basal medium that has been cooled to about 50° C. The red cells are not destroyed at this temperature and they retain their bright red color, thus imparting a red color to SBA. Chocolate agar, on the other hand, is brown in color, thus its name as “chocolate”. Chocolate agar is more enriched than SBA. In fact, some fastidious organisms will grow on chocolate agar but not on SBA. Both media are usually incubated in a carbon dioxide atmosphere.

Question 16

Smooth strains (S) of Streptococcus pneumoniae are encapsulated and are pathogenic. Rough strains (R) are not encapsulated and are generally not pathogenic. When mice are injected with live S strains they die; when injected with live R strains, the mice survive. Mice also survive when injected with dead S strains. Interestingly, when **dead S strains and live R strains are mixed together **and then injected into mice, the mice die and colonies of S and R strains can be isolated from the dead mice.

What is the most likely explanation for these laboratory results?

1 Transportation
2 Transformation
3 Conjugation
4 Transduction
5 Transposition

Answer 16

Transformation

In 1928, an English bacteriologist named Frederick Griffith published the results of the experiment described above. While the results were curious and unusual in 1928, this experiment became a classic example of the genetic event now known as transformation. In transformation, soluble DNA from a donor cell (in this case, the dead S strain) is taken up by a recipient cell of the same species but a different genotype (in this case, the R strain). The DNA from the donor recombines with the DNA of the recipient resulting in the expression of genetic characteristics of the donor (the ability to manufacture capsular material).
Conjugation and transduction are also important mechanisms of DNA transfer, but soluble DNA is not involved in these processes.
Transportation and transposition are unrelated to the genetic events described above.

Question 17

The toxin of Corynebacterium diphtheriae is responsible for the vaccine-preventable childhood illness diphtheria. Toxin production in this organism is mediated by chromosomal genes that originated from a bacteriophage. When incorporated into the DNA of a host cell, bacteriophage DNA is called a prophage. Organisms that contain the prophage produce the toxin and cause diphtheria, while those that do not contain the prophage are probably nonpathogenic.

This mechanism of DNA transfer from one cell to another by way of a bacteriophage is called

1 Transduction
2 Transformation
3 Conjugation
4 Transfection
5 Recombination

Answer 17

Transduction

Transduction is the transfer of a fragment of donor chromosome to a recipient cell by a temperate bacteriophage produced in the donor cell. The recipient may, by recombination, exchange the transduced gene for its own corresponding gene, eliminating the latter. The other two types of genetic transfer, transformation and conjugation, do not involve bacteriophages. Transfection and recombination are not involved in the transfer of DNA between bacterial cells.

Question 18

Prokaryotic cytoplasm consists of

1 Mitochondria
2 Golgi bodies
3 70S ribosomes
4 Endoplasmic reticulum

Answer 18

70S ribosomes

Prokaryotic cytoplasm consists of 70S ribosomes.
Mitochondria, golgibodies, and endoplasmic reticulum are part of eukaryotic cytoplasmic structures.
Protein synthesis in prokaryotes, as well as eukaryotes, takes place on ribosomes that are complex of protein and ribonucleic acid. However, eukaryotic ribosomes are larger and heavier (80S ribosome) than prokaryotic ribosomes (70S ribosomes).

Question 19

Gram positive bacterial cell wall is composed of

1 Peptidoglycan and outer membrane only
2 Peptidoglycan, teichoic acids and lipoteichoic acid
3 Phospholipid membrane and teichoic acids
4 Thin layer of peptidoglycan only
5 Lipids and proteins only

Answer 19

Peptidoglycan, teichoic acids and lipoteichoic acid

Gram positive bacterial cell wall comprises multiple layers of cross-linked peptidoglycan, teichoic acids anchored to the peptidoglycan and lipoteichoic acid anchored to the plasma membrane. Teichoic acids are water soluble polymers of polyol phosphates and are important factors in virulence. Gram-positive cell walls usually lack lipids and proteins with few exceptions. Long-chain fatty acids are part of cell wall of Corynebacterium diptheriae, Mycobacterium tuberculosis, and Nocardia asteroides.
Gram negative bacterial cell wall is made up of two layers. Inner membrane is made up of thin layer of peptidoglycan, and outer membrane consists of proteins, phospholipids, and lipopolysaccharide.

Question 20

Turbidimetric density measurement for bacterial cell concentration involves

1 Serial dilution followed by colony forming units on an agar plate
2 Viable cell count
3 Nonviable cell count
4 A and C
5 B and C

Answer 20

B and C

Cell concentration measurement by viable cell count involves serial dilution, followed by determination of colony forming units on an agar surface. This method is used in determining bacterial cell count in urine cultures.
Turbidimetric density measurement for bacterial cell concentration includes viable cell count and nonviable cell count. This method is used to prepare a standard inoculum for antimicrobial susceptibility testing.

Question 21

The differential growth media

1 Is composed of compounds that select the growth of one type of bacteria over other
2 Contain nutrients and pH indicators to allow visual distinction of bacterial growth
3 Is a type of enrichment media
4 A and C
5 B and C

Answer 21

B and C

Enrichment medium is a growth medium that contains added growth factors like vitamins and blood.
Selective growth medium is a type of enrichment medium that contains additives that select the growth of one type of bacteria and inhibit others.
The differential growth media contains a combination of nutrients and pH indicators to visually differentiate bacteria (like giving a distinctive color) that grow on it.

Question 22

Bacterial metabolism:

1 Is mostly autotrophic for pathogenic bacteria.
2 Does not vary depending on nutritional environment.
3 Is mostly heterotrophic for pathogenic bacteria.
4 A and C
5 B and C

Answer 22

Is mostly heterotrophic for pathogenic bacteria.

Bacterial metabolism refers to all the chemical processes in a bacterial cell. These include the biochemical reactions bacteria use to break down organic compounds, as well as those they use to synthesize new bacterial parts from resulting carbon skeletons.
The process of anabolism and catabolism varies with nutritional environment and is heterotrophic for pathogenic bacteria.

Question 23

Gene expression involves transcription and translation. Transcription

1 Is the assembly of polypeptide chains from mRNA transcript
2 Occurs at the ribosomes
3 Is the synthesis of single stranded RNA mediated by RNA polymerase using one strand of DNA as template
4 Occurs in 3’OH to 5’PO4 direction
5 Is the transfer of bacterial genes by a bacteriophage from one cell to another

Answer 23

Is the synthesis of single stranded RNA mediated by RNA polymerase using one strand of DNA as template

Transcription is the synthesis of single stranded RNA using one strand of DNA as template and is mediated by RNA polymerase and it occurs in template 5’PO4 to 3’ OH direction.
Translation is synthesis of specific protein from the mRNA code.
The transfer of bacterial genes by a bacteriophage from one cell to another is known as transduction.

Question 24

Respiration in bacteria takes place via the

1 Cytoplasmic membrane
2 Nuclear membrane
3 Mitochondria
4 Endoplasmic reticulum
5 Golgi bodies

Answer 24

Cytoplasmic membrane

Prokaryotic or bacterial cells do not contain any internal membranes and therefore rely on cytoplasmic membrane for carrying out all membrane-localized functions, including aerobic respiration by which metabolic energy is produced.
Eukaryotic cells contain multiple internal membrane bound organelles that include mitochondria, endoplasmic reticulum, and golgi apparatus.

Question 25

Identify the bacterial classification that should apply to a cell with the type of envelope in the attached image.

1 Gram-negative
2 Spheroplast
3 Mycoplasma
4 Gram-positive
5 Protoplast

Answer 25

Gram-negative

The gram-negative bacterial cell envelope diagrammed in this illustration contains an inner and outer cell membrane. Between these is the cell wall structure, which is made up of a layer of peptidoglycan surrounded by the periplasmic gel. The outermost layer of the membrane is made up of the bacterial endotoxin LPS (lipopolysaccharide). This component is often directly toxic to humans and animals and plays a major role in the pathology of gram-negative organisms. Gram-positive cells lack an outer cell membrane and have a much thicker cell wall with substantially more complex peptidoglycan layers. Mycoplasma lack a cell wall altogether, as do protoplasts and spheroplasts, which are simply gram-positive or gram-negative cells that have had the cell wall removed artificially.

Question 26

In the accompanying diagram of a bacterial cell envelope, identify the overall structure marked as “2.”

1 Lipoprotein
2 Outer protein coat
3 Peptidoglycan
4 Inner cell membrane
5 Capsule

Answer 26

Lipoprotein

Structure “2” indicates the lipoproteins, which **function to bridge between the outer cell membrane and the peptidoglycan component of gram-negative bacterial cell walls. **

The capsule is a region of material that extends beyond the outer protein coat of most bacteria and helps them adhere to eukaryotic cell surfaces. Bacterial capsules often contribute directly to the pathogenicity of the organism.

The inner cell membrane is a lipid bilayer, which *surrounds and contains the overall cellular cytosol. *

Peptidoglycan is a major structural component of the bacterial cell wall and is independent of the membrane.

The **outer protein coat **refers to the structure found at or near the overall surface of the cell (a capsule is often seen extending beyond this feature).

Question 27

In the accompanying diagram of a bacterial cell envelope, identify the structure marked “1.”

1 Nucleoid
2 Peptidoglycan
3 Outer protein coat
4 Inner cell membrane
5 Polyribosomes

Answer 27

Peptidoglycan

Structure “1” is the peptidoglycan layer of gram-negative bacterial cell walls. This layer is surrounded by the periplasmic gel and is attached to the outer membrane by lipoproteins.
The inner cell membrane surrounds and contains the bacterial cytosol. Polyribosomes are units within the cytosol, which translate mRNA into proteins. The outer protein coat refers to the structure found at or near the overall surface of the cell (a capsule is often seen extending beyond this feature). Nucleoid refers to the bacterial DNA and associated proteins, which appear within discreet areas of the cytosol. Unlike eukaryotic cell nuclei, the nucleoid is not membrane bound.

Question 28

In the accompanying diagram of a bacterial cell envelope, identify the structure marked “5”

1 Nucleoid
2 Peptidoglycan
3 Outer protein coat
4 Inner cell membrane
5 Polyribosomes

Answer 28

Inner cell membrane

Arrow “5”, points to the inner cell membrane which surrounds and contains the bacterial cytosol.
The outer protein coat is the structure found at or near the overall surface of the cell. A capsule is often seen extending beyond this feature and contributes to bacterial pathogenicity and host cell trophism. Peptidoglycan is a major component of the bacterial cell wall, particularly in gram positive species. These complex carbohydrate polymers provide structural integrity to the cell and are a major factor in the Gram stain reaction. Polyribosomes are units within the cytosol which translate mRNA into proteins. Nucleoid, refers to the bacterial DNA and associated proteins which appear within discreet areas of the cytosol. Unlike eukaryotic cell nuclei, the nucleoid is not membrane bound.

Question 29

In the accompanying diagram of a bacterial cell envelope, identify the structure marked “4”

1 Nucleoid
2 Peptidoglycan
3 Outer cell membrane
4 Inner cell membrane
5 Polyribosomes

Answer 29

Outer cell membrane

Arrow “4”, points to the outer cell membrane of the gram-negative bacterial cell envelope. The outer leaflet of this membrane is made up of LPS (lipopolysaccharide) which is also called endotoxin and contributes to gram-negative bacterial pathology. Peptidoglycans are complex carbohydrate polymers which provide structural integrity to the cell wall and are a major factor in the Gram stain reaction. Gram-negative cells have a relatively thin cell wall which contains a compact layer peptidoglycan. The inner cell membrane surrounds and contains the bacterial cytosol. Polyribosomes are units within the cytosol which translate mRNA into proteins. Nucleoid refers to the bacterial DNA and associated proteins which appear within discreet areas of the cytosol. Unlike eukaryotic cell nuclei, the nucleoid is not membrane bound.

Question 30

Gram-negative bacterial cell walls have a number of unique features not associated with gram-positive cells. Which of these is only found in gram-negative bacterial cell walls?

1 Peptidoglycan
2 Teichoic acid
3 Lipoteichoic acid
4 periplasmic gel
5 Murein

Answer 30

periplasmic gel

The periplasmic gel is a layer of non-cross linked murein (peptidoglycan) found between the inner and outer cell membranes of gram-negative cells. Peptidoglycan, teichoic acid, lipoteichoic acid, and murein are all terms for structural components which are present in gram-positive bacterial cell walls.

Question 31

Identify the bacterial classification that applies to a cell with the type of envelope shown in the attached image.

1 Gram-negative
2 Spheroplast
3 Mycoplasma
4 Gram-positive
5 Protoplast

Answer 31

Gram-positive

The gram-positive bacterial cell envelope diagramed in this illustration contains a single inner cell membrane and a very substantial cell wall structure. This wall is anchored to the membrane and displays numerous complex layers. This represents the peptidoglycan layers characteristic of these cells.

**Gram-negative **cells lack such a thick cell wall and have both an inner and outer cell membrane.

Mycoplasma lack a cell wall altogether, as do protoplasts and spheroplasts, which are simply gram-positive or gram-negative cells that have had the cell wall removed artificially.

Question 32

In the accompanying diagram of a bacterial cell envelope, identify the overall structure marked as “2”.

1 Cell wall
2 Outer protein coat
3 Peptidoglycan
4 Inner cell membrane
5 Capsule

Answer 32

Cell wall

Structure “2” is the cell wall which is made up of several layers of complex carbohydrates and associated proteins. This structure is very thick in many species of bacteria and provides the basis for the gram reaction.

The capsule is a region of material which extends beyond the outer protein coat of most bacteria and help them adhere to eukaryotic cell surfaces.
Bacterial capsules often contribute directly to the pathogenicity of an organism.

The inner cell membrane is a lipid bilayer which surrounds and contains the cytosol.

**Peptidoglycan is a major structural component of the bacterial cell wall and is independent of the membrane. ** NOTE: Some sources say its attached to the outer membrane via lipoproteins, it’s composed of alternatine NAG and NAM subunits.

The outer protein coat refers to the structure found at or near the overall surface of the cell (a capsule is often seen extending beyond this feature).

Question 33

In the accompanying diagram of a bacterial cell envelope, identify the structure marked “1.”

1 Nucleoid
2 Peptidoglycan
3 Outer protein coat
4 Inner cell membrane
5 Ribosomes

Answer 33

Inner cell membrane

Structure “1” is the inner cell membrane that surrounds and contains the bacterial cytosol. Ribosomes are units within the cytosol, which translate mRNA into proteins. The outer protein coat refers to the structure found at or near the overall surface of the cell (a capsule is often seen extending beyond this feature). Peptidoglycan is a major structural component of the bacterial cell wall and is independent of the membrane. Nucleoid refers to the bacterial DNA and associated proteins, which appear within discreet areas of the cytosol. Unlike eukaryotic cell nuclei, the nucleoid is not membrane bound.

Question 34

In the accompanying diagram of a bacterial cell envelope, identify the structure marked “5”.

1 Nucleoid
2 Peptidoglycan
3 Outer protein coat
4 Inner cell membrane
5 Polyribosomes

Answer 34

Outer protein coat

Arrow “5”, points to the outer protein coat which is the structure found at or near the overall surface of the cell. A capsule is often seen extending beyond this feature and contributes to bacterial pathogenicity and host cell trophism. Peptidoglycan is a major component of the bacterial cell wall, particularly in gram positive species. These complex carbohydrate polymers provide structural integrity to the cell and are a major factor in the Gram stain reaction. The inner cell membrane surrounds and contains the bacterial cytosol. Polyribosomes are units within the cytosol which translate mRNA into proteins. Nucleoid, refers to the bacterial DNA and associated proteins which appear within discreet areas of the cytosol. Unlike eukaryotic cell nuclei, the nucleoid is not membrane bound.

Question 35

In the accompanying diagram of a bacterial cell envelope, identify the structure marked “4.”

1 Nucleoid
2 Peptidoglycan
3 Outer protein coat
4 Inner cell membrane
5 Ribosomes

Answer 35

Peptidoglycan

Arrow “4” points to the peptidoglycan layers of the bacterial cell wall. These complex carbohydrate polymers provide structural integrity to the cell and are a major factor in the Gram stain reaction. Gram positive cells have a thick cell wall, which contains high levels of peptidoglycan. The inner cell membrane surrounds and contains the bacterial cytosol. Ribosomes are units within the cytosol, which translate mRNA into proteins. The outer protein coat refers to the structure found at or near the overall surface of the cell (a capsule is often seen extending beyond this feature). Nucleoid refers to the bacterial DNA and associated proteins, which appear within discreet areas of the cytosol. Unlike eukaryotic cell nuclei, the nucleoid is not membrane bound.

Question 36

Bacterial cells contain which of the following components

1 Nucleus
2 Membrane-bound organelles
3 70S ribosomes
4 80S ribosomes

Answer 36

70S ribosomes

Ribosomes are internal cellular components which function to translate mRNA into proteins. Although both bacteria and eukaryotic cells have these, there are essential differences in the size and composition of subunits within each. These differences are sufficient to allow many anti-bacteria agents to target 70S ribosomes without substantial eukaryotic cell toxicity. The bacteria ribosome is made up of a 50S and a 30S subunit, while the eukaryotic 80S ribosome is made up of a 60S and a 40S subunit. Bacteria do not contain true membrane-bound organelles as do eukaryotic cells. The bacterial DNA is contained within an area or region of the cytosol called the nucleoid. Once again, this region is distinct within the cytosol, but is not membrane bound as is the eukaryotic nucleus.

Question 37

Peptidoglycan is made up of repeating subunits of which of the following components?

1 N-acetyl glucosamine
2 Glycerol teichoic acid
3 LPS
4 Inner cell membrane

Answer 37

N-acetyl glucosamine

Peptidoglycan is a major structural component of gram-positive bacterial cell walls. It is composed of alternating subunits of N-acetyl glucosamine and N-acetyl muramic acid. Glycerol teichoic acid is another component of the bacterial cell wall. LPS, lipopolysaccharide, is a bacterial toxin present on the surface of gram-negative cells. LPS is also referred to as bacterial endotoxin. The inner cell membrane lies beneath the bacterial cell wall and surrounds the internal cellular components.

Question 38

Bacterial cell walls are unique in nature. Do all bacteria have such structures?

1 No
2 Yes
3 Only gram-negative bacteria
4 Only gram-positive bacteria
5 Only the Mycoplasmas

Answer 38

No

Gram positive and gram-negative bacteria have cell wall structures; however, the mycoplasma lack this component.

Question 39

Thioglycolate is a common primary isolation medium that is used to culture the following type of bacteria:

1 Aerobic
2 Anaerobic
3 Gram-positive
4 Gram-negative
5 Mycobacteria

Answer 39

Anaerobic

Thioglycolate medium contains thioglycolic acid, which provides a reducing environment so that oxygen is depleted from the medium, allowing growth of anaerobes without resorting to special anaerobic techniques. The anaerobic bacteria can be either gram-positive or gram-negative. The Mycobacteria are obligate aerobes and would not survive in thioglycolate medium.

Question 40

Eosin-methylene blue (EMB) agar is used for the selection of

1 Gram-positive
2 Gram-negative
3 Aerobic
4 Anaerobic
5 Facultative anaerobes

Answer 40

Gram-negative

EMB medium is selective for gram-negative bacteria such as Salmonella and Shigella. The dyes in EMB medium interfere with electron transport that takes place in gram-positive bacteria.

Question 41

**Teichoic acids **are polymers of glycerol phosphate or ribitol phosphate and are located in the outer layer of

1 Gram-positive bacteria
2 Gram-negative bacteria
3 Viruses
4 Mitochondria
5 Golgi body

Answer 41

Gram-positive bacteria

Teichoic acids are present in the outer layer of gram-positive bacteria. Their function is unknown. They are antigenic and induce antibodies that are species specific. Gram-negative bacteria, viruses, mitochondria, and golgi body do not contain teichoic acids in their outer layer.

Question 42

A medium is prepared in the laboratory for the isolation of a certain bacterium that requires the particular components used in the preparation. The agar medium contains fresh eggs, potato flour, malachite green, and a basic salts solution. The agent for which this medium is prepared is most likely

1 Brucella abortus
2 Francisella tularensis
3 Rochalimaea henselae
4 Legionella pneumophila
5 Mycobacterium tuberculosis

Answer 42

Mycobacterium tuberculosis

Lowenstein-Jensen medium is used for the isolation of M. Tuberculosis and is an egg-based medium. Other media for laboratory cultivation are agar-based, such as the Middlebrook 7H10 medium.

Question 43

Media inoculated for the isolation of Mycobacterium tuberculosis should be incubated under which atmospheric condition?

1 10% CO2 + 90% air
2 85% N2 + 10% H2 + 5% CO2
3 100% air
4 5% CO2 (Candle jar)
5 Strict anaerobic conditions

Answer 43

10% CO2 + 90% air

All media inoculated for primary isolation of M. tuberculosis should be incubated in an atmosphere of 10% carbon dioxide. A candle jar has only 3-5% carbon dioxide. Since the mycobacteria are not anaerobic, they will not grow in an anaerobic atmosphere such as the 85% nitrogen, 10% hydrogen, 5% carbon dioxide example.

Question 44

A skin lesion is sampled that you suspect is caused by a Mycobacterium marinum. Because of the potential organisms involved, the temperature of choice for incubation is

1 21-24° C
2 33-35° C
3 35-37° C
4 25-33° C
5 40-42° C

Answer 44

25-33° C

The optimum temperature for incubating media inoculated with most specimens from the body is 35-37° C. Exceptions are made for specimens obtained from skin or superficial lesions suspected to contain Mycobacterium marinum, Mycobacterium ulcerans , or Mycobacterium haemophilu These organisms multiply best at 25- 33° C. Cultures suspected to contain Mycobacterium avium or Mycobacterium xenopi exhibit optimum growth at 40-42° C.

Question 45

Prokaryotic cytoplasm consists of

1 Mitochondria
2 Golgi bodies
3 70S ribosomes
4 Endoplasmic reticulum

Answer 45

70S ribosomes

Prokaryotic cytoplasm consists of 70S ribosomes.
Mitochondria, Golgi bodies, and endoplasmic reticulum are part of eukaryotic cytoplasmic structures.
Protein synthesis in prokaryotes as well as eukaryotes takes place on ribosomes that are complex of protein and ribonucleic acid. However, eukaryotic ribosomes are larger and heavier (80S ribosome) than prokaryotic ribosomes (70S ribosomes).

Question 46

Turbidimetric density measurement for bacterial cell concentration involves

1 Serial dilution followed by colony forming units on an agar plate
2 Viable cell count
3 Nonviable cell count
4 A and C
5 B and C

Answer 46

B and C

Cell concentration measurement by viable cell count involves serial dilution followed by determination of colony forming units on an agar surface. This method is used in determining bacterial cell count in urine cultures.
Turbidimetric density measurement for bacterial cell concentration includes viable cell count and nonviable cell count. This method is used to prepare a standard inoculum for antimicrobial susceptibility testing.

Question 47

Growth of the bacteria in a medium stops after some period because

1 Cells exhaust the nutrients
2 Increase in toxic products such as acid
3 Limited in energy source
4 Limited vitamin or amino acids necessary for growth
5 All of the above

Answer 47

All of the above

In log phase, growth is exponential and cells consume half the nutrients. Media becomes unable to support the growth because of the lack of energy source, vitamins, amino acids, and increase in toxic products lead to a decline in bacterial population.

Question 48

Pathogenic bacteria are mostly

1 Mesophiles
2 Psychrophiles
3 Autotrophs
4 A and C only
5 B and C only

Answer 48

Mesophiles

Pathogenic bacteria involve mostly Mesophiles, which grow between 20° C to 45° C. Psychrophiles grow between 0° C to 20° C.
Autotrophs do not require organic compounds because they can synthesize them from inorganic compounds.
Heterotrophs refers to bacteria that require preformed organic compounds for growth.

Question 49

Obligate anaerobes

1 Contain superoxide dismutase
2 Do not use fermentation pathway
3 Require oxygen as a hydrogen acceptor during generation of metabolic energy
4 None of the above
5 A and C

Answer 49

None of the above

Obligate aerobes contain superoxide dismutase and use respiration to generate energy where oxygen is used as final electron acceptor.

**Obligate anaerobes lack superoxide dismutase and therefore are killed by oxygen. **They use **fermentation pathway where the electron acceptor is an organic compound **and not oxygen.

Question 50

Mutations are changes that can occur in the DNA code, which changes the coded protein or prevents its synthesis. A mutation that changes a codon encoding an amino acid to a stop codon is called

1 Silent mutation
2 Missense mutation
3 Nonsense mutation
4 Frameshift mutation
5 Null mutation

Answer 50

Nonsense mutation

Silent mutation is a change at the DNA level that does not result in any change of amino acid in the encoded protein.

Missense mutation results in a different amino acid being inserted in the protein.

Nonsense mutation changes a codon encoding an amino acid to a stop codon.

Frameshift mutation is a result of insertion or deletion in the genome, leading to disruption of the gene and production of incomplete or inactive proteins.

Null mutation occurs when there is extensive insertion or deletion or gross rearrangement of the chromosome structure, which results in complete destruction of gene function.

Question 51

In microbiology, gram stained smears of an infected wound are viewed with the microscope. You note a mixture of cocco-bacilli and white blood cells on the smear. From a biological standpoint, the main difference between prokaryotic and eukaryotic cells is

1 Eukaryotes have cell walls
2 Prokaryotes lack a nuclear membrane
3 The cell wall of a(n) eukaryote is permeable
4 Eukaryotes contain circular DNA
5 Bacteria are an example of Eukaryotes

Answer 51

Prokaryotes lack a nuclear membrane

Prokaryotes are primarily distinguished from Eukaryotes by the fact that they lack a nuclear envelope around their circular DNA. Instead, genetic material is present as a diffuse area of nucleoid, which is contiguous with the cell’s cytoplasm.
Both Prokaryotes (bacteria) and some Eukaryote cells possess cell walls (i.e. plants, fungi and some protistans), but this is not the main difference between them.
The cell wall of a Eukaryote is not permeable, whereas the cell wall of a bacteria (prokaryote) is permeable.
On the contrary, it is Prokaryotes that carry circular DNA. Eukaryotes contain helical DNA molecules known as chromosomes.
Bacteria belong to the Kingdoms Archibacteriae and Eubacteriae. Organisms in Archibacteriae tend to be of a more ancient variety, surviving in extremely harsh environmental conditions where no other organisms could normally exist (i.e. oceanic volcanic vents, hot springs). Eubacteriae encompasses the vast majority of bacterial organisms found in the world.

Question 52

An oncology researcher examined several cancer cell lines for the presence of proteins, which are involved in imparting a phenotype that is resistant to chemotherapeutic agents. Using immunocytochemistry, the relative levels of these proteins were determined. Antibodies to each of the proteins were labeled with a unique fluorescent dye. Imaging and digital image analyses were used to quantify the amounts of protein present in each of the cell types. High expression of which of the following proteins that functions by extruding drugs from the cell is predictive of multidrug resistance in human cancer cell lines?

1 P-glycoprotein
2 Multidrug resistance associated protein
3 Lung resistance related protein
4 Heat shock protein
5 HER2

Answer 52

Multidrug resistance associated protein

All of these proteins have been implicated in conveying resistance of cancer cells to chemotherapeutic agents.
P glycoprotein (Pgp), a plasma membrane protein belonging to the ATP binding cassette (ABC) transporter family and the product of the MDRI gene, was the first multidrug resistance protein-lacking Pgp and conveys resistance of similar types of drugs as does Pgp.
MRP is also a membrane-associated protein of the ABC transporter group. It confers resistance by extruding the drugs from the cell. Certain cell types either lack overexpression of MRP and Pgp or the presence of these two proteins does not do much to establish the multidrug resistant phenotype to the cells. In that group of cells, overexpression of LRP, a 110-kDa protein, is observed. This protein is believed to be the major vault protein.
Over-expression of certain heat shock proteins can confer resistance to specific chemotherapeutic agents to breast cancer cells. However, the mechanism of action is to prevent induction of apoptosis that would normally occur upon treatment with those chemotherapeutic agents.
Over-expression of HER2, a tyrosine kinase belonging to the ErbB family, occurs in several human adenocarcinomas. Treatment of breast cancer cells that over-express HER2 with an antibody directed against this protein inhibit the growth of these tumor cells.

Question 53

A 9-year-old boy presents with a productive cough for the past few days. The patient also has been having fevers of 101°F. The patient has had recurrent bouts of pneumonia. His neonatal course was complicated by** meconium ileus. A sweat chloride test is positive and sputum examination reveals gram-negative rods that are oxidase-positive. The culture has a greenish tint**.

Based on the patient’s underlying condition and laboratory findings, this infection is most likely caused by what organism?

1 Pseudomonas aeruginosa
2 Staphylococcus aureus
3 Streptococcus pneumoniae
4 Haemophilus influenzae
5 Listeria monocytogenes

Answer 53

Pseudomonas aeruginosa

This child who has recurrent bouts of pneumonia, a history of meconium ileus, and a positive sweat chloride test most likely has cystic fibrosis (CF), an autosomal recessive disorder affecting chloride channel protein. Children with cystic fibrosis are very susceptible to infections. Most notable is their susceptibility to infection with Pseudomonas aeruginosa. Pseudomonas is a gram-negative rod that is strictly aerobic and oxidase-positive. Some species produce pigments such as pyocyanin and pyoverdine, which cause a greenish tint on agar growth media.
Staphylococcus species is another common pathogen for CF infections, but it will appear as gram-positive clusters, while Streptococcus species appear as gram-positive diplococci.
Haemophilus influenzae is a small gram-negative (coccobacillary) rod that causes diseases such as sinusitis and pneumonia.
Listeria monocytogenes is a small gram-positive rod most commonly associated with meningitis and sepsis in newborns and immunocompromised individuals, not pneumonia.

Question 54

A 5-year-old boy presents due to redness on the arm. The boy was stung by an insect, was scratching his arm frequently, and then developed redness at the site. There are no other symptoms. The doctor takes a specimen from the lesion for culture and a gram-positive, catalase-positive bacteria growing in clusters is isolated.

What is the most likely causative organism?

1 Bacteroides fragilis
2 Streptococcus pneumoniae
3 Streptococcus pyogenes
4 Streptococcus mutans
5 Staphylococcus aureus

Answer 54

Staphylococcus aureus

The boy most likely presents with impetigo. The characteristics of the bacteria suggest that it is Staphylococcus aureus. This is a facultative anaerobic organism that grows in clusters. It is catalase-positive and usually beta-hemolytic.

Bacteroides fragilis is an anaerobic gram-negative rod that is a normal inhabitant of the stool. They are typically implicated in lower abdominal and pelvic infections.

Streptococcus pneumoniae is a gram-positive lancet-shaped coccus that grows in pairs or chains. They are catalase-negative and typically associated with diseases such as pneumonia.
Streptococcus pyogenes is also a gram-positive, catalase-negative organism that grows in chains. It may cause cellulitis.
Streptococcus mutans is another gram-positive, catalase-negative organism that grows in chains. It is usually associated with abscesses, particularly in the mouth.