Session 1

Question 1

What is an infection?

Answer 1

Invasion of the host tissues by microorganisms

Question 2

What causes disease?

Answer 2

Disease caused by toxins, host response, microbial multiplication.

Question 3

Define disease

Answer 3

A disorder of structure or function in a human, animal, or plant, especially one that produces specific symptoms or that affects a specific location and is not simply a direct result of physical injury.

Question 4

What role does the patient play in the infection model?

Answer 4

Potential carrier of disease. Microorganisms can be carried on skin or mucosal membranes which are normally harmless or beneficial but transfer to other sites may be harmful

Question 5

What are commensals?

Answer 5

Another name is microbiota. It means a collection of microorganisms

Question 6

What role does the contact/intermediary play in the infection model?

Answer 6

The intermediary may be infected and carry the disease and go on to infect the patient instead of being directly from the source. Some infections require contact e.g. STIs. airborne spread may e sufficient e.g. chicken pox. vectors may sometimes be necessary e.g. Malaria. Doctors and nurses can be intermediaries in hospitals.

Question 7

What is transient carriage?

Answer 7

When an intermediary carries a pathogen without being affected by the disease.

Question 8

Do most pathogens cause disease?

Answer 8

No, most pathogens don’t cause disease unless they’re in the wrong place.

Question 9

Define pathogen

Answer 9

B bacterium, virus, or other microorganism that can cause disease

Question 10

What role does the environment play in the infection model?

Answer 10

Transmission from source to patient can occur through the patients environment. transmission can be due to ingestion of contaminated food or water, inhalation of contaminated air, contact with contaminated surfaces including medical devices.

Question 11

What role do animals play in the infection model?

Answer 11

Can act as intermediaries and infect patients. Zoonosis - Disease that can be spread to humans through animals e.g. Mad Cow Disease.

Question 12

What are the types of transmission?

Answer 12

Horizontal and vertical.

Horizontal modes of transmission - contact (direct, indirect or through vectors), inhalation (droplets or aerosol), ingestion (faecal-oral transmission).

Vertical transmission - From mother to child (can be before or during birth)

Question 13

What’s the difference between droplets and aerosol?

Answer 13

Droplets fall quickly like in a cough but aerosols stay suspended in the air

Question 14

How do microorganisms cause disease? (Process)

Answer 14

Exposure 
Adherence
Invasion
Multiplication
Dissemination (spreading)

Question 15

What are virulence factors?

Answer 15

Factors that are produced by a microorganism that add to their survival and effectiveness to cause disease. They can be put into two classes: exotoxins and endotoxins.

Exotoxins are factors that help the survival of the pathogen (e.g. cytolytic factors, AB toxins, super antigens and enzymes).

Endotoxins are factors that the host can recognise as signs of infection.

Question 16

What is host cellular damage?

Answer 16

In order to eliminate the pathogen, the host creates conditions that damage the bacteria but these conditions can also damage host cells. Damage to the host can be direct from the bacteria or as a result of the host’s immune response.

Question 17

What are disease determinants?

Answer 17

Any of a group of variables, such as specific disease agents and environmental factors, that directly or indirectly influence the frequency or distribution of a disease.

Disease determinants from the pathogen - Virulence factors, inoculum size (more pathogens = bigger problem) and antimicrobial resistance.

Disease determinants from the patient - Site of infection and co-morbidities (people with co-morbidities are more susceptible to disease)

Question 18

When reviewing a possible infection what questions must you answer?

Answer 18

Is there an infection?
Where is the infection?
What is the cause?
What is the best treatment?

Question 19

When taking a history what must you ask?

Answer 19

Symptoms (local/systemic)?
Severity of symptoms?
Duration of symptoms?
Potential exposures(e.g contact with animals, travel, swimming in unclean water, food etc.)?

Question 20

What are the types of investigation?

Answer 20

Specific and supportive.

Specific - Specific test for a specific disease E.g presence of bacteria in a culture.
Supportive - More general test. E.g Full blood count (neutrophils and lymphocytes will be higher), C-reactive protein (a biomarker of disease), blood chemistry (looking at liver and kidney function), imaging (x-ray and MRI), histopathology

Question 21

What tests might we do for bacteriology?

Answer 21

Specimen type (swab, fluid or tissue), microscopy (looking at bacterial cells using microscope e.g gram stain, or patient cells e.g cerebrospinal fluid), culture (grow the bacteria), antibiotic susceptibility, antigen detection, nucleic acid detection

Question 22

What tests might we do for virology?

Answer 22

Antigen detection (from the virus), antibody detection (the patient’s response), detecting viral nucleic acids (DNA or RNA)

Question 23

What are the microorganisms that cause disease?

Answer 23

Viruses, Bacteria, Fungi, and Parasites

Question 24

What is the structure of a virus?

Answer 24

Nucleic acid (genetic code - can be RNA or DNA ) in core, surrounded by protein coat which can be coated in a envelope. Has spikes on cell surface for attaching to specific cell surfaces.

Question 25

What are the types of DNA virus?

Answer 25

Single stranded and non-enveloped Double stranded and non-enveloped Double stranded and enveloped

Question 26

What are the types of RNA virus?

Answer 26

Single stranded, Positive strand, Icosahedral, Non-enveloped. Single stranded, Positive strand, icosahedral or helical, enveloped Single stranded, negative strand, Helical, enveloped Double stranded, icosahedral, non enveloped

Question 27

What are bacteriophages?

Answer 27

Viruses for bacteria. They can transmit genes for virulence factors and or resistance between bacteria.

Question 28

What kind of bacteria cause infections?

Answer 28

Eubacteria

Question 29

What kind of viruses tend to be easier to kill?

Answer 29

Enveloped.

Question 30

What is the general structure of a bacterium?

Answer 30

Circular DNA and plasmids in cytoplasm, no nucleus(nucleoid instead), they have a cell wall (peptidoglycan) as well as plasma membrane. Can have capsule and bacterial flagellum too. Pili on cell surface.

Question 31

What are plasmids?

Answer 31

Plasmids are small portions of DNA, many contain genes for virulence factors and resistance. This DNA can be moved to other bacteria and so are considered transferable DNA.

Question 32

What are the bacterial shapes?

Answer 32

Coccus (plural - cocci) - spherical. Bacillus (plural - bacilli) - rod shaped Spirillus (less common) - zig zag shaped.

Question 33

What are the arrangements of cocci?

Answer 33

Clusters or chains

Question 34

What makes a bacteria gram negative or gram positive?

Answer 34

Gram-positive - Outer structure is plasma membrane then periplasmic space then peptidoglycan layer. Stain purple Gram-negative - Outer structure is plasma membrane then periplasmic space, then peptidoglycan, then an additional periplasmic space then an outer membrane of lipopolysaccharide and protein. Stain pink.

Question 35

What is the oxygen tolerance of bacteria?

Answer 35

Aerobes - Can survive in the presence of oxygen. Obligate aerobes - Require oxygen for survival. Anaerobes - Can survive in the absence of oxygen. Obligate anaerobes - Require oxygen free environment for survival (unless they can from spores).

Question 36

How do we name bacteria?

Answer 36

Using Linnaean taxonomy. Genus then species e.g Staphylococcus aureus. Names can often be supplemented by adjectives describing growth or susceptibility e.g. E. coli, 0157, MRSA (methicillin resistant Staph aureus.) Names can change

Question 37

Describe the differences between eukaryotes and prokaryotes.

Answer 37

Prokaryote DNA is circular, usually single stranded. extra chromosomal DNA may also be present in the form of plasmids. Eukaryotes have multiple chromosomes. Prokaryotes don't have a nucleus or nucleoli whereas Eukaryotes do. Prokaryotes don't have membrane-bound organelles whereas as eukaryotes do e.g. mitochondria, Golgi apparatus, endoplasmic reticulum. Prokaryotes usually have a cell wall which may contain peptidoglycan whereas in eukaryotes only plant cells have cell walls which are made from cellulose and no peptidoglycan. in prokaryotes theres no carbohydrate in the plasma membrane and most lack sterols in their plasma membrane whereas in eukaryotes sterols and carbohydrates are present in the plasma membrane. In prokaryotes they have 70s ribosomes but in eukaryotes they have 80s (70s in organelles)

Question 38

What are the types of fungi?

Answer 38

Yeasts - Single celled | Moulds - Multicellular

Question 39

What are the types of parasite?

Answer 39

Protozoa - Single celled | Helminths - Multicellular, worms

Question 40

What is an agar plate?

Answer 40

Agar is a polysaccharide derived from seaweed used to culture bacteria in agar plates. it melts at 85 degrees C but doesn't return to its gel form until it reaches 34-42 degrees. Its clearer than gelatin and resists digestion from bacterial enzymes. The use of agar allows the creation of a medium that can be inoculated at 40 degrees in its cooled molten state (gel) and yet incubated at 60 degrees C without melting. Commonly contain additional ingredients to facilitate growth, including proteins and sodium chloride. Standard agar is red due to the addition of blood. As a result it's considered an enriched medium as the plate with the protein and blood provides sufficient nutrients for a number of common bacteria.

Question 41

What's chocolate agar?

Answer 41

Some bacteria are inhibited by blood. some bacteria e.g Haemophilus influenza will only grow if the blood cells have been lysed releasing their intracellular nutrients. The lysed blood is darker so the agar used is called chocolate agar.

Question 42

What is the cell envelope?

Answer 42

The bacterial “cell envelope” is a term applied to all material external to and enclosing the cytoplasm. It consists of several chemically and functionally distinct layers, the most prominent of which are the cell wall and the cytoplasmic membrane. The cell envelope also includes the capsule or glycocalyx, if present. The cell membrane is composed of phospholipid, the molecules of which form two parallel surfaces (called a lipid bilayer) such that the polar phosphate groups are on the outside of the bilayer and the nonpolar lipid chains are on the inside. The membrane acts as a permeability barrier, restricting the kind and amount of molecules that enter and leave the cell. ALL bacteria have a cell wall. The common feature of the cell wall is a Peptidoglycan layer. The peptidoglycan layer determines the shape of the cell. It is composed of a cross-linked polymeric mesh. The molecular structure of this layer helps render the bacterial wall resistant to host peptidases such as those in the intestine. This helps the bacteria survive in a hostile environment.

Question 43

What's the difference between gram-positive and gram-negative?

Answer 43

Gram-positive: Gram-positive bacteria have thick, multi-layered, peptidoglycan cell walls that are exterior to the cytoplasmic membrane Gram-negative: Gram-negative bacteria have a more complex cell wall structure composed of two membranes (an outer membrane and an inner, that is, cytoplasmic, membrane). The two membranes are separated by the periplasmic space, which contains the peptidoglycan layer. In contrast to gram-positive cells, the peptidoglycan layer of gram-negative cells is thin, and the cells are consequently more susceptible to physical damage. The outer membrane is distinguished by the presence of embedded lipopolysaccharide (LPS) The polysaccharide portion of LPS (O-polysaccharide) is antigenic and can, therefore, be used to identify different strains and species. The lipid portion (called lipid A) is embedded in the membrane and is toxic to humans. Because lipid A is an integral part of the membrane, it is called endotoxin, as opposed to exotoxins, which are secreted substances.

Question 44

Describe the capsule

Answer 44

Many bacteria secrete a sticky, viscous material that forms an extracellular coating around the cell. The material is usually a polysaccharide. This is the CAPSULE. It is not present in all bacteria. It can be found in some gram-positive bacteria and some gram-negative bacteria. The capsule allows cells to adhere to surfaces, protect bacteria from antibodies and phagocytosis, and act as diffusion barriers against some antibiotics, thus contributing to the organisms’ pathogenicity. Capsules can also protect bacteria against dessication, or drying, which facilitates transmission. Later in the Unit you will learn of some specific bacteria that have capsules and the clinical relevance of the capsule. To complete the topic at this stage the important bacteria containing a capsule include: • Streptococcus pneumoniae (pneumococcus) • Neisseria meningitidis (meningococcus) • Group B streptococcus (GBS) • Hemophilus influenzae type b (Hib)

Question 45

What role does the spleen have with bacteria?

Answer 45

The spleen is especially important in recognising these bacteria and dealing with them. If a patient does not have a spleen then they are at extra risk of infections from these bacteria. Such patients will commonly need to be immunised against these bacteria and/or receive prophylactic antibiotics.

Question 46

How is a gram stain prepared?

Answer 46

* Step 1. Prepare a slide * Step 2. Drop a few drops of crystal violet stain onto the fixed culture (enough to cover the specimen) and let it stand for 20 seconds. Then pour off the crystal violet stain and gently rinse the excess stain with distilled water. o The objective of this step is:  To allow the crystal violet stain to bind to the peptidoglycan molecules of the Gram + bacteria (if present). Remember, Gram positive bacteria have a large peptidoglycan layer located outside the bacterial "inner membrane".  To allow the crystal violet stain to bind to the lipopolysaccharide molecules attached to the "outer membrane" of the Gram negative bacteria (if present). Remember, while Gram positive bacteria have no "outer membrane", Gram negative bacteria have lipopolysaccharide molecules attached to their bacterial "outer membrane". • Step 3. Drop a few drops of iodine solution on the smear and let it stand for 20 seconds. Then pour off the iodine solution and rinse the slide with distilled water. o The objective of this step is to fix the crystal violet to the peptidoglycan molecules on the Gram + bacteria. * Step 5. Drop a few drops of decolorizer (acetone or ethanol) Then IMMEDIATELY rinse the slide off with distilled water after 5 seconds. o The objective of this step is to dissolve the lipopolysaccharide membrane in the Gram - bacteria and expose the thin peptidoglycan layer below. * Step 6. Drop a few drops of basic counterstain (fuchsin or safranin) on the slide and let it sit for 20 seconds, then wash off the solution with distilled water. o The objective of this step is to stain the peptidoglycan layer of the Gram negative bacteria a pink / red colour. Remember that the addition of iodine to the crystal violet in Step 4 binds the crystal violet stain in the Gram positive bacteria, so the counterstain is unable to bind to the peptidoglycan wall in the Gram positive bacteria in the specimen. Gram positive bacteria retain the crystal violet dye, = BLUE / PURPLE Gram negative bacteria: a counterstain (commonly safranin or fuchsine) added after the crystal violet gives all Gram-negative bacteria a red or pink coloring. = PINK

Question 47

What is white blood cell count?

Answer 47

• White blood cell (WBC) count is a count of the actual number of white blood cells per volume of blood. Both increases and decreases can be significant.

Question 48

What is the white blood cell differential?

Answer 48

White blood cell differential: looks at the types of white blood cells present. There are five different types of white blood cells, each with its own function. The differential classifies a person's white blood cells into each type: neutrophils (also known as PMNs), lymphocytes, monocytes, eosinophils, and basophils.

Question 49

What might affect white blood cell count?

Answer 49

May be increased with infections, inflammation, cancer, leukaemia; decreased with some medications, some autoimmune conditions, some viral or severe infections, bone marrow failure, enlarged spleen, liver disease, alcohol excess and congenital marrow aplasia (marrow doesn't develop normally).

Question 50

What might affect neutrophil count?

Answer 50

Neutrophil This is a dynamic population that varies somewhat from day to day depending on what is going on in the body. Neutrophils are one of the first lines of defence against bacterial infection. Neutrophils can be increased in some bacterial infections and would support a diagnosis. In some infections the white count and neutrophil count can be suppressed.

Question 51

What might affect lymphocyte count?

Answer 51

Can be increased or decreased by viral infections. Can be increased in conditions such as chronic lymphocytic leukaemia