Flashcards in Bacterial Structure and Classification Deck (55):
What are the two major shapes of bacteria?
rods (bacilli) and spheres (cocci).
Bacilli can be found in chains, but chains of cocci are more often noted and are called _______.
How can cocci present?
Cocci can also be found in pairs (diplococci) in addition to chains, e.g., the classic “kissing kidney beans” appearance of Neisseria gonorrhoeae, or in “grape-like clusters” characteristic of **staphylococci**
What are some other bacterial shapes?
comma-shaped vibrio species, e.g., Vibrio cholerae, the causative agent of a watery diarrhea, and corkscrew-shaped spirilli and spirochetes, for which the etiologic agent of syphilis, Treponema pallidum, is an example.
What are the major differences between eukaryotic and prokaryotic cells (i.e. bacteria)?
1. Bacteria are much smaller (yet are significantly larger than viruses)
2. Bacteria lack a formal nucleus and other membrane-bound organelles associated with eukaryotes. The bacterial genome usually comprises a single, circular DNA chromosome that lacks introns and is not bound by histone proteins.
3. While animal cells lack cell walls, bacteria synthesize cell walls composed of peptidoglycan, which distinguishes them from the cell walls of plants (composed of cellulose) and fungi (chitin)
What structure of bacteria is a major target for therapeutic intervention? Why?
because peptidoglycan is unique to bacteria, the cell wall and its biosynthetic machinery serve as major targets for antimicrobial therapy
NOTE: The ribosome, smaller in bacteria than animal cells, RNA polymerase, and topoisomerases are also good targets
What cell wall characteristics distinguish gram- positive bacteria from gram-negative bacteria?
it is thick and heavily cross-linked in Gram positive bacteria and thin and lightly cross-linked in Gram negative bacteria
Significant points about the cell membranes of bacteria (2 things)
1. Bacteria cell membranes lack sterols in their membranes.
2. Gram negative bacteria possess a second, outer membrane
What is the major structural difference between gram-positive and gram-negative bacteria?
the major difference is the additional outer membrane of Gram negative bacteria
What are the major roles of pili in bacteria?
NOTE: they are found in both gram-positive and negative bacteria, but are much more common in gram-negative bacteria
The “sex pilus” is used to transmit genetic material from one bacterium to another during conjugation.
The major role of pili in pathogenesis, however, is to attach bacteria to host cell surfaces. Without firm attachment to some tissues, colonization is difficult.
What kind of antigen so flagella possess? Do all bacteria have flagella?
flagella bear the “H” antigen used in serotyping. Not all bacteria are motile nor do all possess pili for attachment.
What are capsules and what do they do?
Capsules are external structures composed of either polysaccharide or polypeptides, and serve to protect encapsulated bacteria from phagocytosis.
Do all bacteria contain capsules?
Not all bacteria are encapsulated, but virtually every bacterium that causes **meningitis** contains a capsule.
What are spores?
Spores (or endospores) are dehydrated, dormant forms that allow potential pathogens to survive (in some instances, for centuries) during harsh conditions. The Bacillus species that causes anthrax is a prime example.
What kinds of bacteria can form spores?
What is the composition of gram-positive cell walls?
Up to 50% of the Gram positive cell wall is composed of teichoic acid or lipoteichoic acid, polymers of glycerol phosphate or ribitol phosphate
lesser species such as the carbohydrate Lancefield antigens of streptococci (important in serotyping) and M proteins of group A streptococci.
Where is the peptidoglycan layer of gram-negative bacteria found?
generally only two layers deep, lightly cross-linked, and lies between the outer and cytoplasmic membranes, a region called the periplasmic space
What is the purpose of the outer membrane in gram negative bacteria?
is negatively charged, which helps these bacteria evade phagocytosis, hinder antibiotic uptake, and avoid the action of complement.
How do nutrients enter gram negative bacteria?
To allow the import of nutrients, the outer membrane contains porins, molecular sieves that allow substrates such as sugars to be transported across the membrane. The sugars are then bound by periplasmic binding proteins and delivered to the cytoplasmic membrane, where they are again transported into the cytoplasm.
What is a major component of the OUTER membrane of gram negative bacteria?
lipopolysaccharide (LPS), found in the outer leaflet of the membrane.
What is the composition of LPS?
LPS contains lipid A, a toxic phospholipid also known as endotoxin, a core polysaccharide, and O antigen polysaccharide, the major surface antigen of Gram negative bacteria and an important component for serotyping some species
How does the body react to Lipid A?
Lipid A is recognized by the innate immune response to elicit a “cytokine storm” which leads to sepsis or septic shock.
NOTE: Gram positive bacteria are now the major cause of sepsis despite lacking lipid A; the eliciting antigen is not clearly understood.
T or F. The successful pathogenesis by some bacteria depends on their ability to secrete proteins or other substances from the cell.
How do gram negative bacteria secrete proteins from the cell?
To date, at least five such systems have been described, but three are most significant to viability and pathogenesis: Types I, II, and III
How do Type I secretion systems work in gram-negative bacteria?
Type I secretion systems are members of the ABC (ATP binding cassette) family of transporters. All Gram negative bacteria appear to encode a Type I system, through which proteins are directly secreted into the extracellular environment from the cytoplasm (i.e. don't stop in the periplasm).
What is the significance of Type I secretion systems to bacterial pathogenesis?
Antimicrobial drugs can be expelled from the cell using these systems, thus promoting resistance to the antibiotic.
Are Types I and II secretion systems found in ALL gram negative bacteria?
Yes, ALL. No gram positive bacteria
What are Type II secretory systems used for?
Type II systems serve as the general secretory pathway, used to deliver proteins to the periplasm and extracellular spaces. In the Type II system, proteins are first secreted to the periplasm before being secreted across the outer membrane
Where are Type III systems found?
Type III protein secretion systems are found only in pathogens
Where are Type III systems referred to as “molecular syringes”?
because their structure allows the direct injection of toxins and other virulence factors into the cytosol of a targeted animal cell.
How are gram stains performed?
In the Gram method, a suspension of bacterial cells is dried and heat-fixed to a glass slide. A purple stain, crystal violet, is applied to the
cells, and then “fixed” to the cells by the addition of a mordant, typically iodine. The cells are then destained by the application of alcohol to the cells. The extensive cell wall of Gram positive bacteria retains the crystal violet while the thin layer of peptidoglycan found in Gram negative bacteria does not. As a result, Gram positive bacteria stain purple/blue in the Gram protocol. Gram negative bacteria are counterstained with safranin, a red/pink dye, so that they can be seen under a microscope.
What happens when the cell wall of enzymes is broken down?
When the cell wall is disrupted under physiologic conditions the bacterial cell lyses.
**peptidoglycan is a prominent target for antimicrobial therapy**
What is peptidoglycan composed of?
Peptidoglycan is composed of a backbone of repeating disaccharides comprising N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG)
The disaccharides of peptidoglycan are linked to form long chains by an enzyme termed a _____.
How are peptidoglycan molecules attached to one another?
A pentapeptide composed of L- and D-isomers of amino acids is attached to the NAM residue of each disaccharide, and these pentapeptides cross-link to one another to bridge the sugar backbones.
What enzymes facilitate the cross-linking of peptidoglycan molecules?
transpeptidases and carboxypeptidases, the former often referred to as penicillin-binding proteins because this is how they were first discovered: they are the targets of the antibiotic penicillin.
T or F. The cross-linking differs between Gram positive and Gram negative bacteria
T. it is a simple bond in Gram negative bacteria, but in Gram positive bacteria, a pentaglycine interpeptide links the polypeptides
NAM-pentapeptides contain what at their terminal end?
a terminal D-alanine pair. These amino acids are critical to the cross-linking process and the ultimate D-alanine is removed by the carboxypeptidases. The pair is also a target of antimicrobials.
What does Cycloserine do?
inhibits the incorporation of the a terminal D-alanine pair into the pentapeptide side chain
What do glycoproteins such as Vancomycin do?
bind the terminal D-alanine pair, masking their availability for cross-linking.
How does cell wall assembly occur?
Cell wall assembly begins in the cytoplasm, where individual NAM-NAG disaccharides linked to a pentapeptide side chain are synthesized and attached via a diphosphate linkage to a lipid carrier. The lipid carrier is localized in the cytoplasmic membrane and is used to transport individual disaccharide-pentapeptide subunits across the membrane to the external face of the cytoplasmic membrane. There the subunit is incorporated into the growing cell wall by the action of glycosidases and transpeptidases.
What happens once the lipid carrier drops off its dissacharide to the growing cell wall?
The lipid carrier then recycles to the interior leaflet of the cytoplasmic membrane to recharge with new cargo, but only after dephosphorylation of the carrier has occurred.
What drug targets/inhibits the dephosphorylation of the lipid carrier during cell wall assembly?
What is Lysozyme?
Lysozyme is a natural defense that targets the peptidoglycan cell wall. Lysozyme is a glycosidase that hydrolyzes the bond between NAM and NAG, thus disrupting the sugar backbone instead of the pentapeptide cross-links
Where is lysozyme present?
Lysozyme is present in tears, saliva, and in the lysosomes of phagocytic cells, and is particularly effective against Gram positive bacteria.
What are B-lactams? How do they work?
Antibiotics tat are centered on the β-lactam ring, which structurally resembles the D-alanine D-alanine terminal pair of the pentapeptide side chains. Because of this resemblance, the β-lactams are bound by the transpeptidases (i.e., penicillin-binding proteins) of the bacteria and prevent cross-linking from occurring
Thus, these are competitive inhibitors
What are some examples of B-lactams?
penicillin G, isolated from Penicillium chrysogenum, cephalosporins, carbapenems, and monobactams
How can bacteria be resistant to B-lactams?
The β-lactam ring is cleaved by a number of β-lactamases produced in resistant bacteria OR other resistant bacteria produce mutated transpeptidases that no longer bind β-lactams.
How does Vancomycin resistance occur?
occurs when D-alanine D-lactone is synthesized by bacteria instead of D-alanine D-alanine. The new pair is still recognized by transpeptidases, but not vancomycin.
What do B-lactams require for activity?
Under normal growth conditions, bacterial autolysins act much like lysozyme to cleave cell walls at sites for new subunit insertion. When β-lactams then inhibit the cross-linking of new subunits, cells lyse due to a loss of structural integrity. In non-dividing cells, the autolysins do not act to cleave cell walls, so inhibition by β-lactams does not have deleterious effects.
What types of bacteria do not stain using a gram stain?
Mycoplasma species, the smallest free-living organisms, lack a cell wall, and Chlamydia species lack peptidoglycan even though they contain the double membrane structure of Gram negative bacteria. Neither of these genera stains by the Gram method.
Members of the genus Mycobacterium contain a cell wall that is covered with a waxy coat containing mycolic acid; this structure interferes with proper Gram staining
How are mycobacteria identified?
using a method that allows a red stain, carbol fuchsin, to penetrate the cells.
Once stained, the waxy surface of mycobacteria prevents the cells from being destained with acid-alcohol, a reagent that efficiently destains Gram organisms. Hence, mycobacteria are said to be acid-fast. The red stained cells are generally observed in contrast to a blue- or green-stained background. Prominent Mycobacterium species cause tuberculosis or leprosy.
The growth property most associated with classification is what?
the bacterium’s response to oxygen.
Aerobic bacteria (aerobes) require oxygen and use respiration for growth. Anaerobic bacteria (anaerobes) are inhibited or killed in the presence of oxygen and use fermentation exclusively for metabolism. Facultative bacteria (facultative anaerobes) represent the majority of pathogens and use respiration in the presence of oxygen and use fermentation in its absence.
What are some factors used for classification of bacteria?
• Staining properties
• Growth properties
• Fermentation properties
• Other enzymatic and virulence properties