Bacterial Structure and Genetics

Question 1

Briefly describe the process by which prokaryotes divide.

Answer 1

Transverse fission at a rate of rapid multiplication (a division cycle as short as once every 20 minutes).

Question 2

What is the significance of the bacterial cytoskeleton pertaining to replication?

Answer 2

The bacterial cytoskeleton contains actin homologues (i.e. mreB and parM) required for even chromosome and plasmid segregation between the daughter cells.

Question 3

What is the purpose and the 3 layers of the bacterial envelope?

Answer 3

The main interface with the environment that maintains osmotic integrity of the cell. The 3 components are the PM, cell wall and structures outside cell wall (lipids, carbs).

Question 4

Where does cellular respiration occur in prokaryotes?

Answer 4

Complexes of the respiration chain lie on the PM, since bacteria don’t have mitochondria.

Question 5

Define the structure of peptidoglycan. Describe the monomer. What is the significance?

Answer 5

Long-chain polysaccharides with peptide cross-links that are not prevalent in mammalian cells (making them a prime target for antibiotics). The monomer is a N-acetyl-glucosamine and N-acetyl-Muramic acid dissacharide. Off of the latter hangs a 5 AA side chain composed of D’s and L-AAs.
D-amino acids signals TLRs as non-self

Question 6

What is the importance of cross-linking in peptidoglycan cell wall?

Answer 6

Cross-links from side to side and up-and-down maintains stability in the cell wall. Holes in the cell wall result in osmotic lysis.

Question 7

Describe how penicillin works as an antibiotic. Why is it now ineffective?

Answer 7

Penicillin is homologous to the D-ala-D-ala shape of the bacterial peptidoglycan. This ideally prevents cross-linking leading to loss of stability and osmotic lysis in actively growing cell populations. It’s now outdated because bacteria developed mutations and enzymes that disrupts the functions of penicillin.

Question 8

Describe the importance of the Gram stain procedure.

Answer 8

This important medical procedure is the first step in ID of bacteria. From this, a clinician can ascertain what type of antibiotic would be most effective against a bacterial infection.

Question 9

Describe 3 ways in which Gram + bacteria differ from Gram -.

Answer 9

Gram-positive bacteria are THICCC, have 20-50 layers of peptidoglycan, stain purple and have protein-lipid structures on the outside. This differs from Gram-negative that are thin, have 1-3 layers, stain pink (from Safranin counterstain) after a Gram stain procedure and have a 2nd layer called an outer membrane above the PEP.

Question 10

List the 5 steps of the Gram stain procedure.

Answer 10

1. Heat fix bacterium after applying a loop on a slide.
2. Stain with Gentian/ crystal violet
3. Add Iodine mordant (dark dye) that lodges inside peptdoglycan; (stains Gram + purple; Gram - is clear)
4. Decolorize dye with Alchohol
5. Counterstain with red dye (Safranin) to stain Gram (-)

Question 11

Define the features of protein fibrillae. What is their clinical significance?

Answer 11

Pili are attachment factors that adhere to mammalian cells and ECM, found in both Gram +/- bacteria. They are virulence factors that are covalently linked to PEP and appear as fuzz in an EM. These are essential for virulence, making them a target for antibiotics.

Question 12

Define features of Teichoic acids. What is their clinical significance?

Answer 12

These are polymers of sugar alcohols unique to Gram (+) bacteria that provide stability for the integrity of the cell wall. These are flags for the innate immune system via TLRs.

Question 13

What is the periplasm of bacteria?

Answer 13

The periplasm is a clear-looking space between the inner and outer membranes that contains carrier proteins, polysaccarides and enzymes. Protein assemblies that lie in the periplasm link the inner and outer membranes.

Question 14

Define Lipopolysaccharide and its 3 components.

Answer 14

LPS is a dissacharide with attached FAs that composes the outer leaflet of the outer membrane in Gram (-) bacteria. The O-antigen is a polymer repeat of 3-5 sugars that can be changed (i.e. elongate) to evade the immune system. The Core is a phosphorylated complex oligosaccharide. Lipid A is the endotoxin that anchors the LPS to the outer membrane.

Question 15

Describe the structure and function of porin proteins.

Answer 15

Trimer, barrel-shaped proteins that allow solutes to cross the outer membrane. The interior is hydrophilic and the outside is hydrophobic. The size of the pore can regualte what can enter/ exit, which is signicant for antibiotic molecules (since some organisms have tiny pores).

Question 16

How does modifying the O-antigen help a Gram (-) bacteria evade the immune system?

Answer 16

O-antigen can vary in length depending on the species. The length diverts the formation of the MAC complex that would normally lead to lysis. Very long O-antigens won’t allow C3b to attach to the cell wall.

Question 17

Describe the toxicity of Lipid A.

Answer 17

Lipid A of LPS binding to TLR leads to the production of inflammatory cytokines and to septic shock, organ dysfunction or DIC, if systemic in circulation. The wide sudden use of antibiotics can kill bacteria leading to the Jarish-Herxheimer reaction (fever and hypotension).

Question 18

Describe the Jarish-Herxheimer reaction.

Answer 18

This is a reaction that can be brought upon by sudden use of antibiotics to kill a Gram (-) bacterial infection. Release of endotoxin can result in fevers, chills and hypotension. Common in mass lysis of spirochete or Syphillis infections.

Question 19

Define the function of the structure and function of the flagella.

Answer 19

This structure is composed of a basal body that rotates a filament in the envelope for locomotion. Counter clockwise rotation propels the cell linearly and cw rotation causes random “tumbling” in the process of chemotaxis.

Question 20

How do pili differ from flagella?

Answer 20

Pili enable the bacteria to adhere to host cells or each other. The conjugation pilus is only formed when DNA is transferred via the F-plasmid.

Question 21

What role do capsules play in bacterial survival? What is the clinical relevance of this?

Answer 21

Capsules are K-antigen polysaccharides that require a special stain to see. They protect bacteria from phagocytosis which is why opsonization by antibodies or complement is so important for controlling spread of infection as an immune response.

Question 22

Describe 3 features of Bacteria spores.

Answer 22

1. Formed by Gram (+) rods under starvation or environmentally hazardous conditions
2. inert, suspended animation state that are resistant to boiling
3. Involve the disintegration of mother cell to save the genetic material in an endospore that can only be killed by autoclave technique

Question 23

What is a key feature about spores?

Answer 23

Spores DO NOT Gram-stain due to thick walls and can only be killed by autoclave!

Question 24

Describe the 2 modes of genetic change in Niesseria gonorrhoeae that result in penicillin resistance.

Answer 24

1. Mutations accumulate after antibiotic drug use to prevent inhibition of growth
2. Plasmid-based gene encodes for an enzyme that hydrolyzes penicillin.

Question 25

Explain the importance of genome-size in relation to bacterial life-style.

Answer 25

The smaller the genome, the more obligate the bacterium is as a host parasite. For example, E. coli (4700 genes) makes all required compounds from glucose; whereas, Mycoplasma (470 genes) has no cell wall and requires all small molecules for homeostasis.

Question 26

What are replicons? Describe their significance.

Answer 26

These are molecules with site for initiation of DNA synthesis (i.e. chromosomes, plasmids and viruses). They have sites for partition of replicated DNA into daughter cells.

Question 27

What 3 types of genetic material are not replicons.

Answer 27

Insertion sequences, transposon and pathogenicity islands are not replicons and replicate when integrated into a replicons (i.e. in a plasmid).

Question 28

Describe the significance of a temperate bacteriophage life cycle.

Answer 28

Temperate bacteriophages integrate into bacterial chromosomes as a provirus that can be passed down as copies in progeny cells.

Question 29

Define and explain the significance of an insertion sequence.

Answer 29

Insertion sequences (IS) can “hop” into other parts of chromosomal DNA due to inverted repeats. This is done by the help of transposase enzyme.

Question 30

How do transposon differ from insertion sequences. how do they form?

Answer 30

Transposons (Tn), unlike insertion sequences, contain genes unrelated to transposition (i.e. anti-biotic resistance genes). These are formed when a 2nd copy of IS inserts on the other side of one IS and a resistance gene (per se). Damage of internal inverted repeats locks the whole structure together.

Question 31

Define Pathogenicity Islands (PI).

Answer 31

PI are very large or multiple transposons that can contain a “complete kit” of virulence genes

Question 32

Briefly describe the 3 mechanisms by which DNA transfer btw bacteria occurs.

Answer 32

1. Transformation - DNA released by lysis is picked up by another
2. Conjugation - DNA transfer by cell-to-cell contact (via pili)
3. Transduction - bacterial DNA packaged into a viral particle and transferred to another bacteria via infection.

Question 33

Define merozygote.

Answer 33

A merozygote is an intermediate formed btw the donor cell and recipient cell upon DNA. This contains a whole copy of the recipient chromosome and a fragment from the donor cell.

Question 34

Describe the process of conjugation. What is the significance?

Answer 34

This is a highly efficient process where a “F-fertility” bacterium transfers plasmid DNA to a recipient. A conjugation bridge is made, DsDNA unzips and one strand stays in the donor as other crosses into the recipient. Replication recreates the opposite strand to re-circularize the DNA. This can occur in very distantly related bacterial species

Question 35

What are R-factors and how do they differ from F-factors?

Answer 35

R-factors are F-like plasmids with multiple antibiotic resistance genes. These are the single GREATEST cause of multi drug resistance in nature conferring to superbugs.

Question 36

Describe the 2 modes of transduction.

Answer 36

1. Viral particle may contain bacterial but not viral DNA to transfer upon infection.
2. A viral genome may incorporate one or more bacterial genes to spread throughout a bacterial population.

Question 37

How do proviruses, IS and transposons differ from replicons?

Answer 37

They bypass DNA homology. Can spread virulence genes across unrelated bacteria.

Question 38

Describe antigenic phase variation and it’s clinical importance.

Answer 38

These are programmed DNA alterations that result in new antigenic variants that have the pathogen turn the specificity of the immune response against itself. Main mode that prevents vaccines against Malaria, Trypanosomiasis and Gonorrhea.

Question 39

Describe the 3 variations of phase variation.

Answer 39

1. Inversion if DNA segment
2. Recombination between expressed and silent genes
3. Stuttering by polymerase when copying a repeat

Question 40

Briefly describe the phase variation of Salmonella flagella, in 3 mechanisms.

Answer 40

Salmonella contains 2 genes for flagellin in which him enzyme functions to invert DNA repeats. Both H1 and H2 can’t be expressed simultaneously.
In the 2nd mech, silent genes have crossover recombination with expressed copy resulting in a mosaic gene.
In the 3rd mech, protein PII allows the bacterial cell to stick to epithelial cells (when made in multiples of 3 nucleotides)