Microbial Genetics

Question 1

Methods of bacterial acquisition of new genetic material

Answer 1

Mutation: spontaneous or induced

Gene transfer: transformation, transduction, conjugation, transposition

Question 2

Types of spontaneous mutation

Answer 2

Replication errors, errors in repair mechanisms

Question 3

Types of induced mutations

Answer 3

Radiation, chemical mutagens

Question 4

Types of single base pair mutations

Answer 4

Silent - no phenotype change
Missense - change in protein
Nonsense - premature stop
Suppressor - phenotype reversion

Question 5

Types of deletion/insertion mutations

Answer 5

Frameshift, nonsense, suppressor

Question 6

Examples of mutations leading to antibiotic resistance

Answer 6

Mutation of housekeeping genes: b-lactamase, nutrient transport proteins, binding site alterations

Question 7

Consequences of gene transfer

Answer 7

Acquisition of new genetic information including virulence factors or antibiotic resistance genes

Question 8

Sources of new DNA elements

Answer 8

Chromosomal DNA (bacterial genomic), plasmids (self-replicating episomal DNA), bacteriophages (transduction), transposons/mobile genetic elements

Question 9

Transformation mechanism

Answer 9

Donor cell lysis releases naked DNA fragments & plasmids, fragments enter recipient cell and integrate into DNA

Question 10

Transduction mechanism

Answer 10

Transducing phage containing donor genomic DNA released with donor cell lysis, phage infects recipient cell & donor DNA integrates into recipient DNA

Question 11

Conjugation mechanism

Answer 11

Free plasmid moves from donor to recipient cell via sex (F) pilus
OR
Integrated plasmid (episome) promotes transfer of genomic DNA, which integrates into recipient DNA

Question 12

Transposition mechanism

Answer 12

Tranposon with inverted repeats inserts in between genes at recipient site

Question 13

“Competent cells” of transformation and examples

Answer 13

Intact cells that can efficiently pick up naked DNA during transformation
E.g. S. pneumoniae, N. gonorrheae, H. influenza

Question 14

Generalized vs. specialized transduction

Answer 14

General: bacteriophage DNA directly integrated into bacterial genome, pulls some bacterial genes with removal
Special: pulls out specific chromosomal genes with removal

Question 15

Properties of bacteriophages

Answer 15

Lytic cycle: replication, assembly, lysis, release
Lysogeny: integration of phage DNA into genome
Induction: release of lysogenized phage DNA and start of lytic cycle in response to a number of env stimuli

Question 16

Examples of bacteriophage-mediated virulence factors

Answer 16

S. aureus exfoliative toxin (SSSS)
Diphtheria toxin
Enterohemorrhagic toxin (EHEC)
Botulism toxin (some)

Question 17

Diphtheria toxin

Answer 17

Produced by Corynebacterium diphtheriae; protein synthesis inhibitor causing ADP ribosylation of EF2, blocking protein production and cell death

Question 18

Exfoliative toxin

Answer 18

From S. aureus; a superantigen causing massive cytokine release into systemic circulation -> vascular changes, fever, etc. including sloughing off of skin

Question 19

EHEC toxin & possible mechanism of acquisition

Answer 19

Shigella-like toxin now found in some E. coli

Shingle infected by phage extracts toxin gene during induction and transfers to E. coli

Question 20

F plasmid structure & function

Answer 20

Plasmid allowing formation of sex pilus
F+ cells have pilus, F- have pilus receptor on surface
Encodes endonuclease cleaves at OOT so both get full dsDNA plasmid

Question 21

F plasmid and gene transfer

Answer 21

Bacteria with high frequency of recombination can recombine genes around integrated F plasmid = more mutation, so when it pulls out, it may take other genes and confer antibiotic resistance, toxins, other traits to other bacteria/organisms

Question 22

Examples of plasmid-mediated virulence factors

Answer 22

Anthrax toxin, S. aureus toxins (some), EHEC adhesion factor, Y. entercolitica adhesion factor, Botulism toxin (some), antibiotic resistance factors

Question 23

Where can transposons move?

Answer 23

From one chromosome to another, one location to another on same chromosome, from chromosome to plasmid, etc.

Question 24

Three types of transposons

Answer 24

Insertion sequences: gene inactivation
Complex or composite: carry new genes
Conjugative/phage: G+ and G- can do this

Question 25

Three major controls of virulence genes

Answer 25

Gene amplifications/ rearrangements: Pseudomonas aeruginosa phase variation in CF
Transcriptional regulation (+ and -): of multiple genes like toxins
Post-txn regulation: A-B subunits of cholera, AC toxin of B. pertussis