Microbial Genetics Flashcards
Methods of bacterial acquisition of new genetic material
Mutation: spontaneous or induced
Gene transfer: transformation, transduction, conjugation, transposition
Types of spontaneous mutation
Replication errors, errors in repair mechanisms
Types of induced mutations
Radiation, chemical mutagens
Types of single base pair mutations
Silent - no phenotype change
Missense - change in protein
Nonsense - premature stop
Suppressor - phenotype reversion
Types of deletion/insertion mutations
Frameshift, nonsense, suppressor
Examples of mutations leading to antibiotic resistance
Mutation of housekeeping genes: b-lactamase, nutrient transport proteins, binding site alterations
Consequences of gene transfer
Acquisition of new genetic information including virulence factors or antibiotic resistance genes
Sources of new DNA elements
Chromosomal DNA (bacterial genomic), plasmids (self-replicating episomal DNA), bacteriophages (transduction), transposons/mobile genetic elements
Transformation mechanism
Donor cell lysis releases naked DNA fragments & plasmids, fragments enter recipient cell and integrate into DNA
Transduction mechanism
Transducing phage containing donor genomic DNA released with donor cell lysis, phage infects recipient cell & donor DNA integrates into recipient DNA
Conjugation mechanism
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
Transposition mechanism
Tranposon with inverted repeats inserts in between genes at recipient site
“Competent cells” of transformation and examples
Intact cells that can efficiently pick up naked DNA during transformation
E.g. S. pneumoniae, N. gonorrheae, H. influenza
Generalized vs. specialized transduction
General: bacteriophage DNA directly integrated into bacterial genome, pulls some bacterial genes with removal
Special: pulls out specific chromosomal genes with removal
Properties of bacteriophages
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
Examples of bacteriophage-mediated virulence factors
S. aureus exfoliative toxin (SSSS)
Diphtheria toxin
Enterohemorrhagic toxin (EHEC)
Botulism toxin (some)
Diphtheria toxin
Produced by Corynebacterium diphtheriae; protein synthesis inhibitor causing ADP ribosylation of EF2, blocking protein production and cell death
Exfoliative toxin
From S. aureus; a superantigen causing massive cytokine release into systemic circulation -> vascular changes, fever, etc. including sloughing off of skin
EHEC toxin & possible mechanism of acquisition
Shigella-like toxin now found in some E. coli
Shingle infected by phage extracts toxin gene during induction and transfers to E. coli
F plasmid structure & function
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
F plasmid and gene transfer
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
Examples of plasmid-mediated virulence factors
Anthrax toxin, S. aureus toxins (some), EHEC adhesion factor, Y. entercolitica adhesion factor, Botulism toxin (some), antibiotic resistance factors
Where can transposons move?
From one chromosome to another, one location to another on same chromosome, from chromosome to plasmid, etc.
Three types of transposons
Insertion sequences: gene inactivation
Complex or composite: carry new genes
Conjugative/phage: G+ and G- can do this
Three major controls of virulence genes
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
