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6 Horizontal Gene Transfer

Transfer of DNA from one bacterium to another which is not its offspring


6 Genetic Transformation

Transfer of naked DNA b/n cells

Natural Transformation occurs in bacteria such as Streptococcus, Haemophilus and Neisseria which produce a protein called the competence factor (acquired property of taking up DNA)

ss DNA is integrated into the recipient DNA by homologous recombination mediated by the RecA protein

Result: replacement of the recipient DNA by the donor DNA

Not widespread: only occurs b/n similar, but not identical, species


6 Genetic Transduction

DNA transfer mediated by a bacterial virus (bacteriophage)

A donor cell is infected by a bacterial virus (phage).

Phage replicates in the cell and degrades host DNA.

Phage particles are assembled and some of them contain small pieces of bacterial DNA.

Upon reinfection of a bacterial cell, the DNA from the original cell is integrated into the recipient chromosome by homologous recombination.

Not widespread: only occurs b/n similar, but not identical, species


6 Conjugation

DNA transfer involving direct cell to cell contact

Male cells possess extrachromosomal DNA elements termed conjugative plasmids that enable them to transfer copies of themselves to another bacterium.

Conjugative plasmids are found in both Gram-negative and Gram- positive bacteria.

Conjugation is the most common form of horizontal gene transfer in bacteria.

Widespread: can occur b/n very different types of cells


6 Transposition

The transfer of a segment of DNA from one site to another in the genome


6 Bacterial Plasmids

Ssmall extra chromosomal elements that have the ability to confer new genetic properties on the bacterial cell.

Usually circular, double-stranded DNA molecules.

Frequently contain drug resistance genes.

Main reason for the spread of antibiotic resistant bacteria in nature.

Carry genes that allow them to replicate independently of the bacterial chromosome.

Require host proteins and enzymes for replication.


6 Conjugative Plasmids

Initiate their own transfer from cell to cell

Contain tra genes that encode the sex pilus through which the DNA is transferred

Contain oriT which is nicked by a relaxase to initiate the transfer of a SS plasmid DNA

Usually contain insertion sequences & transposons

Can also promote the conjugative transfer of mobilizable plasmids containing oriT

Highly promiscuous


6 R-Plasmids (R-Factors)

Carry genes that make bacteria resistant to one or more antibiotics

Can transfer b/n different bacterial species

Contain tra genes (encoding the sex pilus), drug resistance, genes, a replication region, insertion sequences, & transposons

oriT: site where transfer of the plasmid is initiated during conjugation


6 Virulence Plasmid

Carry genes that encode toxins & other virulence factors


6 Predict the outcome of continued widespread use of medically-important antibiotics in animal feed

Farmers used to feed antibiotics to animals to protect them

Many antibiotics used are no longer effective in humans

Issue: antibiotic use in animal feed led to a spread of antibiotic-resistance genes


7 Prophylaxis






use of antimicrobial agents to prevent infection

prevent surgical site infections (1 hour before incision)

prevent endocarditis

mefloquine: prevent malaria

TMP/SMX: taken by immunocompromised to prevent infections like Pneumocystis jiroveci (carinii)

- PCN: history of rheumatic fever
- levofloxacin: pts w/ ascites, prevent bacterial peritonitis
- antiretrovirals: after needlestick injury
- antibiotic: after exposure to meningococcal meningitis


7 Pre-emptive therapy

asymptomatic infection which is likely to become symptomatic

valganciclovir: transplant patient with evidence of
cytomegalovirus (CMV) reactivation by increased viral load


7 Empiric therapy

presumed infection but before the etiology of the infection is known

common infections: fever in
neutropenic patients, community-acquired pneumonia and ventilator- associated pneumonia

delaying antibiotic therapy can be detrimental in some but better in others where infeciton may resolve due to immune system



7 Pathogen-directed therapy

organism is known, but antibiotic susceptibility is awaited

broader than when the susceptibility is known


7 Susceptibility-guided therapy

organism and the antibiotic susceptibility are both known

"streamlining: narrowest spectrum, most effective, least toxic, cheapest


7 Common infections for which no antibiotics are required (5)

viral infection (ex. rhinitis)

contamination (ex. coagulase-negative staphylococci)

colonization in absence of infection (ex. VRE from rectal swab)

treatment hasn't shown to hasten resolution (ex. bronchitis)

chronic infection


7 Antibiotic Susceptibility Testing:

Disk diffusion

Broth macrodilution, microdilution and agar dilution


Commercial semi-automated methods

Genetic testing

- antibiotic disks plasced on agar
- greater zone of inhibition = more susceptible bacteria

- varying concs of antibiotic inoculated
- determines minimum inhibitory conc (MIC): lowest conc of antibiotic w/ no visible growth

- strip impregnated w/ dif concs of antibiotics
- zone of inhibition = MIC

- varying concs of antibiotic determine breakpoint (susceptible vs. resistant)

- PCR identifies genes & mutations conferring resistance


7 Concentration vs. Time Dependent Antibiotics

- max efficacy at max concs
- upper conc limit = toxicity
- aminoglycosides, FQs

- max efficacy at max time conc of drug exceeds MIC
- continuous infusion is optimal
- beta-lactams, vancomycin (not ceftriaxone)


8 Mechanisms of Antibacterial Resistance (4)

1) Enzymes that destroy or inactivate antibiotics

2) Development of altered targets

3) Alterations in permeability of bacterial cell

4) Presence of pumps which remove antibiotics

5) Biofilms?


8 Mechanisms of Antibacterial Resistance:
Enzymes that destroy or inactivate antibiotics

Beta-lactamase enzymes mediate antibiotic resistance
- Hydrolysis of beta-lactam ring
- Hydrolyzed antibiotic can't bind to its PBP target

Extended spectrum beta-lactamases
- Affect advanced generation bet-lactams
- Klebsiella, E. coli, other GNR

Broad spectrum beta-lactamases
- Carry ampC genes in chromosomes
- Destroy penicillins & cephalosporins
- Not fully inhibited by beta-lactamase inhibitors
- Carbapenems: only beta-lactams resistant to hydrolysis by ampC beta-lactamases
- Carbapenemases: emerging classes of beta-lactamases


8 Mechanisms of Antibacterial Resistance:
Development of altered targets

Modification of antibiotic target w/ diminished affinity of antibiotic for antibiotic target w/o loss of target's function

1) Beta-lactam resistance
- Alterations in PBPs
- PBPs are responsible for cross-linking cell walls in all bacterial plasma membranes
- When PBPs bind beta-lactams, PBPs can't perform functions --> cell death
- Resistance: PBPs w/ low affinity for beta-lactams can catalyze all steps in cell wall synth

2) Quinolone resistance
- Alteration of DNA gyrase
- Quinolones bind DNA gyrase to prevent its function
- Mutated DNA gyrases from quinolone exposure prevent quinolone binding --> resistance

3) Vancomycin resistance
- Presence of mutant peptidoglycan precursors

4) Macrolide resistance
- Alteration of ribosome


8 Mechanisms of Antibacterial Resistance:
Alterations in permeability of bacterial cell

Impede entrance of antimicrobial agent into bacterial cell & limits its ability to reach & interact w/ its site of action

Porins facilitate transfer of molecules & antibiotics across cell membranes
- Mutation/loss of porins prevent/slow antibiotic entrance into cells
- Decrease rates of entrance favor enzymes which modify antibiotics

Pseudomonas aeruginosa
- Quinolones enter P. aeruginosa through OprD porin
- Decreased PorD expression --> diminished permeability of quinolones


8 Mechanisms of Antibacterial Resistance:
Presence of pumps which remove antibiotics

Efflux pumps remove antibiotics from cells before they can bind targets & kill/inhibit bacteria growth

Worse if also diminished susceptibility to the antimicrobial agent due to other mechanisms of resistance


8 Presence of pumps which remove antibiotics:

Organisms living in biofilms are protected from antibiotics:

1) Poor permeability of antibiotics through biofilms

2) Slow metabolic state of organisms living in biofilms


8 Antimicrobial Resistance:
Escherichia coli



Nosocomial & community-acquired infections (cystitis, pyelonephritis, bacteremia)

- Amoxicillin ("simple" beta-lactamase)


8 Antimicrobial Resistance:



Nosocomial infections (intraabdominal, urinary tract, bacteremia)

- Ampicillin
- Vancomycin


8 Antimicrobial Resistance:
Staphylococcus aureus


Therapeutic agent


Glycopeptide resistance

- Nosocomial infections (skin, wound, bacteremia, hospital/community acquired pneumonia)


- Panton-Valentine Leukocidin

S. aureus w/ reduced susceptibility to vancomycin
- all isolates were MRSA w/ prolonged vancomycin exposure


8 Antimicrobial Resistance:
Classes of glycopeptide in S. auerus

Vancomycin-intermediate S. aureus (VISA)
- Synth of thickened cell wall
- Dipeptides bind vancomycin & reduce its availability to reach targets

Heterogenous vancomycin-intermediate S. aureus (hVISA)
- Variable vancomycin susceptibility
- Unusually thickened cell wall

High-level vancomycin-resistant S. aureus (VRSA)
- transfer of plasmid-mediated VanA gene


8 Antimicrobial Resistance:
Steptococcus pneumoniae



Risk Factors

Bacteremia, community-acquired pneumonia, otitis media, bacterial sinusitis

Penicillin (altered PBP)

Age <2, daycare, recent antibiotic treatment


8 Antimicrobial Resistance:
Group A streptococci

Pharyngitis, skin infection

Target Modification: ribosomal methylation
- MLSi & MLSc
- erm(A) & erm(B) genes

Removal of Antibiotic: macrolide efflux pump
- mef(A)

Target Modification: ribosomal site mutations