LOs: 6-8 Flashcards
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
Perioperative
Endocarditis
Travel
Opportunistic
Other
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
broad
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
E-test
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
Concentration
- max efficacy at max concs
- upper conc limit = toxicity
- aminoglycosides, FQs
Time
- 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:
Biofilms
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
Diseases
Resistance
Nosocomial & community-acquired infections (cystitis, pyelonephritis, bacteremia)
- Amoxicillin (“simple” beta-lactamase)
- TMP/SMZ
8 Antimicrobial Resistance:
Enterococci
Diseases
Resistance
Nosocomial infections (intraabdominal, urinary tract, bacteremia)
- Ampicillin
- Vancomycin
8 Antimicrobial Resistance:
Staphylococcus aureus
Diseases
Therapeutic agent
Genetics
Glycopeptide resistance
- Nosocomial infections (skin, wound, bacteremia, hospital/community acquired pneumonia)
Vancomycin
- SCCMec IV
- 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
Diseases
Resistance
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
8 Emerging Resistance (4)
Dissemination of carbapenemase enzymes in Acinetobacter
Development of quinolone-carbapenem resistance in Klebsiella
Increasing emergence of new “panresistant” strains
Only one new class of therapy against GNR: glycylcyclines
8 What to do about antimicrobial resistance
Can’t depend on pharmaceutical industry
Avoid antibiotics for viral & non-infectious syndromes
Educational & research initiatives (Antimicrobial Stewardship Programs)
Hand hygiene, infection control, vaccines
