15 Prokaryotes strike back Flashcards
Mechanisms of Resistance
b-lactams
Mechanism of action
-Inactivate PBPs (peptidoglycan synthesis)
Major resistance mechanisms
- Beta-lactamases
- Low affinity PBPs
- Decreased transport
Mechanisms of Resistance
Glycopeptides
Mechanism of action
-Bind to precursor of peptidoglycan
Major resistance mechanisms
-Modification of precursor
Mechanisms of Resistance
Aminoglycosides
Mechanism of action
-Inhibit protein synthesis (bind to 30S subunit)
Major resistance mechanisms
-Modifying enzymes (add adenyl or PO4)
Mechanisms of Resistance
Macrolides
Mechanism of action
-Inhibit protein synthesis (bind to 50S subunit)
Major resistance mechanisms
- Methylation of rRNA
- Efflux pumps
Mechanisms of Resistance
Quinolones
Mechanism of action
-Inhibit topoisomerases (DNA synthesis)
Major resistance mechanisms
- Altered target enzyme
- Efflux pumps
Intrinsic Resistance
- Inherent features which prevent antibiotic action
- Usually determined and expressed by genetic material contained within the chromosome
- e.g. b-lactamases in Gram –ve bacteria inactivate b-lactam antibiotics (penicillins,etc)
Acquired Resistance
- Resistant strains emerge from previously sensitive bacterial populations
- Determined by e.g. acquisition of plasmids or transposons, or mutations in chromosomal genes
Horizontal gene transfer
Draw
Acquired Resistance - Transposons
- Discrete genetic elements capable of moving (transposing) within the bacterial genome or from one DNA molecule to another (genome, phage, conjugative plasmid, etc)
- Found in all organisms
- Transposition provides multiple copies
- Not capable of independent replication
- Frequently carry genetic information that encodes for resistance to antibiotics
Composite Transposon
- Conservative transposition
- DNA replication does not occur so when excised the transposon is removed from the site of the original chromosome
- Two copies of the insertion sequence (IS), flanked by inverted repeats
- One active transposase, one inactive transposase
Simple Transposon
- e.g. Tn3, 4,957 bp
- Replicative transposition
- Transposase is responsible for excision and transfer
- Resolvase is responsible for resolution of the transfer
b-Lactam Antibiotics
- Substrate analogues of D-Ala-D-Ala
- Inhibit crosslinking step of peptidoglycan synthesis
Penicillins, Cephalosporins, Carbapenems, Monobactams
Penicillin Binding Proteins (PBPs)
- Catalyse final steps of peptidoglycan synthesis
- Multiple PBPs (4-5) essential for cell viability
- The b-lactams acylate the active site serine residue of PBPs to inhibit transpeptidation
- Activity of b-lactams determined by affinity for PBPs, stability to b-lactamases and membrane permeability
International Spread of PNSP
• MDR-PNSP strain in Iceland, resistant to tetracyclines, chloramphenicol, erythromycin and others
• Icelandic PNSP similar to strain isolated in Spain
• Possible factors for spread of resistance:
• The use of b-lactams in Iceland is low
• High use of other antibiotics may have selected for
multi-drug resistant clone
• 57% of population live in one city, 80% children attend day-care centres
b-Lactam Resistance in S. aureus
- > 90% strains produce b-lactamases
- Plasmid encoded, confers resistance to penicillin, ampicillin and other b-lactams
- These strains are mostly susceptible to penicillinase- resistant penicillins (e.g. methicillin), 1st generation cephalosporins, and b-lactam/b-lactamase inhibitor combinations
- > 40% of S. aureus bacteremias reported in the UK are now resistant to methicillin and other b-lactams
Methicillin Resistant S. aureus (MRSA)
- MRSA contains novel PBP2a; substitutes for native PBPs and has low affinity for all b-lactams
- MRSA chromosome contains ~ 50kb mec region not present in MSSA
- PBP2a is encoded by mecA gene; expression controlled by mecI, mecR1 and other factors
- Most MRSA are multi-drug resistant (MDR-MRSA) but are susceptible to vancomycin
- Epidemic strains EMRSA-15 and -16 most prevalent
Vancomycin
- Member of glycopeptide family of antibiotics
- Binds to D-Ala-D-Ala in peptidoglycan precursors
- Prevents transglycosylation and transpeptidation
- Resistance to b-lactams does not confer cross- resistance to vancomycin
- Only active against Gram +ve bacteria
- Cannot cross outer membrane of Gram –ve bacteria
- Used primarily for MDR Staphylococci infections, patients with penicillin allergy or C. difficile infections
Vancomycin-Resistant Enterococci (VRE)
- Rapid increase in infection/colonization incidence with VRE over the past 20 years, MDR strains appeared
- E. faecium and E. faecalis containing vanA or vanB gene clusters produce modified peptidoglycan containing D-Ala-D-lactate or D- Ala-D-Ser that does not bind vancomycin
- Resistance genes are on mobile elements, have spread widely since 1st reports in late 1980’s and are a major focus of infection control
- Other gene clusters (vanC-vanH) also confer resistance, typically low-level and non-transferable
Map of Tn1546
• VanYB and VanW sit between VanSB and VanHB in Tn1547
VRSA – An Emerging Problem
- S. aureus with reduced susceptibility to vancomycin
- First reported in 1997, now worldwide; Isolates typically obtained from patients with chronic VRE infection
- Decreased susceptibility may be due to increased levels of peptidoglycan and precursors
- vanA has now been observed in S. aureus
Cabapenem resistance
- Carbapenems considered a drug of last resort
- Carbapenem-resistant enterobacteria (CRE) indentified
- By 2007 21% of all K.pneumoniae in New York carried the plasmid
- Specific resistant pathogens have emerged that prevent successful antibiotic intervention
