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PDA > 15 Prokaryotes strike back > Flashcards

15 Prokaryotes strike back Flashcards

1
Q

Mechanisms of Resistance

b-lactams

A

Mechanism of action
-Inactivate PBPs (peptidoglycan synthesis)

Major resistance mechanisms

  • Beta-lactamases
  • Low affinity PBPs
  • Decreased transport
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2
Q

Mechanisms of Resistance

Glycopeptides

A

Mechanism of action
-Bind to precursor of peptidoglycan

Major resistance mechanisms
-Modification of precursor

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3
Q

Mechanisms of Resistance

Aminoglycosides

A

Mechanism of action
-Inhibit protein synthesis (bind to 30S subunit)

Major resistance mechanisms
-Modifying enzymes (add adenyl or PO4)

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4
Q

Mechanisms of Resistance

Macrolides

A

Mechanism of action
-Inhibit protein synthesis (bind to 50S subunit)

Major resistance mechanisms

  • Methylation of rRNA
  • Efflux pumps
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5
Q

Mechanisms of Resistance

Quinolones

A

Mechanism of action
-Inhibit topoisomerases (DNA synthesis)

Major resistance mechanisms

  • Altered target enzyme
  • Efflux pumps
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6
Q

Intrinsic Resistance

A
  • 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)
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7
Q

Acquired Resistance

A
  • Resistant strains emerge from previously sensitive bacterial populations
  • Determined by e.g. acquisition of plasmids or transposons, or mutations in chromosomal genes
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8
Q

Horizontal gene transfer

A

Draw

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9
Q

Acquired Resistance - Transposons

A
  • 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
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10
Q

Composite Transposon

A
  • 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
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11
Q

Simple Transposon

A
  • e.g. Tn3, 4,957 bp
  • Replicative transposition
  • Transposase is responsible for excision and transfer
  • Resolvase is responsible for resolution of the transfer
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12
Q

b-Lactam Antibiotics

A
  • Substrate analogues of D-Ala-D-Ala
  • Inhibit crosslinking step of peptidoglycan synthesis

Penicillins, Cephalosporins, Carbapenems, Monobactams

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13
Q

Penicillin Binding Proteins (PBPs)

A
  • 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
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14
Q

International Spread of PNSP

A

• 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

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15
Q

b-Lactam Resistance in S. aureus

A
  • > 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
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16
Q

Methicillin Resistant S. aureus (MRSA)

A
  • 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
17
Q

Vancomycin

A
  • 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
18
Q

Vancomycin-Resistant Enterococci (VRE)

A
  • 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
19
Q

Map of Tn1546

A

• VanYB and VanW sit between VanSB and VanHB in Tn1547

20
Q

VRSA – An Emerging Problem

A
  • 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
21
Q

Cabapenem resistance

A
  • 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

PDA (18 decks)

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