Antibiotics III: The Glycopeptides Flashcards
Enterococci is
- Ubiquitous in Nature
- Widely distributed in nature
- Normal flora in the GI and GU tract
- Caution in establishing clinical significance
- E. faecium, E. faecalis most prominent in humans
- Increased prevalence as nosocomial pathogens
- Primarily associated with UTI, wound infections and bacteremia, especially following surgical interventions
- High mortality rate among patients with bacteremia caused by Enterococcus sp.
Enterococci and -Lactams
- Intrinsically less susceptible to -lactams
- Penicillin G/ampicillin MICs 10-fold higher than for Streptococci, therefore not bactericidal
- Cephalosporin use can increase the risk of enterococcal infection
- Acquired high-level resistance is a new problem
- High-level ampicillin resistance
- Altered PBPs found in E. faecium
- B -lactamases still rare in Enterococci
What is very powerful antibiotic
Cephalosporin
Glycopeptide Resistant Enterococci
• First reported in UK and France in 1986
• Resistance observed in Enterococci that were already resistant to other antibiotics
• Resistance disseminated to other species, e.g. VRSA • Glycopeptide resistance usually due to thickened
peptidoglycan layer in MRSA (mecA)
• vanA gene cluster has been found in S. aureus
• Difficult to control infections, most VRE are MDR
• Isolation of GRE from humans and animals in European communities linked to veterinary use of antibiotics
- powerful antibiotic can treat pretty much any infection
Vancomycin
- Member of glycopeptide family of antibiotics
- Binds to D-Ala-D-Ala in peptidoglycan precursors
- Prevents transglycosylation and transpeptidation
- Resistance to -lactams does not confer cross- resistance to vancomycin
- Only active against Gram +ve bacteria (have a very complex cell wall)
- Cannot cross outer membrane of Gram –ve bacteria
- Used primarily for MDR Staphylococci infections, patients with penicillin allergy or C. difficile infections
Glycopeptide Antibiotics structure
- alcohol chain is hydrophobic and helps to anchor drug into cell wall
- all contain the same peptide backbone. All have a aromatic residue
Vancomycin Mode Of Action
form H bonds onto alanine residues - form 5 bonds
- if you take a H away it will bond far less strongly
Vancomycin Resistance
- removing an h bond will weaken the association of vancomycin with the d-ala d-ala chain
- ester bond to break but beccause there is no h bond donor is d-ala backbone. there is a network of 4 h bonds. vancomycin will no longer bind strongly to the cross link precursors
- Vancomycin resistant bacteria mutate the D-Ala-D-Ala termination of the pentapeptide to D-Ala-D-Lac (VanA, VanB, VanD, VanF) or to D-Ala-D-Ser (VanC, VanE, VanG)
- This results in loss of a H-bond and much lower affinity of vancomycin for this peptide in the resistant bacteria
Mechanism of VRE
- Aquired high-level resistance
- 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
- MDR E. faecium poses therapeutic challenge
- Other gene clusters (vanC-vanH) also confer resistance, typically low-level and non-transferable
Tn1546
- contains genes which code for Van a type resistance
- Conjugative plasmid pIP816 on E. faecium BM4147
- 10,851 bp transposable element
- Confers high-level VanA vancomycin resistance
- Encodes 9 polypeptides in 4 functional groups
- Transposition functions: ORF1, ORF2
- Regulation of resistance gene expression: VanR, VanS • Resistance to glycopeptides: VanH, VanA, VanX
- Accessory proteins: VanY, VanZ
•VanYB and VanW sit between VanSB and VanHB in Tn1547
VanA and VanH
VanA
• Enzyme homologous to bacterial ligase
• Catalyzes the formation of ester bonds between D-Ala and D-Lac to produce the depsipeptide D-Ala-D-Lac preferentially to the usual dipeptide D-Ala-D-Ala
bacterium will have pyruvate present as part of normal metabolism. VanH reduces lactate
VanH
• Dehydrogenase, reduces pyruvate to D-Lac, the substrate for VanA
• Affects cross-linking of precursors to the growing peptidoglycan processed by the PBPs
VanX
- D,D-dipeptidase that hydrolyzes D-Ala-D-Ala but can not hydrolyse D-Ala-D-Lac
- Prevents synthesis of precursors ending in D-Ala
hydrolyses any d-ala d-ala that are already there
VanR, VanS, VanY and VanZ
- vanA, vanH, and vanX resistance genes are regulated at the transcriptional level by the VanR- VanS two-component regulatory system
- Two regulatory proteins are probably involved in transmitting a signal in response to the presence of vancomycin at the surface of the cell
- D,D-carboxypeptidase, accessory protein contributes to resistance by cleaving the terminal D-Ala of late peptidoglycan precursors resulting from incorporation of D-Ala-D-Ala that escaped hydrolysis by VanX
- VanZ confers low-level resistance to teicoplanin
Van Z helps to
Van Z helps to resist the anchoring of the alcohol group into the cell wall
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
