Lecture 4

Vancomycin resistance

VREVISA / hVISA / VRSA

Thursday – 1/17/2008

Enterococcus • Gr+ Cocci (in chains).

• Two species infecting human:

E. faecium and E. faecalis.

• Initially considered harmless GI commensal.• Infection typically follows GI colonization.

• Low pathogenicity• Yet, can cause high mortality in patients with

bacteremia.

Enterococcus infections

• E. faecalis vs. E. faecium• Nosocomial infections, mostly in debilitated

patients

• Common cause of nosocomial urinary tract infection

• Currently 3rd leading cause of bloodstream infections.

• Serious complication: endocarditis

Nosocomial transmission

• Mode of transmission:– HCW– Environmental sources (medical devices)

• Carriage:– GI tract– Duration: months - years

Enterococcus and antibiotic resistance

• Enterococci intrinsically resistant to various antimicrobial classes

– Low level aminoglycoside (low ability to penetrate cell wall)– Relative resistance to β-lactams (Cephalosporines + penicillin) (PBP5) – macrolides (low level)– TMP-SMX

• Acquire high level antibiotic resistance through horizontal transfer of relevant genes.

• 1980’s: emergence of beta-lactam and high gentamycin resistance.

• 1986 fist report of VRE in Europe, 1989 first report in US.

• Currently: alarming situation in US hospitals, rare in Europe

Glycopeptidesvancomycin, teicoplanin, telavancin

• Antibiotic class used to treat Gram positive bacteria.

• Mode of action: disruption of peptidoglycan polymerization

• Bind to the amino-acids within the cell wall. (D-ala-D-ala).

• Prevent the addition of new units to the peptidoglycan.

VRE - mechanism of resistance

1. Modifying enzymes

2. Degrading enzymes

3. Target Change

4. Efflux pumps

Cell wall synthesis in enterococci:

Mechanism of resistance• Operons that encode enzymes for

– synthesis of low-affinity precursors (D-ala-D-lac vs. D-ala-D-ala).– Elimination of normal high-affinity precursors (removing the

vancomycin-binding target)

• Operon encoded on a transposon

VanA operon

Glycopeptide resistance gene operons: vanA-vanE and vanG

• Enterococci – acquisition of VanA, B, D, E and G phenotypes.

• VanC – affords intrinsic resistance, and is chromosomal (arises in less virulent enterococci).

• VanD, E, G: reported only sporadically.

• VanC and VanE: D-Ala-D-Ser

• VanA & VanD: resistant to vancomycin + teicoplanin.

• VanA & VanB: most common • The phenotype is accomplished by multiple proteins in gene

clusters.

The vanA gene cluster

Adapted from Courvalin et al. 2006

vanA (ligase) and vanH (dehydrogenase) are responsible for the synthesis of the modified depsipeptide (D-Ala-D-Lac).

vanX (D,D-Dipeptidase) and vanY (D,D-Carboxypeptidase) cleave normal peptidoglycan substrates.

• VanA type: inducible high levels of vanco-R and teicoplanin-R (Tn1546)– VanH - dehydrogenase (pyruvate->lactate)– VanA - ligase catalyses bond of D-Ala-D-Lac.– VanX and VanY - remove c-terminal D-Ala to

eliminate normal precursor

• VanB different in its regulation (and no vanZ, additional vanW - function unknown).

Transformation

Plasmidtransfer

Genetic Mechanisms of Resistance Acquisition

Mutation

Genetic Mechanisms of VRE

• Resistant genes clustered in operons

• Operons located on transposons

• Transposons transmitted– Directly to chromosome – via plasmids

Rapid spread of VRE

Tn1546 Transposon (vanA)

• A small mobile genetic element (6625 bp)

• More mobile than a plasmid

• Plasmids that carry Tn1546 - highly efficient conjugative plasmids (~65 Kbp)

Virulence genes associated with VRE

Enterococcal surface protein variant (esp)

hylEFM

Variant esp gene (esp- / espEfm) enterococcal surface protein

• Enhanced adherence

• Associated with hospital infections (VREF-100% and VSEF~50%).

• Prevalent in E. faecalis strains related with infections.

• Absent in community isolates.

• Not more virulent in mouse model.

Another virulent factor: hylEfm

• Significant identity with hyaluronidase genes

• Hyaluronidase: a virulent factor in S. aureus, S. pneumoniae, GAS

• Predominant in VREF strains

• Associated with espEfm

Epidemiology of VRE infections

Prevalence of VRE among enterococci in nosocomial infections

in ICU patients

Bonten et al. Lancet Infect. Dis. 2001

Vancomycin-resistant enterococci (VRE)

E. faecium• Rare, but emerging cause

of infection (15% ->~30%)

• Most (>90%) are VRE (VREF)

• Colonization of hospitalized patients 1.5%-32%

E. faecalis• Common cause of

infection (>90% ->70%).

• But most not VRE.

VRE: European vs. US epidemiology

• 1986 - first case.• Uncommon as nosocomial

infection (<3%) only sporadic outbreaks

• Widely prevalent in European livestock and in healthy people in the community

• Inciting factor: animal use of glycopeptides– Avoparcin as a growth factor

since 1970s, banned in 1997.

• 1989 – first case.• Epidemic spread:

– Small outbreaks– Northeast -> West coast– By 1995 – high endemicity

in ICUs.

• Community reservoir – absent.

• Inciting factor: use of oral vancomycin.

Europe USA

VRE: European vs. US epidemiology

• Sporadic nosocomial outbreaks.

• Highly prevalent in healthy humans and livestock.

• ‘Non-epidemic strains’.

• Do not have esp gene.

• Endemic in US ICUs

• No community reservoir

• ‘Epidemic strains’.

• Contain esp gene.

Europe USA

Vancomycin use in US vs. Europe

Bonten et al. Lancet Infect. Dis. 2001

Bloodstream isolates of VREF SENTRY antimicrobial surveillance program

Deshpande et al. Diag. Microbiol. Infect Dis. 2007

Genetic capitalism (the rich become richer)

The success of a highly adaptive clone.

Leavis et al. Curr. Opion. Microbiol 2006

Emergence of CC17 in the Netherlands / Top et al. JCM 2008

Is the European epidemic following the US epidemic in a 10 y delay?

Factors influencing VRE spread:

• Patient colonization– GI tract– Groin– Skin

• Colonization pressure– Number/density of colonized

patients– Admission/transfer of

colonized patients– Proximity of colonized

patients– Shared care givers

• Contaminated environment– Stethoscopes/ BP cuffs, etc..

• Antibiotic pressure– Vanco– Cephalosporins– Antianaerobic

• Bacterial virulence factors– esp gene

Suggested strategies for IC of VRE / Bonten et al. Lancet Infec Dis 2001

Infection Control and/or Ab control

• When not clonal spread, strict IC - not efficient.

• Then probably resriction of Ab classes but which?

• Studies show: RF for VRE:– Vancomycin (IV - controversial)– Extended-spectrum cephalosporin– Other B-lactam-B-lactam inhibitors (controversial which)– Anti-anaerobic regimens.

Vancomycin resistant S. aureus (VRSA)

Vancomycin intermediate

S. aureus (VISA, hVISA)

Vancomycin resistance in S. aureus VISA (vancomycin intermediate S. aureus)

• First case 1996

• MIC 4-8µg/ml

• Thick cell wall (reduces vanco penetration through cell wall).

• Accumulates multiple mutations that activate pathways for cell wall synthesis & change cell physiology

• High fitness cost

hetero-VISA (hVISA)

• hVISA (hetero-VISA)– Appear to be susceptible, but

consists of subpopulations that have MIC≥4µg/ml

– Difficult to detect.

VRSA - yet a very rare event

• 2002 Michigan – 1st case vanA mediated since then 6 more cases.

• vanA resistance in VRSA rare, most occurred in the same geographical area (Michigan)

• Only very few descriptions of vanA gene cluster in MRSA, though in lab – years ago it was demonstrated.

• Gene cluster found on a plasmid-specified transposon (Tn1546)

Currently most worrisome:

• Concomitant carriage of VRE and MRSA is increasing.