Lecture 3

MRSAMethicillin resistant S. aureus

Tues – 1/15/2008

S. aureus – the pathogen

• Microbiology – Gr+ cocci with many virulent factors (toxins and enzymes)

• Frequent nosocomial- and community-acquired pathogen

• Mode of transmission – contact

• Clinical manifestations:– Skin and soft tissue infections– Pneumonia– Osteomyelitis / Arthritis– Bacteremia / Sepsis – Endocarditis– Toxin-mediated disease: TSS, Food

poisining

S. aureus - Epidemiology

• Epidemiologic niche: – Nasal carriage (anterior nares)– GI tract (rectal)– Perineal– Throat

• Nasal carriage – 30% of adults– 20% Persistant carriers– 60% Transient carriers– 20% Never carriers

• Nosocomial transmission – transient hand carriage

Risk groups with high carriage rates

• Diabetes Mellitus• Dialysis patients• HIV• Chronic skin diseases• IV Drug abusers• Health care workers (?)

Antimicrobial resistance of S. aureus - history

200320001990198019701960

Introduction of Methicillin – ‘59

Epidemic spread of MRSA, Europe, India, Australia, USA

MRSA single clone theory Lacey & Grinsted, ‘73

Cloning of mecA

Matsuhashi ‘86

SCCmec sequenced Ito ‘99

1st MRSA isolate ‘61

2nd wave of epidemic MRSA (MDR), USA,

Australia, Ireland

Worldwide dissemination

Increasing reports - CA-MRSA

1st VISA, Japan ‘97 1st VRSA, USA ‘02

SA genome sequence, Kuroda ‘01

CA-MRSA sequence, Baba ‘02

CA-MRSA in Australia

MRSA – mechanism – I

• Horizontally transferred DNA element - SCCmec.

• Site specific recombination.

• mecA gene encodes PBP2a.

• PBP2a = 78 KDa PBP - capable of cell wall synthesis.

• PBP2a has low affinity for all -lactams.

MRSA - mechanism of resistance

1. Modifying enzymes

2. Degrading enzymes

3. Target Change

4. Efflux pumps

Transformation

Plasmidtransfer

Genetic Mechanisms Horizontal vs. Vertical transmission

Mutation

Large genetic mobile elements (cassettes)

MRSA – mechanism-II

• mecA is part of a large, mobile, genetic element – Staphylococcal cassette chromosome mec (SCCmec)

SCCmec cassette

• A unique class of mobile genetic element (21-67kb) • Resembles a pathogenicity island, but with no virulence

genes.• Ccr complex: ccrA & ccrB encode recombinase A & B enable

SCCmec to integrate into the chromosome in correct orientation.

• Mec complex: encodes β-lactam resistance and its inducible regulation + transposons + integrated copies of plasmids that carry various resistance genes (non-b-lactam)

ccr complex (type2)Mec complex (class B)

orfX

IS 1272mecR1

mecA IS431mec

ccrA3ccrB3

Tn554 mecImecR1

mecAIS431mec

pT181

IS431 IS431

merTn554

orfXccr complexmec complex (class A)

ccr complex (type3)

Type III SCCmec (67kb)

ccrA1 ccrB1R-IIS1272

mecR1

mecAIS431mec

orfXTypeI SCCmec (34kb)

mec complex (class B)

ccr complex (type 1)

ccrA2 ccrB2 Tn554 mecImecR1

TypeII SCCmec (53kb)

mec complex (class A)orfX

IS431mec

pUB110

IS431mec

ccr complex (type 2)

ccr complex (type2)Mec complex (class B)

orfX

IS 1272mecR1

mecA IS431mec

Type IV SCCmec (24kb)

mecA

Genetic organization of SCCmec type I-VI de Lencastre et al. 2007

Origin of SCCmec and the mec gene

• Single clonal origin theory• Hiramatsu et al. 1996: Clonal diversity:

different strains developed independently

• Origin of mecA gene - horizontal transfer from:– SCN – S. scuiri– Enterococcus hiriae

Prevalence of MRSA in USA (cumulative data 1998-2005) / Shorr CID 2007

MRSA among S. aureus isolates in Europe

MRSA – a nosocomial pathogenUntil ~1996

CA-MRSA – an emerging infection

JAMA 1998

EID 2003

CID 2004

CA-MRSA: 1996-2008Changing definitions

• No contact with health-care facilities in prior 6-12 m.– Maybe more than 1y.

• Resistant only to b-lactams, but not to other classes.– Resistant to quinolones, macrolides and

others

• SCCmec IV– and V … and VI…

X X

Community acquired MRSA (CA-MRSA)/ Weber. CID 2005

Risk factors for MRSA

CA-MRSA

• Skin, soft tissue infection

• ???

HA-MRSA

• Previous contact with health care system

• Longer hospitalization• ICU admission or

invasive procedures• Ab Rx.

Clonal spread of MRSA

• Spread is mainly clonal. Only few clones are the cause of most infections.

• Major cause for clonal spread: lapses in IC

• Yet - role of Ab pressure:…

Antibiotic consumption and MRSA, an ecologic study (EID 2004)

Changing Epidemiology of MRSA / Crum et al. Am. J. Med 2006

CA-MRSA infections in Texas (2002-2004) / Kaplan et al. CID 2005

MRSA in the Netherlands

How did CA-MRSA evolve?

• Recent evolution of CA-MRSA from common MSSA?

• “Hospital escape” of unsuccessful HA-MRSA

SCCmec Type IV = “Mobile mec”

• Novel SCCmec type

• Smaller – more efficient horizontal transfer

ccr complex (type2)Mec complex (class B)

orfX

IS 1272mecR1

mecA IS431mec

24kb

Small Size

Resistance and virulenceUS300

• Major CA-MRSA clones in US: US300 & US400

• US300 – the most common single clone of CA-MRSA

• SCCmec IV• Resistant to ciprofloxacin (mutation in gyrA)• Many strains acquired MDR by plasmides (tetK, erm )

• Several mobile genetic elements• Several Toxins

Resistance and virulencePanton Valentine leukocidine

• A pore forming cytotoxin• Strains containing pvl genes were associated

with severe SST – infections• Direct role of pvl – still controversial

ACME – arc gene cluster Complete genome sequence of US300

/ Diep et al. Lancet 2006

• Arginine Catabolic Mobile Element: virulence/strain survival factor

• Different from native arc gene carried by all S. aureus• Highly similar to ACME from S. epidermidis

• Arginine deiminase pathway – Inhibits the nitric oxide production– Allows survival in low ph, anaerobic conditions

• Enhances fitness: enhances potential to grow and survive within a host

ACME (Arginine Catabolic Mobile Element) – arc gene cluster

ACME positive isolates in UK / Ellington et al. JAC 2008

ST97

ST8

ACME neg

ST8 (US300)

How do we control MRSA?

• Hospitals: – Infection control!!!– Antibiotic control??

• Community:– ?????

Treatment of MRSA

CA-MRSA

• Clindamycin ??– (high ery-R suggests

inducible clinda-R)

• TMP-SMX?• Rifampin?• Vancomycin

HA-MRSA

• Vancomycin

• Linezolid• Daptomycin