1. Lecture 3 MRSA Methicillin resistant S. aureus
Tues – 1/15/2008

2. 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
Toxins: cytotoxins, superantigens, enterotoxins, TSST-1, exfoliative toxins, Panton-Valentine leukocidin
Enzymes: protease, lipase, hyaluronidase

3. 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

4. Risk groups with high carriage rates
Diabetes Mellitus
Dialysis patients
HIV
Chronic skin diseases
IV Drug abusers
Health care workers (?)

5. Antimicrobial resistance of S. aureus - history
SA genome sequence, Kuroda ‘01
CA-MRSA sequence, Baba ‘02
Cloning of mecA
Matsuhashi ‘86
SCCmec sequenced Ito ‘99
MRSA single clone theory Lacey & Grinsted, ‘73
1960
1970
1980
1990
2000
2003
1st MRSA isolate ‘61
1950 penicillinase found in E.coli (before use of penicillin) rapidly moved to S.aureus
59 introduction in Europe, 61 first MRSA in UK.
61 introduction in USa – FIRST mrsa in USA – 1968.
2004 – 60% of SA in ICU in USA = MRSA.
Epidemic spread of MRSA, Europe, India, Australia, USA
2nd wave of epidemic MRSA (MDR), USA, Australia, Ireland
Increasing reports - CA-MRSA
Worldwide dissemination
CA-MRSA in Australia
Introduction of Methicillin – ‘59
1st VISA, Japan ‘97
1st VRSA, USA ‘02

6. 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.
capable of cell wall synthesis in absence of all other PBPs.

7. MRSA - mechanism of resistance
Modifying enzymes
Degrading enzymes
Target Change
Efflux pumps

8. Genetic Mechanisms Horizontal vs. Vertical transmission
Mutation
Plasmid
transfer
Transformation
Large genetic mobile elements (cassettes)

9. MRSA – mechanism-II
mecA is part of a large, mobile, genetic element – Staphylococcal cassette chromosome mec (SCCmec)

10. SCCmec cassette A unique class of mobile genetic element (21-67kb)
ccr complex (type2)
Mec complex (class B)
orfX
IS 1272
mecR1
mecA
IS431mec
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)
Primary mode of spread – clonal expansion, but how is an MRSA strain “born”:
Transduction by one of the many phages. It has been estimated that only 20 times did SA acquire SCCmec
A unique class of mobile genetic element that Contains no phage-related genes, no transoposases.
Origin of SCCmec - unknown, but they are disseminated widely among Staphylococcal strains.
Location of integration site of SCCmec near the origin of replication
Ccr complex enables movement of the cassette

11. The mec Complex S. aureus contains only class A & B.
ccr complex (type2)
Mec complex (class B)
orfX
IS 1272
mecR1
mecA
IS431mec
S. aureus contains only class A & B.
Class C mainly in S. haemolyticus
Class D in S. hominis

12.     TypeI SCCmec (34kb) TypeII SCCmec (53kb)
ccr complex (type 1)
mec complex (class B)
TypeI SCCmec (34kb)
orfX
mecR1 
R-I
ccrA1
ccrB1
mecA 
IS1272
IS431mec
TypeII SCCmec (53kb)
mec complex (class A)
ccr complex (type 2)
orfX
pUB110 
ccrA2
ccrB2
Tn554
IS431mec 
mecI
mecR1
IS431mec
mecA
Type III SCCmec (67kb)
mec complex (class A)
ccr complex
ccr complex (type3)
orfX
SCCmecI= Mec complex type B: mecI and 3’ region of mecR1 are deleted
SCCmec2 & 3: mec Complex A
ISI431 encode tobra Res
Tn554 - encode Eryth R
ŸTn544 cadium R
ccrA3
Tn554
mecI
mecA
pT181
mer
Tn554
ccrB3
mecR1
IS431mec
IS431
IS431
ccr complex (type2)
Mec complex (class B)
orfX
IS 1272
mecR1
mecA
IS431mec
Type IV SCCmec (24kb)  

13. Genetic organization of SCCmec type I-VI de Lencastre et al. 2007
SCCmecI= Mec complex type B: mecI and 3’ region of mecR1 are deleted
SCCmec2 & 3: mec Complex A
ISI431 encode tobra Res
Tn554 - encode Eryth R
ŸTn544 cadium R

14. 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

16. Prevalence of MRSA in USA (cumulative data 1998-2005) / Shorr CID 2007
Ip – in patient
Op - outpatient

17. MRSA among S. aureus isolates in Europe

18. MRSA – a nosocomial pathogen Until ~1996

19. CA-MRSA – an emerging infection
JAMA 1998
CID 2004
EID 2003

20. CA-MRSA: 1996-2008 Changing 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

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

22. 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.

23. 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:…
(closely related genetically strains)

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

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

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

27. MRSA in the Netherlands
76 isolates randomly picked.
SCCmec in 46.
Most are CA-MRSA – emerging!
HA-MRSA – most from abroad.

28. How did CA-MRSA evolve? Recent evolution of CA-MRSA from common MSSA?
“Hospital escape” of unsuccessful HA-MRSA

29. SCCmec Type IV = “Mobile mec”
ccr complex (type2)
Mec complex (class B)
orfX
IS 1272
mecR1
mecA
IS431mec
24kb
Small Size
Novel SCCmec type
Smaller – more efficient horizontal transfer

30. Resistance and virulence US300
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

31. Resistance and virulence Panton Valentine leukocidine
A pore forming cytotoxin
Strains containing pvl genes were associated with severe SST – infections
Direct role of pvl – still controversial

32. ACME – arc gene cluster Complete genome sequence of US300 / Diep et al
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

33. ACME (Arginine Catabolic Mobile Element) – arc gene cluster

34. ACME positive isolates in UK / Ellington et al. JAC 2008
ST8 (US300)
ST8
ACME neg
ST97

35. How do we control MRSA? Hospitals: Community: Infection control!!!
Antibiotic control??
Community:
?????

36. Treatment of MRSA CA-MRSA Clindamycin ?? TMP-SMX? Rifampin? Vancomycin
(high ery-R suggests inducible clinda-R)
TMP-SMX?
Rifampin?
Vancomycin
HA-MRSA
Vancomycin
Linezolid
Daptomycin
Daptomycin – cyclic lipopeptide
Oxazolidinone – linezolid (zyvox)
2000 approved, 2001 first case of resistance