Methicillin resistant Staphylococcus aureus (MRSA) demonstrates resistance to methicillin and other beta-lactam antibiotics. There are two main types - hospital associated MRSA (HA-MRSA) which circulates in healthcare facilities, and community associated MRSA (CA-MRSA) which is found in the community without recent healthcare exposure. CA-MRSA strains first emerged in the 1990s and seem to have evolved from MSSA by acquiring small SCCmec cassettes. While usually associated with community infections, some studies have found that CA-MRSA strains can also cause healthcare-associated infections.

1. METHICILLIN RESISTANT
STAPHYLOCOCCUS AUREUS
Dr.Riyaz Sheriff 1

2. What is MRSA?
• Methicillin Resistant Staphylococcus aureus
demonstrates resistance to semisynthetic
penicillins – Methicillin, Cloxacillin & Oxacillin.
• MRSA also exhibit resistance to
cephalosporins, monobactams and
carbepenamases
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3. • Hospital associated MRSA (HA-MRSA)  MRSA strains
that circulate and are transmitted to individuals in
health care facilities
• Risk factors for HA-MRSA
– Isolation of MRSA after 48hours of hospital admission
– History of Hospitalization, Surgery, Dialysis
– History of long term admission within one year of the
MRSA culture date
– Presence of indwelling catheter or percutaneous device at
the time of culture or previous to isolation of MRSA
• Community associated MRSA (CA-MRSA) 
– MRSA isolates obtained from individuals in community
who have not had recent exposure to the health care
system
– Patients in health care facilities in whom the infection was
present or incubating at the time of admission
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4. Methicillin / Meticillin
• Narrow spectrum antibiotic of penicillin class
introduced in 1959
• Has ortho-dimethoxy phenyl group attached
to side chain carbonyl group of penicillin
nucleus.
• Has been renamed as meticillin to have a
common international nonproprietary name.
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5. PENICILLIN METHICILLIN
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6. Dr.Riyaz Sheriff 6

7. Why MRSA?
• Methicillin was initially used to detect
resistance to beta lactamase stable penicillins
hence the name MRSA.
• Oxacillin is used as alternative to methicillin
 ORSA (oxacillin resistant Staphylococcus
aureus)
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8. Mechanism of resistance to penicillins
• Presence of mecA gene
– mecA gene is carried on
Staphylococcal cassette
chromosome mec (SCCmec)
– SCC have mecA and ccr
(cassette chromosome
recombinase) and attach it
to the 3’ end of
staphylococcal chromosome
– Codes for modified penicillin
binding protein 2a (PBP 2a
OR PBP 2’)
– Has low affinity for Beta
lactams
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9. Mechanism of resistance to penicillins
• Production of Beta lactamase enzyme
– Some strains produce excessive Beta lactamase which makes it
appear borderline resistant to oxacillin (BORSA). Difficult to detect.
– MRSA with minor alterations to existing PBP  Moderately
resistant Staphylococcus aureus (MODSA)
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10. Virulence factors of Staphylococcus aureus
• Capsular polysacchrides • Hemolysins
• Peptidoglycan and Teichoic acids – Alpha Hemolysin
• Protein A – Beta Hemolysin
• Enzymes – Gamma Hemolysin
– Catalase – Delta Hemolysin
– Clumping factor – Panton-Valentine Leukocidin
– Coagulase • Toxins
– Fibrinolysins – Epidermolytic toxins
– Hyaluronidase – Enterotoxins A,B,C,D,E,H & I
– Lipases • Superantigens
– Phosphatidylinositol specific – Toxic shock syndrome toxin I
phospholipase C
– Nuclease
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11. Virulence Factors in CA-MRSA
Category Toxin
Cytolytic toxins Panton-Valentine leukocidin S and F
Fibronectin binding proteins A and B
Leukocidin R
Superantigenic toxins Enterotoxins (A to J)
Epidermolytic toxins
Toxic shock syndrome toxin -1
Enhanced growth and survival Arginine catabolic mobile element
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12. Nomenclature of MRSA
US pulse field Canadian Multi locus SCCmec type PVL presence
gel MRSA sequence
electrophoresi typing
s
USA 100 2 5 II NEGATIVE
USA 200 4 36 II NEGATIVE
USA 300 10 8 IVa POSITIVE
USA 400 7 1 IVa POSITIVE
USA 500 5,9 8 IV NEGATIVE
USA 600 1 45 II NEGATIVE
USA 700 72 IVa NEGATIVE
USA 800 2 5 IV NEGATIVE
USA 1000 59 IV POSITIVE
USA1100 30 IV POSITIVE
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13. Detection of MRSA
• Phenotypic detection systems
– Disc diffusion test
• Colony suspension prepared from 5colonies and plated on muller
hinton agar containing 2-4% NaCl at neutral pH.
• Oxacillin disc (1ug) placed and incubated at 35°C for 24 hours
• <10mm is considered resistant, >13mm is considered sensitive.
• For isolates with intermediate results
– Test for mec A, PBP2a
– Cefoxitin disc test
– Oxacillin MIC test or
– Oxacillin salt agar screen test may be performed.
• Any growth within the zone of inhibition indicates oxacillin
resistance
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14. Other detection methods
• Agar dilution test
– 0.5 McFarland preparation of isolate is spot inoculated on MHA with
2% NaCl containing 256 – 0.125 ug oxacillin/ml in serial doubling
dilutions
– MIC of >4ug/ml is considered resistant , MIC <2ug/ml is considered
susceptible
• Broth microdilution
– Muller hinton broth inoculated with inoculum density of 5 X 10 5 cfu/ml
• Breakpoint methods
– Includes both agar and broth methods but test only the breakpoint
concentration (2mg/L oxacillin, 4mg/L methicillin)
• E-test oxacillin MIC test
– Easy to set up as a disc diffusion test
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15. • Oxacillin screen agar
– MHA with 4%NaCl and 6ug/ml oxacillin
– Inoculated with 10uL of 0.5Mcfarland preparation, streaked in one
quadrant and incubated at 35°C for 24-48 hours
– Any growth after 24hrs is considered oxacillin resistant
• Cefoxitin disc diffusion test
– Cefoxitin – potent inducerof mecA regulatory system
– Used as a surrogate marker for detection of mecA gene mediated
methicillin resistance
– Inducible resistance to methicillin is better seen with cefoxitin than
oxacillin. This is due to enhanced induction of PBP 2a by cefoxitin.
– 30ug cefoxitin disc is used.( < 21mm is resistant and > 22mm
susceptible)
• PBP 2a latex agglutination kit
– PBP2a extacted from suspension of colonies and react with latex
particles coated with monoclonal antibody against PBP 2a
– Visible agglutination indicates positive result and presence of PBP 2a
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16. Chromogenic Screening medium
Media Components/ action Colony colour
CHROM ID CEFOXITIN 4MG/L GREEN
TARGETS ALPHA
GLUCOSIDASE
ORSAB MANNITOL SALT AGAR BLUE
OXACILLIN 2MG/L
POLYMYXIN
ANILINE BLUE
MRSA INDENT AGAR CEFOXITIN RUBY COLOURED
CHROMOGENIC
PHOSPHATASE SUBSTRATE
DENIM BLUE AGAR CEFOXITIN DENIM BLUE COLONIES
PHOPHATIDASE ACTIVITY
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17. • Molecular methods
– Detection of mecA gene by PCR is considered gold
standard
• DNA extraction
• mecA gene amplified using specific primers
– 30 cycles of denaturation at 94°C for 45seconds
– Anneling at 50°C for 45 seconds
– Extension at 72°C for 1 minute
– Final extension step at 72°C for 3mins
• PCR products visualized on 2%agarose gel with
ethidium bromide dye under U-V transluminator
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18. Dr.Riyaz Sheriff 18

19. Community associated meticillin-
resistant Staphylococcus aureus
strains as a cause of healthcare-
associated infection
J.A Otter, G.L. French
Journal Of Hospital Infection 79
(2011) 189-193
Impact factor 3.393
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20. Introduction
• CA-MRSA infections were first reported in early
1990s from western Australia, New Zealand and
in America.
• CA-MRSA seems to have evolved from MSSA
acquiring SCCmec cassettes.
• Have the ability to affect younger, healthy people
and spread rapidly in community settings.
• CA-MRSA are generally susceptible to non Beta
Lactam antibiotics, Have small SCCmec cassettes
(type IV or V), Many of them produce PVL
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22. CA-MRSA in Hospital outbreaks
• Nosocomial outbreaks reported from 2003 from North
America, Germany, Israel, Switzerland, Greece and UK.
• Most outbreaks have been caused by single cross
infecting strain
• Infections in health care workers and transmission to
their household contacts have occurred in several of
these outbreaks
• PVL + CA-MRSA has been isolated in most of the
outbreaks but one outbreak in Israel and two
outbreaks in UK were caused by PVL negative strains
 CA-MRSA do not need PVL to cause nosocomial
infections
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23. Control Measures for CA-MRSA
• Isolation of affected patients
• Screening other patients in the unit for asymptomatic
carriage
• Reinforcement of standard infection control
procedures
– Hand hygiene
– Screening of staff members for colonization
– Swabbing of environmental surfaces and equipments
– Improved environmental cleaning
• Closure of unit to new admissions
• Screening of household contacts of health care
workers and appropriate management during CA-
MRSA outbreaks
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24. CA-MRSA role in endemic HAI
CA-MRSA is overtaking HA-MRSA as a cause of HAI
Initially CA-MRSA was reported in new born infections
and post operative prosthetic joint infections
Recent reports suggest CA-MRSA as a cause of HA
bacteraemias.
16% of hospital onset infections and 22% of health
care associated infections are caused by CA-MRSA
Many countries have reported 10fold increase in CA-
MRSA from 1999 to 2006
In Greece PVL positive CA-MRSA accounted for 45% of
HA-MRSA infections at several hospitals during 2001-
2003
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25. CA-MRSA implications
 CA-MRSA is emerging as a cause of hospital associated infections in Asia,
Africa & South America. The reason for this emergence is not clearly
understood.
 CA-MRSA have the ability to affect healthy individuals
 CA-MRSA strain with PVL are on the rise resulting in higher virulence.
 Studies have shown that CA-MRSA behave like HA-MRSA once inside the
hospitals but cause more invasive infections than uncomplicated SSTI.
 CA-MRSA exposed to antibiotics is more likely to acquire resistance genes
and become MDR strains like HA-MRSA
 Definitions for HA-MRSA and CA-MRSA become more confused.
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26. CA-MRSA implications (Cont’d)
 Control of MRSA in hospital settings will be more difficult.
 With lack of clear knowledge on epidemiology of CA-MRSA the current
infection control policies may not be successful and needs to be
modified.
 Hospitals should type MRSA strains involved in outbreaks and there
should also be periodic assessment of the antibiotic resistance profiles
among CA-MRSA strains.
 The success of CA-MRSA in spreading in various subsets of the
community is a major concern.
 Finally there is an urgent need to measure the prevalence and
epidemiology of CA-MRSA and also develop systems to identify and
control these organisms in the community, hospitals and other health
care facilities.
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27. Conclusion
 CA-MRSA has emerged as a Nosocomial pathogen in the recent years and
are beginning to overtake HA-MRSA in hospital infection.
 CA-MRSA infections puts a wider group of people at risk. This includes
hospitalized patients, Health workers and their community contacts
 Classification of CA-MRSA and HA-MRSA is becoming more confusing
 Western countries report a MRSA incidence ranging from 10.4% to 40%
 In Tamil Nadu MRSA prevalence in clinical samples is estimated to be
around 31.1% in clinical and 37.9% in carrier samples
 MDR among MRSA in clinical isolates was estimated around 63.6% and
23% carrier samples
 There is an urgent need to measure the prevalence and epidemiology of
CA-MRSA and also develop systems to identify and control these
organisms in the community, hospitals and other health care facilities.
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28. References
• Koneman’s atlas and textbook of diagnostic microbiology
• Essentials of medical pharmacology KD tripathi 5th edition
• Michelle barton et al Guidelines for prevention and management of
community - associated methicillin - resistant Staphylococcus
aureus: a perspective for Canadian health care practitioners,
Canadian journal of infectious disease and medical microbiology vol
17, supplement C 2006 page 4C-24C
• Rajadurapandi et al prevalence and antimicrobial susceptibility
pattern of methicillin resistant Staphylococcus aureus: a
multicentre study, IJMM (2006) 24 (1): 34-38
• Benjamin etal Reduced Vancomycin Susceptibility in Staphylococcus
aureus, Including Vancomycin-Intermediate and Heterogeneous
Vancomycin- Intermediate Strains: Resistance Mechanisms,
Laboratory Detection, and Clinical Implications, CMR 2010 p.99-139
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