Methicillin-Resistant Staphylococcus aureus (MRSA)
Detection and Identification Methods
- Gram +ve coccus
- Grape like clusters of cells
- Resitant to methicillin and other penicillins
- May also be referred to as ORSA [oxacillin resistant]
aureus (MRSA) were first reported in the early 1960's and are now regarded
as a major hospital acquired pathogen worldwide. The term methicillin resistant
is historically used to describe resistance to any of this class of
antimicrobials. Today in the USA approx. 35% of hospital strains of S.
aureus are resistant to methicillin (or other penicillin antibiotics), and
in recent years the emergence of vancomycin-resistant S. aureus (VRSA)
has caused additional concern.
Resistance occurs when the organism has a mecA gene producing an altered
penicillin binding protein, PBP2a (also known as PBP2') and either an oxacillin
MIC of 2mg/l or a methicillin MIC of 4mg/l.
Infected and colonised patients are the reservoir of MRSA both in hospitals and
the community with transmission generally being via contact with health
Effective, rapid laboratory diagnosis and susceptibility testing is critical in
treating, managing and preventing MRSA infections.
for these items:
Laboratory screening for MRSA is a complex
balance between speed of result, sensitivity, specificity and cost.
Currently the majority of screening is carried out using plate based methods.
Surveys suggest that this methodology group accounts for >90% of the
screening tests performed.
However a number of alternative methods including broth based methods,
chromogenic media, rapid screening kits, molecular assays and automated systems
are increasingly being used. Isolation from screening swabs can be a lengthy
procedure, due to the number of 'contaminating' organisms that are present in
swabs from non-sterile sites.
Broth based enrichment media are commonly employed to increase
sensitivity. However this is at the expense of speed of result. NaCl is
generally added to the base broth along with either methicillin, oxacillin,
[and more recently] cefoxitin. Indicator compounds can also be used to give an
early indication of the presence of MRSA.
Solid Agar Media: There are no universal standardised methods for
screening and isolation of MRSA using solid agar media. Many selective media
are available, and these rely on inhibitors such as NaCl and/or antibiotics to
aid selection, together with a pH indicator to highlight presumptives. Examples
are Mannitol Salt Agar containing 7% NaCl with either 4mg/L methicillin or
2mg/L oxacillin; Oxacillin Resistant Screening Agar with 5.5% NaCl and 2mg/L
oxacillin; Baird Parker Medium with 8mg/L ciprofloxacin; Mueller Hinton Agar
with 4% NaCl and 6mg/L oxacillin. Sensitivity at 24hrs incubation is variable
with 48hrs incubation often required for an acceptable result.
Recently developed chromogenic media combine primary growth and
selectivity with differentiation from coagulase negative staphylococci. These
media show improved specificity when compared with traditional media.
Sensitivity is also improved but requires 48hrs incubation to achieve >85%.
The majority of molecular methods used for the detection of MRSA are
in-house, relying on multiplexed PCR primers detecting genes specific for S.
aureus (nuc,fem) and mecA detecting methicillin resistance. Most are only
suitable for use with pure cultures and not screening of swabs due to the
presence of coagulase negative staphylococci carrying the methicillin resistant
gene mecA. Newer commercially available amplification assays targeting mecA in
combination with other specific markers such as coagulase and have shown
There have been a number of developments with bioluminescence in
particular the use of adenylate kinase (AK), an enzyme found in all cells that
produce ATP from ADP. AK measurement is more sensitive than ATP-based systems
and allows routine detection of 50 organisms or more in a sample. Early
performance data shows results equivalent to conventional plate culture methods
whilst providing results within 5 hours.
for these items:
Traditionally confirmation of S. aureus
is performed using the slide coagulase test (clumping factor) and the tube
coagulase test (free coagulase). Positives on the slide
coagulase test should be confirmed with the tube coagulase test. DNase media
plates can also be used but positives require additional confirmation.
Agglutination kits are widely available and can be used to confirm S.
aureus by detecting protein A and clumping factor, although some strains of
MRSA have low levels of these proteins. Newer kits now work by also detecting
surface antigen. Other latex kits detect PBP2a which occurs within the cell
membrane and requires lysis of the cells for detection.
A wide range of commercial biochemical kits are available, both manual
and automated. These are based on an array of biochemical tests giving a
profile assessed against databases/tables. Many automated systems combine
biochemical identification of S. aureus with antibiotic sensitivity
panels for the confirmation of MRSA.
Antibiotic Sensitivity Methods
for these items:
Methods for methicillin and oxacillin
susceptibility testing are extensive and published data is contradictory with
regard to recommendations.
There is no single method that is suitable for all MRSA strains. Standard
methods are published by the British Society for Antimicrobial Chemotherapy
(BSAC) and in the USA, by the Clinical Laboratory Standards Institute (CLSI),
previously known as NCCLS.
Minimum Inhibitory Concentration [MIC] by the dilution method has traditionally
been the reference method.
BSAC recommend the use of Mueller Hinton or Columbia agar with 2% NaCl and
104 cfu/ml inoculum incubated at 30°C. CLSI recommend Mueller Hinton
Agar with 2% NaCl and 104 cfu/ml inoculum incubated at 33-35°C.
Molecular methods which detect the mecA gene are replacing MIC as the
Antibiotic sensitivity testing using disc diffusion methods remain the
most widely used but results are influenced by a range of factors including
medium, NaCl concentration, temperature, inoculum and test agent.
A number of recent studies using cefoxitin disc diffusion method suggest
greater reliability than with oxacillin. No special medium or incubation
temperature is required and the test is less affected by hyper-producers of
The most recent CLSI supplement (M100-S14) suggests the use of 30µg
cefoxitin discs using a breakpoint of <= 19mm as indicative of resistance of
S. aureus to oxacillin. Other media based methods include agar, broth
based breakpoint methods (2mg/L oxacillin, 4mg/L methicillin) and agar
screening methods recommended by CLSI (Approved standard M7-A6).
MRSA is no longer only an infection that is acquired in
hospitals [HA-MRSA], although this remains a primary source of transmission.
Increasingly MRSA can be acquired in the community [CA-MRSA] and indeed from
pets [PA-MRSA?]. This perhaps is the more worrying trend due to the potentially
large host population and emphasises the need for significantly increased
controls of how and when antibiotics are used.
Widespread delivery of antibiotics outside clinically significant uses can only
lead to further selection of organisms resitant to higher levels of