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Biotyping MRSA
User Manual
The contents of the User Manual are displayed on these web pages, but, if you prefer a printed version, it can be downloaded through the link below. The second link shows the flow of samples through the laboratory and the decision processes affecting the treatment of the sample.
Tests performed on all isolates received
Phenotypic confirmation of MRSA status is carried out on all isolates received. Isolates are phenotypically typed using biotype and antibiogram. Genotyping is currently by PCR-ribotyping, PFGE and SPA typing. Phenotypic characteristics of Scottish MRSA are still being monitored.
Confirmation of MRSA status
Isolates giving equivocal results with the standard tests for species identification of Staphylococcus aureus are tested by PCR to detect the species specific nuc gene. Isolates with borderline resistance to methicillin are tested by oxacillin and cefoxitin E-tests and by PCR to detect the mecA gene.
Antibiotic susceptibility monitoring
Isolates are tested for resistance to 21 antibiotics - (using VITEK 2 and several others by disc diffusion). The resulting antibiogram is part of the phenotyping pattern; it is also used to monitor changing resistance patterns. Isolates with reduced susceptibility to mupirocin are further tested by PCR for the presence of the mupA gene to distinguish between high and low level mupirocin resistance.
GISA (Glycopeptide intermediate resistant Staphylococcus aureus) screen
All referred isolates are screened, using a Break Point plate assay, for reduced susceptibility to teicoplanin. Isolates positive on screening are tested by E-test for vancomycin and teicoplanin and by PCR for the presence of vanA/B genes.
Epidemiological typing
By phenotypic typing (using biotype and antibiogram), more than 90% of MRSA isolates are assigned to known epidemic strains. Genotyping by PCR-ribotyping (PCR-R), pulsed field gel electrophoresis (PFGE) typing and SPA typing is carried out where appropriate. PFGE type of MSSA and CNS is also performed if appropriate. PFGE is the more discriminatory technique and is particularly valuable in the investigation of localised outbreaks. PCR-R is a rapid method and, although less descriminatory, is useful for identifying clonal groups and can also be used for speciation of CNS.
Other sequence-based methods, e.g. MLST, MLVA, are carried out on selected isolates. MLVA is undergoing evaluation and may be used more widely in the future.
Toxin Testing
PCR tests are routinely available for detection of 9 staphylococcal toxin genes: five enterotoxin genes (sea, seb, sec, sed and see), the toxic shock syndrome toxin-1 (tst), two exofoliative toxins (eta and etb) and the Panton-Valentine Leukocidin gene. Other toxin genes are under evaluation.
Snapshot Programme
Because hospitals referral policies differ, a "Snapshot" programme is carried out, in collaboration with HPS, to obtain a more comprehensive picture of MRSA epidemiology in Scotland. In a rolling program, and by mutual agreement, each hospital is allocated a period when all "new" MRSA isolates are sent to the MRSA reference laboratory with epidemiological information.
Control of infection advice
Advice on the various aspects of the control of MRSA infection is available.
Staphylococci other than MRSA
Many of the tests performed in the Reference Laboratory are useful in the investigation of other Staphylococci. Epidemiological typing of MSSA and CNS by PFGE and species identification of CNS by phenotypic and genotypic (PCR based) methods will be undertaken on request.