| Summary: | MMed (Clinical Microbiology), Faculty of Health Sciences, University of the Witwatersrand, 2009 === Introduction: The clinical laboratory is required to rapidly identify Staphylococcus aureus as a
cause of bacteraemia, and in particular, to detect methicillin resistance amongst bacteraemic
isolates, to facilitate prompt initiation of appropriate therapy which may directly impact on
patient survival, and to allow for implementation of appropriate infection control measures.
Hence, the laboratory needs to choose tests to detect methicillin-resistant S. aureus (MRSA)
bacteraemia which are rapid, accurate, simple, cost-effective and appropriate for the setting.
Primary study objective: To determine the accuracy of four phenotypic susceptibility tests to
directly detect MRSA from blood culture specimens (BC) compared with detection of the
mecA gene by the polymerase chain reaction (PCR) from S. aureus cultured from the same
BC.
Materials and Methods: BCs were selected from patients with incident, S. aureus bacteraemic
episodes at two hospitals, during January and February 2006. S. aureus was identified by
standard phenotypic tests, including the presence of a deoxyribonuclease (DNAse). Direct
susceptibility tests (DST) were performed (oxacillin (1μg) and cefoxitin (30μg) disk diffusion
(DD), oxacillin Etest® (AB bioMérieux) and CHROMagar®-MRSA (CHROMagar®
Microbiology)), and repeated on stored cultures. Detection of nuc and mecA genes by PCR
confirmed S. aureus and methicillin resistance respectively. The sensitivity and specificity of
the DST were calculated with reference to the mecA PCR result, to fulfil the primary study
objective.
Results: During the two-month study period, 9,400 BC were submitted to the clinical
laboratories at the 2 hospitals; S. aureus was isolated from 156 specimens. Of these, 89 BC
from 89 incident cases were included in the study, and 65 were subjected to all tests,
including PCR. Of the 65 nuc-positive S. aureus isolates from 65 BC, all were positive with
the direct DNAse test, and 25 (38%) were mecA positive. Compared to PCR, sensitivity and
specificity for the direct oxacillin DD, cefoxitin DD, oxacillin Etest® and CHROMagar®-
MRSA was 100% and 90%, 98% and 100%, 100% and 100%, and 96% and 42%
respectively.
Discussion: In this study, we found that, compared to PCR for the nuc and mecA genes, the
combination of a direct DNAse test and oxacillin Etest®, facilitated accurate detection of
MRSA bacteraemia. The direct oxacillin Etest® result did not appear to be influenced by a
non-standardised inoculum, in contrast to the other direct tests, and provided an oxacillin
minimum inhibitory concentration. The direct cefoxitin DD test produced more accurate
results than the direct oxacillin DD test, was easier to read and distinguished MRSA from
MSSA with zone diameters clustering into more clearly defined susceptibility categories.
Although the chromogenic agar performed well when used to identify methicillin resistance
amongst cultured S. aureus isolates, it was apparent that this test, read at 24 hours, could not
be used reliably as a DST. Since the Etest® is more costly than the DD test; its use should be
reserved for BC from patients in “high-risk” hospital areas, e.g. intensive care units. The
direct cefoxitin DD could be used for all BC positive for GPCC, and could be used without a
direct identification test because of its lower cost; it is further recommended that the direct
cefoxitin DD test replace the direct oxacillin test.
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