Comparison Of Microdilution Technique And Phenotypic Method For The Detection Of Staphylococcus Aureus Penicillinase Production In Japan

Background: Beta lactamase resistant antistaphylococcal penicillin is not available in Japan. However, cefazolin is not appropriate antibiotics for some S. aureus infectious disease such as bacterial meningitis. Although CLSI and EUCAST recommend the zone-edge test or zone diameter measuring to detect beta lactamase production, their sensitivity and specificity depend on technician's experience. Detection of blaZ gene is not available in most laboratory, whereas microdilution technique is available in most laboratory and does not depend technician‘s experience. The aim of our study is to compare the sensitivity and specificity of conventional phenotypic methods with microdilution technique. Methods & Materials: We used stored S. aureus blood culture isolates which had been reported as penicillin susceptible (using MIC ≦0.12 mcg/mL) from January 2010 to May 2017. All the isolates were checked MICs, nitrocefin test, zone diameter measuring and zone-edge determination test. All the strains were checked blaZ producion by PCR. MICs were checked by EIKEN disk. Chromogenic nitrocefin test was performed with nitrocefin disk. Zone-edge determination test and Zone diameter measuring were performed by three microbiology laboratory technicians who have different experience. Each technicians’ experience were under 2 years (A), 10 years (B) and 25 years (C). Benzylpenicillin 10U disk was used for both disk methods. Results: Total of 107 stored S. aureus isolates were collected. There were no isolates from same patient. Three isolates were positive for blaZ. Two false negative nitrocefin tests were determined by all three technicians. One false negative zone edge test was determined by under 2 years experienced technician. In all the blaZ positive isolates MIC were less than or equal to 0.03 mcg/mL. Conclusion: These results suggest that nitrocefin test is not useful even if it is performed by trained technician and zone edge test has the same sensitivity as previous report only when trained technician performed it. In our study all the MIC ≦0.03 mcg/mL strains were negative for blaZ and it may suggest that MIC cutoff≦0.03 mcg/mL may be used for detection of penicillinase negative S. aureus. Further study should be necessary to assess the value of this finding.