PubMed · 3431371
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H Brown. 1987. Antibiotic sensitivity.. https://pubmed.ncbi.nlm.nih.gov/3431371/
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108 Staphylococcus spp. strains from 300 clinical specimens from hospitalized patients were isolated. Identification and drug resistance were determined using automated ATB system. 37 S. aureus strains, 44 S. epidermides strains and 27 strains of other coagulase-negative staphylococci were cultured. Sensitivity to methicillin of S. aureus was determined with four methods: ATB system, disc-diffusion (Oxa 1 microgram), Crystal MRSA ID System and agar screen test in TSA medium with methicillin (25 micrograms/ml). 13 S. aureus strains (about 1/3 of strains) were methicillin-resistant (MRSA). Complete conformity of the results was obtained with Crystal MRSA ID, disc-diffusion and agar screen tests. In the case of three S. aureus strains the results of determination in ATB system were not consistent with the results obtained with the use of the methods mentioned above. Susceptibility to methicillin of 71 coagulase-negative strains (CNS) was determined using two methods at first: ATB and disc-diffusion. In the case of 25 methicillin-resistant strains identical results were obtained. For 20 coagulase-negative strains non-conformity with the results of these two methods was observed. As the decisive method, the agar screen test (TSA-MET) was applied. 18 of these 20 CNS strains were categorized as methicillin-resistant. Finally, 43 MRCNS (i.e. 60%) were detected among 71 coagulase-negative strains. The results of methicillin resistance determination of staphylococci in an automated system should be confirmed with a second test such as agar screen, disc-diffusion or Crystal MRSA ID System (in the case of S. aureus).
In the tests bacteriological laboratories participated in 45 sanitary-epidemiological stations. Each station was given two strains of staphylococci: S. epidermidis and S. aureus homogeneously resistant to methicillin (MRSA) or S. aureus sensitive to methicillin (MSSA), and S. aureus coagulase-negative and clumping-factor-positive strain. The analysis was carried out of the results of control identifications of strains of the determinations of the sensitivity of the identified strains to antibiotics. Among the studied strains the greatest difficulty in identification were caused by the coagulase-negative strain of S. aureus. In the determination of the sensitivity of the control strains to chemotherapeutic agents abnormalities were found in the technique of antibiogram performing and incorrect selection of discs for antibiograms as well as erroneous interpretation of the results.
The Showa disk susceptibility test using two penicillinase-resistant penicillins, oxacillin and methicillin, was evaluated to discriminate between the strains of Staphylococcus aureus resistant to methicillin (MRSA) and those susceptible (MSSA) in the multi-center trials. The study included 651 clinical isolates of S. aureus, comprising of 329 MRSA and 322 MSSA isolates. The inhibitory zone diameters by Showa disks to oxacillin and methicillin highly correlated with minimum inhibitory concentrations (MICs) determined by standard agar dilutions with 0.961 and 0.930 correlation coefficients, respectively. Of 651 duplicate MIC determinations, 79.9% (oxacillin) and 80.3% (methicillin) were within +/- 1 log2 dilutions with each other. When Showa oxacillin and methicillin disks were incubated at 35 degrees C, sensitivity and specificity of oxacillin to detect MRSA were 95.4% and 98.1%, and those of methicillin were 94.8% and 95.2%. When tested on agar plates supplemented with 5% NaCl, sensitivity and specificity markedly improved to > 97%. Also, when incubated at 30 degrees C, sensitivity and specificity became to nearly 100%. Of 329 MRSA isolates, the interpretive criteria combined with incubation at 30 degrees C and testing onto 5% NaCl supplemented agar plates could correctly identify 324 (98.5%) and 329 (100%) isolates, respectively. In conclusion, when the Showa oxacillin and methicillin disk susceptibility tests were employed exactly according to the manufacturer's instruction, the test performances to detect MRSA were enough reliable to screen MRSA isolates in clinical microbiology laboratories.