SUMMARY The detection of methicillin resistance was examined in 51 strains of Staphylococcus aureus and 135 strains of coagulase negative staphylococci using Isosensitest, Diagnostic Sensitivity Test (DST), Mueller-Hinton (MH), Columbia, and Sensitest agars. MH agar with 5% added sodium chloride incubated at 35°C was the most effective in detecting resistance in S aureus, and Columbia agar with 5% added sodium chloride incubated at 35°C was most effective for coagulase negative staphylococci. For clinical purposes, a provisional report of sensitivity for S aureus could be issued after 18 hours; with coagulase negative staphylococci, only resistant strains could be reported at this time. For definitive results cultures must be examined after 40 hours of incubation.Resistance to methicillin in Staphylococcus aureus is usually heterogeneous and due to the presence of a component which sometimes only manifests resistance if grown on osmotically supportive media' or if cultured at 30'C2 for up to 48 hours.3 Snell et al4 recommended that incubation at 30°C or adding 5% sodium chloride to the medium, or both, be used for methicillin testing. Despite the heterogeneous nature of the resistance in S aureus to methicillin it has been shown to be clinically relevant in that it impairs the response to treatment with methicillin.5 Resistance to methicillin in coagulase negative staphylococci has also been shown to be due to heterogeneous populations and to be enhanced by growth on hypertonic medium or incubation at 30°C or for 48 hours.6In addition to these factors, the detection of methicillin resistance can be affected by the basal test medium used. Hindler and Inderleid7 showed that when MH, an undefined medium, was used from five different manufacturers there were differences in the detection of resistance of S aureus to methicillin.Brown and Kothari8 determined the minimum inhibitory concentration (MIC) of strains of S aureus using MH, DST, Sensitest, and Wellcotest agars, and all gave similar results when incubated at 30°C for 18 hours. In view of the increasing clinical importance of
Aims-To test 10 culture media for their ability to detect resistance and sensitivity of staphylococci to methicillin by disc diffusion. Methods-Fifty strains of Staphylococcus aureus and 135 strains of coagulase negative staphylococci were tested using Columbia, Diagnostic Sensitivity Test, Mueller Hinton, Sensitest and Isosensitest agars with and without 5% added sodium chloride. Cultures were examined after 18 and 40 hours of incubation. The diameter of the zone and its characteristics were recorded and these media were assessed for their ability to produce clear, readable zones of inhibition. Changes in the variables which determined resistance were investigated. Results were analysed allowing a zone diameter reduction of 8 mm and 10 mm compared with the control in addition to the standard 6 mm.
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