Effect of the source of Mueller-Hinton agar and resistance frequency on the detection of methicillin-resistant Staphylococcus aureus

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Abstract
Inconsistencies in the results fo disk diffusion tests fo oxacillin against S. aureus that occurred when using commercially prepared Mueller-Hinton agar from different sources led to the evaluation of the ability of media from different sources to detect resistance to oxacillin, methicillin and nafcillin in S. aureus. Mueller-Hinton agar from 5 manufacturers was prepared in this laboratory and used for standard disk diffusion and agar dilution tests. Ten oxacillin-resistant S. aureus isolates, of which 3 were definitive-resistant and 7 were occult-resistant, were examined. All definitive-resistant strains were resistant to all 3 antimicrobial agents on 4 of 5 agars. The occult-resistant strains were consistently detected as resistant on only 1 of the agars. With only slight differences, oxacillin, methicillin and nafcillin resistance was more readily detected by disk diffusion and agar dilution when initially incubated at 30.degree. C, and extended incubation improved the detection. The frequency of resistance within a population of occult-resistant cells was low compared with the frequency within a population of definitively resistant cells. The heterogeneity of colony morphology and apparent growth rates within a population of occult-resistant cells contributed to the problem of detecting some resistant isolates. Definitive-resistant isolates were characterized by a very high and stable frequency of resistance. Occult-resistant strains were characterized by a lower frequency of resistance, although the true frequency of resistance may be difficult to ascertain because of heterogeneity in growth rates.