[Demonstration of methicillin-resistant Staphylococci using the broth microdilution method].
The increasing incidence of methicillin resistant staphylococci rendered more important the problem of their detection, by the susceptibility tests, particularly by the broth microdilution methods. The effects of the addition of 2% NaCl to the Cation Supplemented Mueller Hinton Broth (CSMHB) on the MICs of penicillinase resistant penicillins (PRP) and of other antibiotics have been studied on 100 strains of staphylococci isolated by pathological materials. In 13% of the studied strains the MIC of oxacillin in CSMHB plus 2% NaCl was at least 4 times greater than the value found in the absence of NaCl. In the same experimental conditions a ratio between MICs equal or greater than 4 dilutions resulted in 69% of the strains for the gentamicin and in 93% of the strains for the amikacin. Such variation in the value of MIC determined a change in the category of susceptibility (from sensitive to intermediate or resistant and from intermediate to resistant) in 10% of the strains for the oxacillin, in 28% for the gentamicin and 41% for the amikacin. To improve detection of methicillin resistant staphylococci without altering the MICs of other antibiotics (aminoglycosides), it is necessary to perform the broth microdilution tests with CSMHB and with CSMHB plus 2% NaCl. CSMHB plus 2% NaCl will be used for testing PPR and cephalosporin, while CSMHB for the other antibiotics.