A simple broth-disk elution test for screening methicillin-resistant (heteroresistant) staphylococci.

Forty-six strains of Staphylococcus aureus and 47 strains of unspeciated coagulase-negative staphylococci were inoculated in tubes containing a cation-supplemented Mueller-Hinton broth with 4% (wt/vol) NaCl. A critical concentration of methicillin (10 micrograms/ml) or oxacillin (6 micrograms/ml) was achieved in each tube by adding the appropriate number of antibiotic disks. Growth was interpreted as resistance. Results obtained with fully resistant or fully sensitive strains showed complete agreement with the reference method. Four strains with intermediate sensitivity (two S. aureus and two coagulase-negative staphylococci) were classified as resistant by the broth-disk elution test.