Successful use of broth microdilution in susceptibility tests for methicillin-resistant (heteroresistant) staphylococci

Abstract

The broth microdilution antimicrobial susceptibility testing procedure was studied to see whether it could be made reliable for determining resistance of staphylococci to methicillin, oxacillin, nafcillin and cephalothin. With 45 selected strains of Staphylococcus aureus and 12 selected strains of S. epidermidis the addition of 2% NaCl to cation-supplemented Mueller-Hinton broth permitted reliable descrimination between resistant and susceptible organisms. A screening test in which resistant staphylococci grew on agar containing 4% NaCl and methicillin (10 .mu.g/ml), oxacillin (6 .mu.g/ml) or nafcillin (6 .mu.g/ml) incubated at 35.degree. C for 24 h (additional 24 h if no growth) was also reliable. In vitro cephalothin resistance occurred in heteroresistant S. aureus but usually did not occur in heteroresistant S. epidermidis.