Antibiogram-resistogram typing scheme for methicillin-resistant Staphylococcus aureus

An antibiogram-resistogram (AR) typing scheme that can simply and rapidly differentiate methicillin-resistant Staphylococcus aureus (MRSA) isolates has been devised. Susceptibility to antibiotics and chemicals was determined by disk diffusion testing. Three disk diffusion methods and three control S. aureus strains were evaluated. A modified Stokes' technique in which S. aureus ATCC 25923 replaced S. aureus NCTC 6571 as the control organism was chosen. Susceptibility patterns against 18 antibiotics and four chemicals were used to determine AR types. AR subtypes were determined with reference to knowledge of the local MRSA population so that plasmid loss would not result in misclassification. AR typing was compared with phage typing and plasmid profiling and found to be more discriminatory than either of these typing methods. Representative isolates of the most frequently occurring AR patterns were further characterised by investigating enterotoxin production, MICs of gentamicin and amikacin, and restriction endonuclease analysis of plasma DNA, to determine whether apparently different strains could have the same AR pattern and to devise confirmatory tests for any such similar patterns. One pattern was shared by two strains but isolates could be differentiated by susceptibility to minocycline. This typing scheme can be used in the diagnostic laboratory and results may be obtained within 24 h.