HETEROGENEOUS EXPRESSION OF FUSIDIC ACID RESISTANCE IN STAPHYLOCOCCUS-AUREUS WITH PLASMID OR CHROMOSOMALLY ENCODED FUSIDIC ACID RESISTANCE GENES

Fusidic acid resistance expression in a methicillin susceptible Staphy lococcus aureus strain (WBG1576), which carries fusidic acid resistanc e on plasmid pUB101, and a prevalent Western Australian methicillin-fu sidic acid resistant strain (WBG8287) were compared. WBG8287 carries f usidic acid resistance on the chromosome and its plasmid content has n o effect on the levels of this resistance, WBG1576 and WBG8287 exhibit ed similar heterogeneous populations in respect to fusidic acid resist ance levels in population analyses. A high-level fusidic acid resistan t mutant of WBG1576 (BE8) had alterations in SmaI chromosomal profiles , but not in plasmid size or resistance expression. Mutations causing increased fusidic acid resistance in WBG1576 are chromosomally located . A high-level fusidic acid resistant mutant of WBG8287 (BE3) had no a lterations in SmaI chromosomal profiles, or plasmid content and resist ances. Comparison of resistance levels to kanamycin and spectinomycin, between high-level resistant colonies of WBG8287 and WBG8287, indicat e that mutations in the chromosomal gene fusA, which encodes elongatio n factor-G, are probably the cause of the increased resistance levels observed in these mutant strains.