Cross-resistance of clinical isolates of methicillin-resistant Staphylococcus aureus to aminoglycosides and fluoroquinolones

Susceptibilities of 117 clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) to aminoglycosides and fluoroquinolones were investigated during 1987 and 1990. Gentamicin, tobramycin, sisomicin, micronomicin, and astromicin showed the bimodal antimicrobial activity pattern against MRSA, revealing cross-resistance to these antibiotics. However, amikacin, netilmicin, isepamicin and arbekacin (ABK) exhibited the monomodal antimicrobial activity pattern, suggesting the presence of less resistant strains. All 117 MRSA strains were susceptible to ABK with MICs less than 3 micrograms/ml. MRSA also showed the bimodal antimicrobial susceptibility pattern to fluoroquinolones such as ofloxacin (OFLX), ciprofloxacin (CPFX) and tosufloxacin (TFLX). Frequencies of TFLX susceptible MRSA isolates decreased progressively from 1987 to 1989 when this drug was not even in general clinical use. The cross-resistance of MRSA between TFLX and OFLX or CPFX persisted. Cross-resistance between aminoglycosides and fluoroquinolones, however, was less frequently observed.