Antagonism between Aminoglycosides and β-Lactams in a Methicillin-Resistant Staphylococcus aureus Isolate Involves Induction of an Aminoglycoside-Modifying Enzyme

We encountered three clinical isolates of methicillin-resistant Staphylococcus aureus which were susceptible to netilmicin and arbekacin in the absence of β-lactam antibiotics but which were resistant to them in the presence of β-lactam antibiotics. One of these strains, KU5801, was used to further investigate the antagonism between aminoglycosides and β-lactam antibiotics. β-Lactam antibiotics induced bacterial synthesis of aminoglycoside-6′-N-acetyltransferase and 2"-O-phosphotransferase [AAC(6′)-APH(2")] in association with decreased antimicrobial activities of aminoglycosides. A 14.4-kb EcoRI fragment that included the genes that control for β-lactam-inducible aminoglycoside resistance was cloned from a 31-kb conjugative plasmid present in KU5801. Restriction fragment mapping and PCR analysis suggested that a Tn4001-like element containing a gene encoding AAC(6′)-APH(2") was located downstream from a truncated blaZ gene. The DNA sequence between blaR1 and a Tn4001-like element was determined. The Tn4001-IS257 hybrid structure was cointegrated into the blaZ gene, and the typical sequences for the termination of transcription were not found between these regions. We deduced that antagonism of aminoglycosides by β-lactam antibiotics in isolate KU5801 involved transcription of the aac(6′)-Ie-aph(2")-Ia gene under the influence of the system regulating penicillinase production.