Plasmid-mediated beta-lactamases among aminoglycoside resistant gram-negative bacilli

Plasmid-mediated beta-lactamases were characterized by DNA hybridization in 371 aminoglycoside resistant gram-negative bacilli with known aminoglycoside resistance mechanism. Positive hybridization was detected in 50% to a TEM-1 probe, in 2% to a SHV-1 probe, and in 3% to both probes simultaneously. No hybridization was obtained to OXA-1, OXA-2, PSE-1/PSE-4/CARB-3 or PSE-2 beta-lactamase probes. TEM-1 beta-lactamase occurred simultaneously in 82% of strains showing the AAC(3)-V type of aminoglycoside resistance mechanism. Using isoelectric focusing as a control method, we found potentially plasmid-encoded beta-lactamases, other than TEM-1 and SHV-1, at various pIs in 13% of 288 randomly selected strains. The pIs of these strains or strains showing positive hybridizations did not fit to pIs of recently characterized plasmid-mediated enzymes against third-generation cephalosporins (e.g. CTX-1). In addition, the strains did not show resistance to cefotaxime or ceftazidime. According to the in vitro susceptibility data ceftazidime and cefotaxime were active against most of the aminoglycoside resistant strains studied. In contrast, the activity of piperacillin was much lower than that of the cephalosporins tested.