Klebsiella oxytoca: Resistance to aztreonam by overproduction of the chromosomally encoded β-lactamase

Abstract Aztreonam-resistant Klebsiella oxytoca strain SL7811 was selected on agar containing 1 μg of aztreonam per ml from a susceptible strain SL781. The MICs for the resistant mutant towards penicillins, aztreonam and ceftriaxone were much higher, to cefotaxime slightly higher and to ceftazidime unchanged. Synthesis of β-lactamase was 223-fold greater in the mutant compared with the susceptible strain. SL781 and its resistant mutant SL7811 produced β-lactamase with the same isoelectric point and substrate profile. The β-lactamase genes from SL781 and SL7811 were cloned in plasmid pBGS18 giving pBOF-1 and pBOF-4 respectively. The sequences of the two putative promoters indicated two modifications in the resistant plasmid pBOF-4: a transversion (G → T) in the first base of the − 10 consensus sequence and a deletion of one C residue four base pairs upstream of the − 10 hexamer.