Two novel plasmid‐mediated β‐lactamases (CTX‐M‐5 and CTX‐M‐6) produced by Salmonella typhimurium clinical strains were characterized. The enzymes exhibited a pI of 8.4, hydrolyzed oxyimino‐β‐lactams and were susceptible to mechanism‐based β‐lactamase inhibitors. The respective bla genes were cloned and sequenced. The deduced amino acid sequences showed a high degree of homology with those of the previously described plasmid class A CTX‐M‐type enzymes and appeared related to the chromosomal β‐lactamases of Klebsiella oxytoca.
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