Chromosomally Encoded AmpC-Type β-Lactamase in a Clinical Isolate of Proteus mirabilis

ABSTRACT A clinical strain of Proteus mirabilis (CF09) isolated from urine specimens of a patient displayed resistance to amoxicillin (MIC >4,096 μg/ml), ticarcillin (4,096 μg/ml), cefoxitin (64 μg/ml), cefotaxime (256 μg/ml), and ceftazidime (128 μg/ml) and required an elevated MIC of aztreonam (4 μg/ml). Clavulanic acid did not act synergistically with cephalosporins. Two β-lactamases with apparent pIs of 5.6 and 9.0 were identified by isoelectric focusing on a gel. Substrate and inhibition profiles were characteristic of an AmpC-type β-lactamase with a pI of 9.0. Amplification by PCR with primers for ampC genes ( Escherichia coli , Enterobacter cloacae , and Citrobacter freundii ) of a 756-bp DNA fragment from strain CF09 was obtained only with C. freundii -specific primers. Hybridization results showed that the ampC gene is only chromosomally located while the TEM gene is plasmid located. After cloning of the gene, analysis of the complete nucleotide sequence (1,146 bp) showed that this ampC gene is close to bla CMY-2 , from which it differs by three point mutations leading to amino acid substitutions Glu → Gly at position 22, Trp → Arg at position 201, and Ser → Asn at position 343. AmpC β-lactamases derived from that of C. freundii (LAT-1, LAT-2, BIL-1, and CMY-2) have been found in Klebsiella pneumoniae , E. coli , and Enterobacter aerogenes and have been reported to be plasmid borne. This is the first example of a chromosomally encoded AmpC-type β-lactamase observed in P. mirabilis . We suggest that it be designated CMY-3.