SALMONELLA-ENTERITIDIS - AMPC PLASMID-MEDIATED INDUCIBLE BETA-LACTAMASE (DHA-1) WITH AN AMPR GENE FROM MORGANELLA-MORGANII

DHA-1, a plasmid-mediated cephalosporinase from a single clinical Salm onella enteritidis isolate, conferred resistance to oxyimino-cephalosp orins (cefotaxime and ceftazidime) and cephamycins (cefoxitin and moxa lactam), and this resistance was transferable to Escherichia coli HB10 1. An antagonism was observed between cefoxitin and aztreonam by the d iffusion method. Transformation of the transconjugant E. coli strain w ith plasmid pNH5 carrying the ampD gene (whose product decreases the l evel of expression of ampC) resulted in an eightfold decrease in the M IC of cefoxitin. A clone with the same AmpC susceptibility pattern wit h antagonism was obtained, clone E. coli JM101(pSAL2-ind), and its nuc leotide sequence was determined. It contained an open reading frame wi th 98.7% DNA sequence identity with the ampC gene of Morganella morgan ii. DNA sequence analysis also identified a gene upstream of ampC whos e sequence was 97% identical to the partial sequence of the ampR gene (435 bp) from M. morganii. The gene encoded a protein with an amino-te rminal DNA-binding domain typical of transcriptional activators of the LysR family. Moreover, the intercistronic region between the ampC and ampR genes was 98% identical to the corresponding region from M. morg anii DNA. AmpR was shown to be functional by enzyme induction and a ge l mobility-shift assay. An ampG gene was also detected in a Southern b lot of DNA from the S. enteritidis isolate. These findings suggest tha t this inducible plasmid-mediated AmpC type beta-lactamase, DHA-1, pro bably originated from M. morganii.