Abstract: The aim was to investigate the relationship between the expression of AmpC beta -lactamase i n Enterobacter cloacae and ampD-ampR regulatory gene. Phenotype screening test, PCR amplification, gene clone and DNA sequencings were performed in the 58 stra ins of Enterobacter cloacae. From the 58 strains of Enterobacter cloacae, the nu mberog de repressed strains were 50 and that of hyperinducible mutant strains were 3, resp ecti vely. 49 of the 58 strains harbored bla ampD gene and 51 of 58 strains harb ored bla_ampR gene. The results of bla_ampD gene sequencings indicated t hat hyperinducible mutant strains had an amino acid change in 95 codon(Try-95→Arg) and eight of the derepressed strains had amino acid substitutions or deletion i n the carboxy-terminal of ampD. The results of bla_ampR gene sequencin gs i ndicated that only five of the derepressed strains had the suspicious mutants in the ampR gene. The deletion and amino acid substitutions in the carboxy-termin al of ampD may result in the stably depressed expression of AmpC beta-lactamase in Enterobacter cloacae. The amino acid substitutions of ampR maybe influence t he expression of AmpC beta-lactamase.

Key words: Enterobacter cloacae, AmpC beta-lact amase, gene analysis, ampD