ABSTRACT
Objective To investigate the influence of plasmid spread and ampD mutation to Enterobacter cloacae that leads to the AmpC β-lactamase change from inducible type to constitutive type. Methods The Enterobacter cloacae were isolated from the patients with nosocomial infection. The inducible type isolations and their constitutive type changers were put into the same group. The plasmid ampC gene and chromatin ampD gene in pairs in each group were amplified, sequenced and compared. Results Of 195 patients infected by Enterobacter cloacae of inducible type, 25 (12.82%) were changed to the ones of constitutive high type. In these 25 changed groups, 10 were caused by plasmid spread, 10 by ampD mutation, 1 by both, and 4 by neither. Twelve changed constitutive type strains had ampD significant mutations, in which 7 were frame-shift mutations and 5 were spot mutations. Conclusions The change ratio of Enterobacter cloacae from inducible type to constitutive type is rather high. Both plasmid spread and ampD mutation are possibly the mechanism of such change. Plasmid mediated AmpC β-lactamase spreads among different species and interregionally. The mutation rate of chromatin ampD gene is also higher than the natural mutation rate. These two mechanisms should be considered in clinical treatment.