Increased expression of ampC in Pseudomonas aeruginosa mutants selected with ciprofloxacin

Two Pseudomonas aeruginosa mutants exhibiting increased expression of ampC were selected during exposure to ciprofloxacin. These mutants also exhibited significant increases in mexCD-oprJ expression, but further studies failed to show a link between the increased expression of mexCD-oprJ and ampC. Increased ampC expression was not related to mutations within ampR, the ampC-ampR intergenic region, ampD, ampDh2, or ampDh3 or to changes in the levels of expression of these amidase genes. However, ampD complementation restored wild-type levels of ampC expression and ceftazidime susceptibility, suggesting alternative mechanisms of ampC regulation.