Characterization of a DHA-1-producing Klebsiella pneumoniae strain involved in an outbreak and role of the AmpR regulator in virulence.

A clonal strain of Klebsiella pneumoniae producing the plasmid-encoded cephalosporinase DHA-1 was isolated from four patients admitted to the teaching hospital of Clermont-Ferrand, France, in 2006. It was responsible for severe infections in three of the patients; the fourth was colonized only in the gastrointestinal tract. The strain had at least two plasmids encoding resistance to antibiotics (quinolones, aminoglycosides, chloramphenicol, sulfonamides, and trimethoprim), as shown by disk diffusion assay, and harbored only a few genes for virulence factors (wabG and mrkD), as shown by PCRs. DHA-1 synthesis is regulated by an upstream, divergently transcribed gene, ampR, which is also involved in the expression of virulence factors in Pseudomonas aeruginosa. To investigate the role of AmpR in K. pneumoniae, we cloned the wild-type ampR gene from the DHA-1 clonal isolate into a previously characterized K. pneumoniae background plasmid-cured strain, CH608. ampR was also introduced into a CH608 isogenic mutant deleted of ampD, in which AmpR is present only in its activator form, resulting in constitutive hyperproduction of the β-lactamase. We showed that ampR was involved in the upregulation of capsule synthesis and therefore in resistance to killing by serum. AmpR also modulated biofilm formation and type 3 fimbrial gene expression, as well as colonization of the murine gastrointestinal tract and adhesion to HT-29 intestinal epithelial cells. These results show the pleiotropic role of ampR in the pathogenesis process of K. pneumoniae.