Biological cost of AmpC production for Salmonella enterica serotype typhimurium

Chromosomally mediated AmpC-type beta -lactamases are frequently found amon g Enterobacteriaceae, Hyperproduction of AmpC beta -lactamase results in hi gh-level resistance to beta -lactam antibiotics. One striking feature of Sa lmonella is the absence of the structural ampC gene, encoding AmpC beta -la ctamase, in contrast with other members in the Enterobacteriaceae family, s uch as Escherichia, Citrobacter, or Enterobacter, The horizontal acquisitio n of ampC genes is one of the causes of the increased resistance to extende d-spectrum cephalosporins and beta -lactamase inhibitors among gram-negativ e rods, Nevertheless, despite the high number of beta -lactam-resistant Sal monella isolates so far described, only two strains expressing resistance t o cephalosporin and beta -lactamase inhibitors which is mediated by AmpC-ty pe enzymes have been found. In this work, data are provided which support t he possibility that the maintenance and expression of the ampC gene may rep resent an unbearable cost for Salmonella in terms of reduction of some of i ts lifestyle attributes, such as growth rate and invasiveness, The deleteri ous AmpC burden can be eliminated by decreasing the production of AmpC when both the regulatory gene, ampR, and ampC are present in Salmonella, Thus, it is suggested that the two genes have to be acquired together by Salmonel la, leading to an inducible beta -lactam resistance phenotype, AmpC synthes is did not produce major variations in the peptidoglycan composition of Sal monella.