Mutations in the ampC promoter of Escherichia coli isolates resistant to oxyiminocephalosporins without extended spectrum β-lactamase production

Escherichia coli strains showing an increased resistance to oxyiminocephalosporins without an extended spectrum β-lactamase production have been screened for mutations in their ampC β-lactamase gene promoter. Mutations were found by direct sequencing of seven promoters at positions −42, −32 (box −35), −18, −1, +5, +24 (attenuator), +31 (attenuator) and +58. By using rapid and simple methods, three of these mutations (−42, −32 and +24), which could enhance transcription, were searched in 37 resistant and 25 sensitive isolates. The −42 mutation was present in 33 of the 37 promoters from the resistant isolates. The −32 and +24 mutations were present only in three and two promoters, respectively, and they were combined in the most resistant strain of the study. The +24 mutation was detected in another strain associated with a 1-bp insertion between the −35 and −10 conserved sequences. None of these mutations was detected in the ampC β-lactamase from the sensitive isolates.