Cephamycin Resistance in Clinical Isolates and Laboratory-derived Strains of Escherichia coli, Nova Scotia, Canada

β-lactamase, altered porins, or both are usually responsible for cefoxitin resistance in Escherichia coli. We examined the relative importance of each. We studied 18 strains of clinical isolates with reduced cefoxitin susceptibility and 10 initially-susceptible strains passaged through cefoxitin-gradient plates. Of 18 wild-resistant strains, 9 had identical promoter mutations (including creation of a consensus 17-bp spacer) and related pulsed-field gel elec-trophoresis patterns; the other 9 strains were unrelated. Nine strains had attenuator mutations; two strains did not express OmpC or OmpF. After serial passage, 8 of 10 strains developed cefoxitin resistance, none developed promoter or attenuator mutations, 6 lost both the OmpC and OmpF porin proteins, and 1 showed decreased production of both. One strain had neither porin alteration or increased AmpC production. Porin mutants may occur more commonly and be less fit and less inclined to spread or cause disease than strains with increased β-lactamase expression. T he development of antibiotic resistance in Escherichia coli has important clinical implications. E. coli is among the most frequently isolated bacterium in a variety of clinical settings. The development of resistance to older agents such as ampicillin and trimethoprim-sulfamethoxa-zole, as well as the emerging problem of fluoroquinolone resistance, may substantially limit our antibiotic choices (1,2). Although cephamycin-resistant E. coli is relatively uncommon, widespread use of β-lactam antiboties may contribute to the development and spread of these strains. In 1999, Sahm et al. reported that 0.16% of E. coli were resistant to cephamycins (3). At a local level, unpublished data from the Queen Elizabeth II Health Science Centre in Halifax, Nova Scotia indicated that, of the 5,767 strains of E. coli processed from urine samples, 0.4% were cephamycin resistant. All strains of E. coli possess a gene that encodes an AmpC β-lactamase. Usually, almost no β-lactamase is produced because the gene is preceded by a weak promoter and a strong attenuator (4). Surveys of resistance mechanisms in cephamycin-resistant strains have most often identified promoter or attenuator mutations, which results in an up-regulation of AmpC β-lactamase production (5–7). Occasionally, cephamycin-resistant strains bear mobilized β-lactamases derived from bacteria such as Citrobacter feundii (8). In addition, mutation or altered expression of outer membrane proteins constituting porins can also contribute to cephamycin resistance. To our knowledge, no investigators have concurrently looked for alterations in porins in addition to promoter-attenuator mutations. Porin alterations might work together to produce a higher level of resistance. In addition, porin alterations may …