Mechanisms of decreased susceptibility to cefpodoxime in Escherichia coli

Abstract

Cefpodoxime is one of five antimicrobial agents recommended by the National Committee for Clinical Laboratory Standards for screening isolates of Klebsiella spp. and Escherichia coli for extended-spectrum beta-lactamase (ESBL) production. In a prior study, we noted that among 131 E. coli isolates for which the MIC of at least one extended-spectrum cephalosporin (ESC) or aztreonam was > or =2 micro g/ml (suggesting the presence of ESBL production), there were 59 isolates (45.0%) for which the MIC of cefpodoxime was 2 to 4 micro g/ml (i.e., a positive ESBL screening test), but the MICs of ceftazidime, cefotaxime, and ceftriaxone were < or =1 micro g/ml (below the ESBL screening breakpoint). Thus, the results appeared to be false-positive ESBL screening tests. These 59 isolates were divided into five phenotypic groups based on the susceptibility patterns of the organisms to a variety of beta-lactam agents and further characterized. The first group (32 isolates) all produced a TEM-1 beta-lactamase, and changes in the major outer membrane proteins were detected in representative strains. The second group (18 isolates) lacked bla(TEM) but showed a number of porin changes; some also showed a modest elevation in production of the AmpC chromosomal beta-lactamase. In the third phenotypic group (seven isolates) all expressed an OXA-30 beta-lactamase. Some also harbored altered porins. The two remaining phenotypes each had a distinct pattern of porin changes with or without beta-lactamase production. These data indicate that several factors are associated with decreased susceptibility to cefpodoxime in E. coli, but none of the mechanisms are related to ESBL production. Current screening methods produced false-positive ESBL results for these isolates. Such isolates should not be classified as containing ESBLs, nor should interpretations of ESCs or aztreonam susceptibility be changed to resistant on test reports for these isolates.

FIG. 1.

OMP profiles of selected E. coli isolates and control strains grown in nutrient broth. Proteins were separated on 10% polyacrylamide-8 M urea gels and stained with Coomassie blue. (A) Comparison of isolates with phenotypes designated as A (EC3589, EC3437, EC922, and EC3095) or B (EC689, EC3194, EC3076) and control strains E. coli C600 (wild type), E. coli MH225 (OmpC⁻), and CDC3100 (enhanced AmpC producer). (B) Comparison of isolates with phenotypes designated as C (EC251), D (EC4081), A (EC3006), or E (EC1552) and E. coli C600 and E. coli MH225 (OmpC⁻).

FIG. 1.

OMP profiles of selected E. coli isolates and control strains grown in nutrient broth. Proteins were separated on 10% polyacrylamide-8 M urea gels and stained with Coomassie blue. (A) Comparison of isolates with phenotypes designated as A (EC3589, EC3437, EC922, and EC3095) or B (EC689, EC3194, EC3076) and control strains E. coli C600 (wild type), E. coli MH225 (OmpC⁻), and CDC3100 (enhanced AmpC producer). (B) Comparison of isolates with phenotypes designated as C (EC251), D (EC4081), A (EC3006), or E (EC1552) and E. coli C600 and E. coli MH225 (OmpC⁻).

FIG. 2.

DNA sequence of the ampC promoter and attenuator regions from selected E. coli isolates compared with analogous sequences of K12 and DH5α (4, 5). The −35 and −10 consensus promoter sequences and the translation initiation codon for AmpC β-lactamase are underlined. Arrows indicate the transcription attenuator region. The amino acid sequence of the initial segment of the AmpC leader region is shown below the corresponding nucleotide sequence.