1 Department of Microbiology, University Hospital Virgen Macarena and School of Medicine, University of Seville, Apdo. 914, 41009 Seville, Spain; 2 Service of Microbiology, University Hospital Marqués de Valdecilla, Santander, Spain
Keywords: E. coli , AmpC , mutations , antimicrobial resistance
Sir,
The expression of the chromosomally encoded AmpC ß-lactamase of Escherichia coli is not inducible, because of the absence of the regulatory gene ampR. The ampC gene of E. coli is regulated by a weak promoter and a transcriptional attenuator. Hyperproduction of AmpC can arise by gene amplification of ampC or mutations at either the promoter and/or the attenuator of ampC, which increase the transcription rate of ampC.1
Constitutive low-level expression of the AmpC ß-lactamase of E. coli does not contribute to a clinically relevant level of resistance to ß-lactams. However, hyperproduction of this enzyme causes resistance to penicillins, first- and second-generation cephalosporins, and cephamycins, and may decrease the activity of oxyimino-cephalosporins. As AmpC is not inhibited by clavulanic acid and similar inhibitors of class A ß-lactamases, strains hyperproducing this enzyme are also resistant to the combinations of penicillins with these ß-lactamase inhibitors.
In this study we describe a novel mutation in the attenuator of the ampC gene of E. coli 47/94, an AmpC-hyperproducing clinical strain isolated in 1994 from a urine sample of a patient at the Clinical Microbiology Department, University Hospital Virgen Macarena, Seville, Spain.2
Mutations in the promoter/attenuator region of the ampC gene were identified as described previously3 by DNA sequencing of the PCR products in both directions using the Big dye terminator v3.0 sequencing kit (Applied Biosystems, Foster City, CA, USA). Sequence analysis was performed on a 3700 DNA Analyzer (Applied Biosystems).
Five point mutations at positions 88 (CT), 82 (A
G), 42 (C
T), 18 (G
A) and 1 (C
T) of the ampC promoter of E. coli 47/94 were observed (Figure 1). All these mutations have been described previously in clinical isolates of E. coli hyperproducing AmpC.36
The role of the mutation at 42 in the hyperproduction of AmpC has been studied in more detail than the mutations at 88, 82, 18 and 1. The C
T transition at position 42 creates a perfect TTGACA box upstream of the normal 35 box, generating a strong ampC promoter and modifying the transcription initiation site.
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In conclusion, the hyperproduction of AmpC in E. coli 47/94 was associated with point mutations in the ampC promoter and a deletion of 30 bp affecting the attenuator region of ampC.
References
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2.
Martínez-Martínez L, Conejo MC, Pascual A et al. Activities of imipenem and cephalosporins against clonally related strains of Escherichia coli hyperproducing chromosomal ß-lactamase and showing altered porin profiles. Antimicrob Agents Chemother 2000; 44: 25346.
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Caroff N, Espaze E, Gautreau D et al. Analysis of the effects of 42 and 32 ampC promoter mutations in clinical isolates of Escherichia coli hyperproducing AmpC. J Antimicrob Chemother 2000; 45: 7838.
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