Mutators are not responsible for the association between b-lactam and quinolone resistance in Escherichia coli with AmpC hyperproduction phenotype

Abstract number: 1733_364

Conejo M., Amblar G., Perea E., Martínez-Martínez L.

Objective: Clinical isolates of Esherichia coli (Eco) resistant to quinolones displaying an AmpC hyperproduction phenotype (resistance to both cefoxitin and amoxicillin-clavulanate in the absence of extended-spectrum b-lactamases) have been increasingly observed in our institution. The aim of this study was to determine if the frequency of hypermutable strains is higher in this group of organisms than in isolates without combined resistance to both quinolones and b-lactams.

Method: Rifampin (Rif) resistance mutation frequency (MF) was studied in 97 Eco isolates from the Univ. Hosp. V. Macarena, Seville, Spain, including 60 isolates showing an AmpC hyperproduction phenotype resulting from several point mutations in the promoter and or the attenuator of the chromosomal ampC gene (ACBL+) and co-resistance to ciprofloxacin and nalidixic acid (QUINR), 9 isolates ACBL+ and susceptible to both quinolones (QUINS), and 28 isolates susceptible to both cefoxitin and amoxicillin-clavulanate (ACBL-) and either QUINR or QUINS (15 and 13 strains respectively). Three Luria-Bertani (LB) tubes for each strain were inoculated with an independent colony from a blood agar plate. After 24 h of incubation, appropriate dilutions were seeded onto LB agar plates with and without Rif (100 mg/L), and colony counts were performed after 48 h. MFs were reported as a proportion of the number of Rif-resistant colonies to the total viable count. The results were the mean value obtained in three independent experiments that were repeated in cases of discrepancies. Eco AB1157 mutS::Tn10, with a defective mutS gene, was used as a strong mutator control, and Eco ATCC25922 as a non hypermutable control. Statistical differences were determined by the chi-square test (p < 0.05: significant).

Results: Strains were considered non hypermutable when the MF was <4×10-8, weak mutators if 4×10-8 ≤ MF < 4×10-7 and strong mutators if MF ≥ 4×10-7. Of the 97 strains, 65 were non hypermutable, 29 were weak mutators and 3 were strong mutators. Hypermutable strains were detected in higher proportions in the isolates with ACBL+QUINS and ACBL-QUINR (54.5% and 40%, respectively) than in isolates with ACBL+QUINR (29.7%). Among ACBL-QUINS isolates only 15% were hypermutable. No significant differences were observed among groups.

Conclusion: Hypermutability could not explain the association between b-lactam and quinolone resistance in clinical isolates of Eco with AmpC hyperproduction phenotype.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
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