Effect of the efflux pump inhibitor Phe-Arg- -naphthylamide on the MIC values of the quinolones, tetracycline and chloramphenicol, in Escherichia coli isolates of different origin

Abstract

Sir, The main mechanisms of quinolone resistance in Escherichia coli include mutations in the gyrA and parC target genes and decreased intracellular accumulation.1 Active efflux pumps are important for intrinsic and acquired antibiotic resistance, and overexpression of efflux pumps affecting quinolones is becoming increasingly common in E. coli. Recently, efflux pump inhibitors have been investigated with a view to improving and potentiating the activity of exported antibiotics.2 Phe-Arg-β-naphthylamide has been described as a broad-spectrum efflux pump inhibitor in Pseudomonas aeruginosa.2 The effect of this inhibitor on fluoroquinolone susceptibilities in Enterobacter aerogenes,3 E. coli,4 P. aeruginosa,2 Acinetobacter baumannii5 and Stenotrophomonas maltophilia5 has also been studied, but not for nalidixic acid and other antimicrobial agents in E. coli. Therefore in this study, we analysed the influence of the Phe-Arg-β-naphthylamide (Sigma) inhibitor on the activity of quinolones, chloramphenicol and tetracycline against a collection of 60 E. coli isolates previously characterized with respect to amino acid changes in both GyrA and ParC proteins.6,7 The collection includes the strain Nor5, an in vitro selected quinolone-resistant mutant, in which the overexpression of MarA, SoxS and AcrA has been shown (J. Vila, B. Peter, J. Sanchez-Cespedes & M. Tario, unpublished data). The remaining 59 isolates were recovered from foods (n = 11), humans (n = 25) and animals (n = 23). Susceptibilities to nalidixic acid, norfloxacin, chloramphenicol, tetracycline and ciprofloxacin were studied by agar dilution in the presence or absence of 20 mg/L of Phe-Arg-β-naphthylamide, in accordance with NCCLS guidelines. All of the 60 E. coli isolates showed an MIC value of >256 mg/L to Phe-Arg-β-naphthylamide. Table 1 shows the effect of 20 mg/L of the inhibitor Phe-Arg-β-naphthylamide on the MIC values of the five tested antibiotics in 60 E. coli isolates with different numbers of amino acid changes in GyrA and T ab le 1 . M IC v al ue s (i n m g/ L ) of th e fi ve te st ed a nt ib io tic s in th e pr es en ce o r ab se nc e of 2 0 m g/ L o f P he -A rg β -n ap ht hy la m id e in a s er ie s of 6 0 E . c ol i i so la te s or ga ni ze d