Antimicrobial resistance and integrons of commensal Escherichia coli strains from healthy humans in China

Antibiotic therapy can affect not only the pathogenic bacteria, but also commensal microorganisms in the gut flora of humans, which might serve as a reservoir of antimicrobial resistance genes. Escherichia coli can be found widely in nature, and can also be implicated in human infectious diseases. Therefore, fecal E. coli is often considered as a good indicator for selection pressure imposed by antimicrobial use. Recently, many researchers began showing interest in the surveillance of antimicrobial resistance in normal flora of humans. In China, however, very little is known about the prevalence of antibiotic resistant genes in intestinal tracts of healthy people. The aim of our study is to investigate the prevalence of antimicrobial resistance genes and diversity of integrons in fecal E. coli strains recovered from healthy persons in China. In 2009, we performed a research on the fecal E. coli strains isolated from asymptomatic healthy unrelated human subjects in Fuzhou, China. And in this study, 150 fecal E. coli strains were randomly selected from those that were collected in 2009. E. coli ATCC 25922 was used as a positive control. Susceptibility to antibiotics was assessed by the disc diffusion method according to the CLSI. The following 18 antimicrobial agents were tested in this study: ampicillin (AMP, 10 mg), ampicillin plus sulbactam (SAM, 10/10 mg), cefazolin (CZO, 30 mg), cefuroxime (CXM, 30 mg), ceftazidime (CAZ, 30 mg), cefotaxime (CTX, 30 mg), cefepime (FEP, 30 mg), cefoxitin (FOX), aztreonam (ATM, 30 mg), imipenem (IPM, 10 mg), amikacin (AMK, 30 mg), kanamycin (KAN, 30 mg), streptomycin (STR, 10 mg), nalidixic acid (NA), ciprofloxacin (CIP, 5 mg), trimethoprimsulfamethoxazole (SXT, 1.25/23.75 mg), chloramphenicol (CHL, 30 mg), and tetracycline (TET, 30 mg). The discs were obtained from Oxoid Ltd (Basingstoke, Hampshire, England). The presence of genes associated with ampicillin (blaTEM, blaSHV, blaOXA-1), tetracycline (tetA–tetE), streptomycin (aadA), sulphonamide (sul1, sul2 and sul3), kanamycin (aphA1 and aphA2), chloramphenicol resistance (cmlA and floR), and plasmid-borne quinolone resistance genes (qnrA, qnrB, and qnrS) was also analyzed by PCR. The presence of class 1, 2, and 3 integrons was characterized by PCR in all E. coli strains. The characterization of the variable region of class 1 and 2 integrons was performed by PCR and subsequent DNA sequencing. As shown in Table 1, the resistances to STR (79.3%), NA (53.3%), TET (50%), AMP (44%), and SXT (32%) were higher than those of the other antimicrobials (0–19.3%). Of these, streptomycin and tetracycline have been widely used as growth promoters or prophylactic agents in animal husbandry. The high resistance of gut flora may be due to the extensive and long-term use of these antibiotics in humans and in livestock. Our study also found that the resistance rate of human fecal E. coli strains to third-generation cephalosporins such as CTX (15.3%) and CAZ (4.0%) was higher than that in other countries. In China, beta-lactams, aminoglycosides, and quinolones are often used to cure Enterobacteriaceae infections. Therefore, it is not surprising that the high resistance of commensal flora may be due to the extensive use of these antimicrobials. Of the 150 E. coli strains tested, 40 (26.7%) carried integrons, among which 39 (97.5%) carried intI1 and only 1 strain carried intI2. Class 3 integrons were not detected in the study. The prevalence of the integrase gene (26.7%) in this study was similar to that in Spain (29%), however, it is lower than that found in clinical Enterobacteriaceae isolates in China (59.9%). All Correspondence to: Ying-ping Cao, Department of Clinical Laboratory, Fujian Medical University Union Hospital, 29# Xinquan Rd, Fuzhou, Fujian 350001, People’s Republic of China. Email: [email protected]