Antimicrobial Resistance of Entrotoxigenic and Non-Entrotoxigenic Escherichia coli Isolated From Diarrheic Calves in Iran

Among diarrheagenic strains of Escherichia coli, entrotoxigenic Escherichia coli (ETEC) is most commonly associated with diarrhea in calves and lambs. Broad use of antimicrobials in agriculture selects for resistant bacteria that may enter the food chain, and potentially result in foodborne disease in humans that is less responsive to treatment with conventional antibiotics.
This study was carried out to identify antimicrobial resistance in ETEC and non-ETEC isolated from diarrheic calves.
Disk diffusion methods and PCR were used to detect antimicrobial resistance. Antimicrobial susceptibility testing was performed by using the standards recommended by the clinical and laboratory standard institute (CLSI). Multiplex or monoplex PCR amplification was used to identify eight antibiotic-resistant genes, including bla SHV, tet(A), Sul1, aac(3)-IV, ere(A), catA1, cmlA, aadA1 and qnr(A), which confer resistance to penicillin, tetracycline, sulfonamide, gentamicin, erythromycin, chloramphenicol, streptomycin, and fluoroquinolone, respectively.
Antimicrobial resistance rates for ETEC isolates were detected against penicillin (100%), tetracycline (90.9%), erythromycin (90.9%), streptomycin (90.9%), sulfonamide (63.6%), chloramphenicol (63.6%), gentamicin (45.4%) and fluoroquinolone (36.3%). Furthermore, according to the results, antimicrobial resistance for non-ETEC isolates was detected against penicillin (100%), followed by erythromycin (97.6%), tetracycline (93%), streptomycin (91.8%), sulfonamide (73.2%), chloramphenicol (51.1%), fluoroquinolone (44.1%), and gentamicin (34.8%). In addition, the distribution of the resistant genes for ETEC isolates were ere(A) (100%), catA1 (100%), cmlA (100%), aadA1 (100%), Sul1 (72.7%), tet(A) (54.5%), aac(3)-IV (54.5%), bla SHV (36.3%), and qnr(A) (9%). For non-ETEC isolates they were ere(A) (100%), aadA1 (100%), Sul1 (87.2%), catA1 (67.4%), cmlA (67.4%), tet(A) (48.8%), aac(3)-IV (48.8%), bla SHV (41.8%), and qnr(A) (3.4%).
Among the eight antimicrobial agents examined in this investigation, the least resistance was observed against gentamicin and fluoroquinolone in both ETEC and non-ETEC isolates. Therefore, carrying out antimicrobial susceptibility tests before drug prescription seems necessary.