Impact of feed supplementation with antimicrobial agents on growth performance of broiler chickens, Clostridium perfringens and enterococcus counts, and antibiotic resistance phenotypes and distribution of antimicrobial resistance determinants in Escherichia coli isolates

Abstract

The effects of feed supplementation with the approved antimicrobial agents bambermycin, penicillin, salinomycin, and bacitracin or a combination of salinomycin plus bacitracin were evaluated for the incidence and distribution of antibiotic resistance in 197 commensal Escherichia coli isolates from broiler chickens over 35 days. All isolates showed some degree of multiple antibiotic resistance. Resistance to tetracycline (68.5%), amoxicillin (61.4%), ceftiofur (51.3%), spectinomycin (47.2%), and sulfonamides (42%) was most frequent. The levels of resistance to streptomycin, chloramphenicol, and gentamicin were 33.5, 35.5, and 25.3%, respectively. The overall resistance levels decreased from day 7 to day 35 (P < 0.001). Comparing treatments, the levels of resistance to ceftiofur, spectinomycin, and gentamicin (except for resistance to bacitracin treatment) were significantly higher in isolates from chickens receiving feed supplemented with salinomycin than from the other feeds (P < 0.001). Using a DNA microarray analysis capable of detecting commonly found antimicrobial resistance genes, we characterized 104 tetracycline-resistant E. coli isolates from 7- to 28-day-old chickens fed different growth promoters. Results showed a decrease in the incidence of isolates harboring tet(B), bla(TEM), sulI, and aadA and class 1 integron from days 7 to 35 (P < 0.01). Of the 84 tetracycline-ceftiofur-resistant E. coli isolates, 76 (90.5%) were positive for bla(CMY-2). The proportions of isolates positive for sulI, aadA, and integron class 1 were significantly higher in salinomycin-treated chickens than in the control or other treatment groups (P < 0.05). These data demonstrate that multiantibiotic-resistant E. coli isolates can be found in broiler chickens regardless of the antimicrobial growth promoters used. However, the phenotype and the distribution of resistance determinants in E. coli can be modulated by feed supplementation with some of the antimicrobial agents used in broiler chicken production.

FIG. 1.

Effect of age on resistance profiles of 197 E. coli isolates from broiler chickens. The percentage of resistance to most antibiotics decreased significantly (P < 0.001) from day 7 to day 35). Asterisks indicate the antibiotics against which the resistance percentages between treatments were statistically different (P < 0.001).

FIG. 2.

Effect of growth promoter diet agents on the frequency of antibiotic resistance levels in 197 E. coli isolates from broiler chickens. The percentage of resistance to most antibiotics decreased significantly (P < 0.001) from day 7 to day 35. Asterisks indicate the antibiotics against which the resistance percentages between treatments were statistically different (P < 0.001).

FIG. 3.

Prevalence of the bla (CMY-2, TEM, and SHV) genes in 84 tetracycline-ceftiofur-resistant E. coli isolates (14, 12, 14, 19, 13, and 12 for treatment control, bambermycin, penicillin, salinomycin, bacitracin, and salinomycin plus bacitracin, respectively). Detection of the bla_CMY-2 gene was performed by PCR as previously described (17).