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.

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Leafy vegetables, including lettuce and spinach, have been implicated in several outbreaks of foodborne disease caused by Escherichia coli O157:H7, a pathogen of increasing public health significance because of the severity of the gastrointestinal illness and long-term, chronic sequelae that can result from infection. A definitive association between the consumption of leafy vegetables and human disease provides implicit evidence of transfer from animal sources to field crops and retail commodities, including minimally processed or fresh-cut products. Understanding the behavior of E. coli O157:H7 in leafy vegetables during production, after harvest, in storage, during processing, and in packaged fresh-cut products is essential for the development of effective control measures. To this end, previous research on the fate of the species at each step in the production of market-ready leafy vegetables is reviewed in this study. Several critical gaps in knowledge are identified, notably uncertainty about the location of contaminating cells on or in plant tissues, behavior in packaged products stored at low temperatures, and the influence of environmental stresses on growth and infectivity.

Pure culture experiments were conducted to assess the bacteriostatic and bactericidal effects of phlorotannins (PT) isolated from Ascophyllum nodosum (brown seaweed) on Escherichia coli O157:H7. In Exp. 1, one non-O157:H7 strain (25922) and three strains of E. coli O157:H7 (3081, EDL933 and E318N) were cultured in M9 medium with PT included at 0 (control), 25, 50 or 100 μg/ml (n = 3). Bacterial growth was monitored by OD 600 at 0, 4, 6, 12 and 24 h, and by dilution plating at 0, 4, 6 and 24 h. All strains were inhibited (p<0.001) by PT to varying degrees. At 50 or 100 μg/ml, PT prevented growth of all four strains. At 25 μg PT/ml, growth of 25922, 3081, E318N and EDL933 was inhibited for 6, 12 and 24 h, respectively, but 25922 and 3081 resumed growth by 12 and 24 h. Direct plating confirmed bactericidal effects of PT on all four strains at 100 μg/ml, and on EDL933 and E318N at 50 μg/ml. In Exp. 2, strains 25922 and 3081 were incubated with no tannins or with 50 μg/ml of PT, purified condensed tannins (CT) from Quebracho (Schinopsis balansaei), or purified tannic acid from Rhus semialata (Anacardiaceae) as hydrolysable tannins (HT). Strain 3081 was unaffected by HT or CT, but was completely inhibited (p<0.001) by PT at 4, 6 and 24 h. Strain 25922 was unaffected by HT, slightly inhibited by CT, and almost eradicated by PT at 4 and 6 h. Transmission electron microscopy revealed tannin-mediated alterations to bacterial cell walls. Phlorotannins from A. nodosum exhibit growth-inhibiting and bactericidal effects in vitro against the strains of E. coli O157:H7 investigated. Anti-E. coli efficacy of A. nodosum PT is superior to that of terrestrial tannins purified from Quebracho and from Rhus semialata.

Antibiotic resistance patterns and the presence of antibiotic and virulence determinants in 74 sorbitol-negative Escherichia coli and 62 Salmonella isolates from nine different broiler chicken farms were investigated. Each farm was supplied by one of three companies that used different antimicrobial agents in feed for growth promotion. The isolates were identified by API 20E for E. coli and by serological tests for Salmonella. The susceptibility of the isolates to antibiotics was determined by Sensititre using the Clinical and Laboratory Standards Institute's breakpoints. Fifty-two E. coli isolates (70.3%) and nine Salmonella isolates (14.52%) were multiresistant to at least nine antibiotics. The multiresistant isolates were evaluated for the presence of tetracycline resistance, integron class 1, and blacMY 2 genes by PCR. Of the 74 E. coli isolates, 55 were resistant to amoxicillin and ceftiofur. Among these 55 resistant E. coli isolates, 45 (81.8%) and 22 (40.0%) were positive for blacMY-2 and qacEdeltal-Sull genes, respectively. Tetracycline resistance was found in 56 isolates (75.8%) among which 12 (21.4%) and 24 (42.9%) gave positive results for tetA and tetB, respectively. Virulence genes (iss, tsh, and traT), aerobactin operon (iucC), and the eaeA gene were detected in some E. coli strains. Among the 27 amoxicillin- and ceftiofur-resistant Salmonella isolates, the blacMY-2 gene was detected in 22 isolates. The class 1 integron gene (qacEdeltal-Sull) was not detected in any Salmonella isolates, whereas the invasin (inv) and virulence (spy) genes were found in 61 (98.4%) and 26 (42%) of the Salmonella isolates, respectively. This study indicated that multiple antibiotic-resistant commensal E. coli and Salmonella strains carrying virulence genes can be found on commercial broiler chicken farms and may provide a reservoir for these genes in chicken production facilities. Except for the presence of tetB, there was no significant effect of feed formulations on the phenotypic or genotypic characteristics of the isolates.