| The use of antibiotics in agriculture remains controversial due to the development of antibiotic resistant bacteria. Most bacteria of agricultural importance develop antibiotic resistance through the acquisition of foreign DNA. Recent research has shown that, in many cases, the acquisition of multiple resistance genes arises through integrons, genetic elements that direct the site-specific recombination of foreign antibiotic resistance gene cassettes. The studies described herein examined integron-mediated antibiotic resistance in three bacterial populations associated with livestock and food safety. Approximately 10% of generic Escherichia coli (E. coli) isolated from swine contained integron gene sequences. A higher percentage of integron containing isolates were obtained from pigs on farms where antibiotics were used regularly (14.1%) versus farms where antibiotics were excluded (5.8%). All integron positive isolates contained gene cassettes involved in resistance to streptomycin (aadA) and, in some cases, the streptomycin resistance phenotype and integron genotype were transferable to other bacterial strains. Macro-restriction profiling of integron-positive isolates revealed several unrelated pulse-types. A similar examination of clinical Salmonella samples revealed a higher percentage (30.5%) of integron-positive isolates. Isolates containing integrons represented various serovars and were acquired from several different species of animal. In each case, the integron-positive isolates contained either an aadA (streptomycin resistance) or pse1 (ampicillin resistance) or both as integrated gene cassettes. Intregron-containing isolates were more frequently resistant to tetracyclines, chloramphenicol, kanamycin, streptomycin, sulfonamides and ampicillin (P < 0.05). A strain of Salmonella enterica var Meleagridis was found to contain the Salmonella Genomic Island 1 (SGI1), a cluster of genes responsible for the penta-drug resistant phenotype common in isolates of the Salmonella enterica var Typhimurium DT104 strain. A final study showed that a comparatively low percentage (7.3%) of E. coli belonging to common Shiga-toxin (Stx) producing serogroups carried integron gene sequences. Moreover, of the integron-containing isolates, none produced Stx. The E. coli isolates found to contain integrons each possessed gene cassettes involved in streptomycin resistance. Overall, the experiments demonstrated: (1) the ubiquity of integron gene sequences in several bacterial population associated with both livestock and food safety; (2) the prevalence of aadA genes as integrated gene cassettes; and (3) the dissemination of SGI1 into a rare Salmonella serovar. |
