Objective—To assess antimicrobial resistance and transfer of virulence genes facilitated by subtherapeutic concentrations of antimicrobials in swine intestines.
Animals—20 anesthetized pigs experimentally inoculated with donor and recipient bacteria.
Procedures—4 recipient pathogenic bacteria (Salmonella enterica serotype Typhimurium, Yersinia enterocolitica, Shigella flexneri, or Proteus mirabilis) were incubated with donor bacteria in the presence of subinhibitory concentrations of 1 of 16 antimicrobials in isolated ligated intestinal loops in swine. Donor Escherichia coli contained transferrable antimicrobial resistance or virulence genes. After coincubations, intestinal contents were removed and assessed for pathogens that acquired new antimicrobial resistance or virulence genes following exposure to the subtherapeutic concentrations of antimicrobials.
Results—3 antimicrobials (apramycin, lincomycin, and neomycin) enhanced transfer of an antimicrobial resistance plasmid from commensal E coli organisms to Yersinia and Proteus organisms, whereas 7 antimicrobials (florfenicol, hygromycin, penicillin G, roxarsone, sulfamethazine, tetracycline, and tylosin) exacerbated transfer of an integron (Salmonella genomic island 1) from Salmonella organisms to Yersinia organisms. Sulfamethazine induced the transfer of Salmonella pathogenicity island 1 from pathogenic to nonpathogenic Salmonella organisms. Six antimicrobials (bacitracin, carbadox, erythromycin, sulfathiazole, tiamulin, and virginiamycin) did not mediate any transfer events. Sulfamethazine was the only antimicrobial implicated in 2 types of transfer events.
Conclusions and Clinical Relevance—10 of 16 antimicrobials at subinhibitory or subtherapeutic concentrations augmented specific antimicrobial resistance or transfer of virulence genes into pathogenic bacteria in isolated intestinal loops in swine. Use of subtherapeutic antimicrobials in animal feed may be associated with unwanted collateral effects.
Objective—To determine whether an anti-Salmonella bacterium is involved in control of pathogen load in persistently infected cattle herds.
Animals—24 Holstein calves experimentally infected and 39 Holstein cows naturally infected with Salmonella spp.
Procedures—An Escherichia coli (designated as P8E5) that possessed anti-Salmonella activity was isolated from Salmonella-negative bovine feces obtained from a herd with endemic Salmonella infection. In vitro analysis involved enumerating Salmonella enterica serovar Typhimurium coincubated with E coli P8E5. In vivo analysis involved coadministration of Salmonella spp and E coli P8E5 or an E coli control strain to neonatal Holstein calves. Fecal samples were collected on multiple days after inoculation, and quantitative PCR assay was performed by use of Salmonella-specific primers.
Results—E coli P8E5 reduced viability of Salmonella spp in vitro. Shedding of Salmonella organisms was diminished in calves administered E coli P8E5, whereas the control strain of E coli had no effect on shedding of Salmonella organisms.
Conclusions and Clinical Relevance—In this study, an E coli strain was identified that possessed bacteriocin-like activity and was able to decrease viability of Salmonella organisms in vitro and in vivo. Therefore, it is possible that this organism could be representative of native microbiota that dampen Salmonella spp in persistently infected cattle herds.
Objective—To assess in pigs the pathogenicity and virulence of 3 strains of Salmonella spp capable of causing atypical salmonellosis in cattle.
Animals—36 Holstein calves and 72 pigs experimentally infected with Salmonella spp
Procedures—Representative Salmonella strains associated with 3 new disease phenotypes (protozoa-mediated hypervirulence, multisystemic cytopathicity, and encephalopathy) that have been characterized in cattle during the past 10 years were orally inoculated into pigs. Clinical manifestations were compared with those observed in cattle. Samples were collected from various tissues, and the presence of Salmonella organisms was assessed qualitatively and quantitatively by use of Salmonella-selective media
Results—Of the 3 unique Salmonella disease phenotypes observed in cattle, only protozoa-mediated hypervirulence was observed in pigs. Hypervirulence was related to a more rapid onset of disease and higher pathogen burden in pigs than in cattle. This phenotype was observed in pigs inoculated with multiresistant Salmonella enterica serotypes Typhimurium or Choleraesuis bearing the Salmonella genomic island 1 (SGI1) integron.
Conclusions and Clinical Relevance—Salmonella hypervirulence was identified in pigs noculated with SGI1-bearing strains exposed to free-living protozoa. Additionally, an SGI1-bearing strain of Salmonella Choleraesuis was detected that resulted in augmented virulence in pigs. Therefore, it appeared that protozoa-associated salmonellosis was analogous in pigs and cattle. Salmonella-mediated encephalopathy and multisystemic cytopathicity did not appear to be relevant diseases in pigs. (Am J Vet Res 2010;71:1170-1177)