Date Full Report Received09/01/2017
Date Abstract Report Received09/01/2017
Antibiotic resistance is a major health concern in both human and veterinary medicine. Antibiotics already used to maintain healthy swine may also play a role in reducing Salmonella; however, the relationship between antibiotics and antibiotic resistant bacterial populations needs to be further understood. The overall objective of this study was to investigate the effects of antibiotics on the intestinal and extra-intestinal bacterial populations of swine challenged – both orally and transdermally – with pan-susceptible and multidrug-resistant (MDR) Salmonella. A Salmonella challenge experiment was conducted on 32 pigs conducted in two replicates. In each replicate four pigs were assigned to each of the four treatment groups; chlortetracycline treatment, ceftiofur treatment, ceftiofur and chlortetracycline treatment, and an untreated control group. Pigs were challenged on days 1 and 3 with a 104 culture of pan-susceptible and MDR Salmonella enterica serovar Senftenberg transdermally and 104 culture of pan-susceptible and MDR Salmonella enterica serovar Derby orally. A single dose of ceftiofur was given on day 5 and chlortetracycline was given in feed from day 5 to day 18 for the maximum treatment period of 14 days. Intra-rectal fecal samples were taken daily throughout the study and on day 19 the pigs were euthanized and tonsils, mandibular lymph nodes, peripheral lymph nodes (superficial front- and hind-limbs), intestines, and gut-associated lymph nodes were collected. Samples were cultured for Salmonella using standard enrichment and selective techniques. Salmonella isolates were confirmed and further characterization included serogrouping and PCR of antimicrobial resistance genes specific to the challenge strains. 16S rRNA metagenomics analyses of fecal samples from days 1, 5, 12, and 19 were conducted to observe differences in the bacterial communities. The overall fecal prevalence of Salmonella between treatment groups varied significantly for non-enriched samples (p<0.05) but did not vary significantly for enriched samples (p=0.61). The overall Salmonella prevalence in lymph nodes was 8.1% for non-enriched samples and 20.3% for enriched samples and prevalence in lymph nodes using enrichment methods did not vary significantly (p=0.14) between treatment groups. All 658 fecal isolates were serogroup B which is indicative of Salmonella Derby. We found 98 serogroup B lymph node isolates and 13 serogroup E4 (indicative of Salmonella Senftenberg) lymph node isolates. The majority of the antimicrobial resistance gene PCR results matched well with the expected serogroup and we found that 49.7% of the fecal isolates harbored qnrB. Among the lymph node serogroup B isolates, 69.4% harbored qnrB and 5 of the 14 serogroup E4 isolates harbored blaSHV. The greatest difference in microbial communities was found between the control and the ceftiofur/clortetracycline treatment groups and between days 5 and 19, though the differences were not significant. Overall we found that Salmonella infection can quickly and efficiently create persistent enteric infections with constant or intermittent shedding in a controlled environment, and that the intradermal route does not appears to be an efficient route of infection for this bacteria. However, even in this scenario, Salmonella was recovered from peripheral lymph nodes after 14 days of antibiotic treatment. We found that antibiotic treatment reduced the number of Salmonella in feces below detection limits but did not eliminate Salmonella shedding. Further analyses are needed to investigate differences in the isolates recovered between treatment groups to explore differences in the pan-susceptible and multi-drug resistant Salmonella populations.