Date Full Report Received


Date Abstract Report Received



Primary Investigator:

Multidrug-resistant (MDR) Salmonella are isolates that are resistant to three or more antibiotics. MDR isolates can lead to greater levels of illness in both humans and animals compared to antibiotic-sensitive isolates; it is currently unknown why this occurs, but some of our previous work indicates that certain antibiotics may enhance Salmonella virulence. Therefore, the current project was performed to assess the potential impact on swine health and food safety in pigs infected with MDR Salmonella that are treated with antibiotics. We did this by determining if chlortetracycline (CTC) treatment influences 1) the amount of MDR Salmonella found in the feces, 2) the amount of Salmonella found in various internal tissues, and 3) changes in the gut bacteria and porcine immune response over time. The two treatment scenarios included 1) treating pigs with CTC before Salmonella exposure (this represents pigs that could have an altered gut microbiota due to antibiotic treatment that are subsequently exposed to Salmonella) and 2) inoculating pigs with Salmonella then treating with CTC (this represents antibiotic treatment to pigs that are unknowingly infected with MDR Salmonella).
Piglets were randomly separated into different groups post-weaning. At six weeks of age, two groups were given chlortetracycline CTC in feed (400 g/ton) continuously for 12 days. At day 5 of treatment, one CTC group and one untreated group were inoculated with MDR S. Typhimurium DT104 isolate 530. Additionally, a group was inoculated with the same isolate, but CTC was administered (400 g/ton) in feed 2 days following infection, rather than prior. Fecal and blood samples were collected throughout the experiment. All pigs were humanely euthanized and necropsied 7 days after Salmonella inoculation (12 days after initiating CTC in the pre-treatment swine), and the following tissues were collected: cecal contents, cecal mucosa, ileocecal lymph nodes, ileal mucosa, and tonsils.
CTC treatment following Salmonella infection did not alter Salmonella shedding levels. A significant increase in colonization of the ileocecal lymph nodes in the pigs treated with CTC after Salmonella inoculation compared to the pigs that did receive CTC after inoculation with Salmonella was observed. On the other hand, CTC treatment before MDR Salmonella inoculation resulted in more Salmonella found in the feces at 2 days post inoculation compared to pigs that were not treated with CTC prior to Salmonella exposure. Also, CTC-treated pigs had a 1,000 fold increase in Salmonella colonization of swine tonsils. Furthermore, CTC treatment had a minor effect on altering the gut bacteria. As CTC has been associated with prolonged MDR Salmonella shedding in swine, the tonsils may be a site of antibiotic-enhanced and prolonged colonization. This information is useful to the swine industry because it shows that CTC treatment can affect Salmonella colonization and shedding in pigs.
Contact information: shawn.bearson@ars.usda.gov