The Source of Salmonella

Salmonella is a common bacterium in animals, and it can make people sick. About 42,000 cases of Salmonella infection are reported in the United States each year, but the actual number may be much greater, according to the U.S. Centers for Disease Control and Prevention. Salmonella can be transmitted in meat and eggs, but the bacteria are neutralized by cooking foods to established food-safety temperatures.

Agricultural Research Service scientists have been taking a close look at the prevalence of Salmonella in feedlot (non-grazing) cattle. Studies coauthored by ARS scientists in Texas and Nebraska have indicated that interventions to reduce Salmonella contamination deep in the cow’s tissues are important—particularly since beef cattle don’t get clinically sick from Salmonella.

When feedlot cows get a low-grade Salmonella infection, their immune system escorts the bacteria to peripheral lymph nodes (PLN) located deep within muscle and fat tissue.

At the ARS Food and Feed Safety Research Unit in College Station, Texas, microbiologist Thomas Edrington and ARS colleagues in Kerrville, Texas, have been investigating the route of entry used by Salmonella when infecting cows. “We are looking into how Salmonella are initially transmitted to feedlot cattle so that we can better assess how Salmonella reaches the PLN,” says Edrington. “This will allow us to design effective preharvest interventions to control the pathogen.” The ARS team collaborated with researchers from Texas Tech University in Lubbock, Texas, and the Roslin Institute at the University of Edinburgh, Scotland.

Their studies indicate that Salmonella reach cows’ PLN via two routes: oral ingestion and insect bites.

For the team’s proof-of-principle study on transmission via insect bites, they used a skin-allergy instrument that contained specific serotypes, or groups, of Salmonella to inoculate various regions of the cow’s skin.

“We were able to recover Salmonella from PLN near regions of the skin-challenge test for up to 8 days after exposure,” says Edrington. In addition, specific serotypes—applied to chosen regions of skin—were only recovered from the PLN draining those chosen inoculation sites. The model is helping the team determine how long cows are infected, and it will allow evaluation of interventions that may shorten the duration of infection.

The team also conducted a series of experiments to investigate whether Salmonella ingested via drinking water would spread from the cow’s gastrointestinal tract to the PLN. Results showed that cows whose drinking water had been inoculated with Salmonella for 14 days had detectable levels of the challenge strain of Salmonella in their PLN.

These ARS modeling studies will be used to evaluate the effectiveness of vaccines in preventing Salmonella spread to PLN after oral and skin exposure. “One potential Salmonella vaccine has already been tested, and while it was not completely effective, it showed merit in reducing the prevalence of Salmonella-infected PLN,” says Edrington. “We are gaining food-safety knowledge that will help with developing approaches to prevent Salmonella in PLN of cattle.”—By Rosalie Marion Bliss, Agricultural Research Service Information Staff.

“The Source of Salmonella” was published in the April 2015 issue of AgResearch Magazine.