In an animal research "first," disease-causing bacteria have been found to gain strength from interaction with single-celled organisms called protozoa that are naturally present inside animals. This finding suggests that the protozoa in animals' digestive tracts may be a place where dangerous bacteria can lurk and develop.

In studies at the Agricultural Research Service (ARS) National Animal Disease Center (NADC) in Ames, Iowa, veterinary medical officer Steven Carlson and microbiologist Mark Rasmussen discovered that an antibiotic-resistant strain of Salmonella becomes particularly virulent when tucked inside protozoa in the rumen, or first stomach, of cattle.

Until now, protozoa had never been thought of as reservoirs of disease in animals, as per Rasmussen.

The scientists set out to study the relationship between rumen protozoa and Salmonella's virulence and resistance to antibiotics. They focused on an S. enterica strain named DT104 that's a foodborne pathogen thought to bemore virulent than its antibiotic-sensitive counterparts.

In animals, salmonellosis is commonly a diarrheal disease that the animals recover from without requiring antimicrobial treatment. But antibiotics are needed when severe diarrhea or systemic infections occur. Unfortunately, a number of Salmonella strains have become resistant to a number of antibiotics, as per Carlson.

The strengthening of disease-causing bacteria as they occupy protozoa is a process that's been seen with free-living protozoa in places such as water-cooling towers and ponds, as per Rasmussen. This strengthening process was discovered when it was linked to an infamous and deadly 1976 outbreak of Legionnaires' disease. But the recent study marks the first time the process has been seen inside an animal, Rasmussen added.

Carlson and Rasmussen also found a way to combat DT104 by using a cleansing process, called "defaunation," which rids the rumen of protozoa.

Postdoctoral molecular biologist Zoe McCuddin and microbiologist Sharon Franklin assisted with this study. All of the scientists in this study work in NADC's Pre-Harvest Food Safety and Enteric Diseases Research Unit.