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 especially 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, according to Rasmussen.

The researchers 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 believed to be more virulent than its antibiotic-sensitive counterparts.

In animals, salmonellosis is usually a diarrheal disease that the animals recover from without requiring antimicrobial therapy. But antibiotics are needed when severe diarrhea or systemic infections occur. Unfortunately, many Salmonella strains have become resistant to many antibiotics, according to Carlson.