Every year, hundreds of thousands of orphaned and abandoned kittens are fostered by U.S. animal shelters. Tiny and vulnerable to infectious disease, ~15% of these kittens will die from diarrhea before they reach an age of 8 weeks. Because of the large number of different pathogens that can cause diarrhea, there is a growing interest in the role of the normal intestinal bacteria in providing defense against these challengers. We have previously shown that healthy foster kittens harbor a specific intestinal bacteria, called Enterococcus hiraethat forms a living shield against invading pathogens. Sick foster kittens have lower numbers of E. hirae and greater intestinal colonization by pathogenic bacteria. The objective of this study is to determine if a probiotic formulation of E. hirae can protect foster kittens from acquiring intestinal infections, developing diarrhea, and death. This objective will be accomplished by partnering with a large, county animal shelter to conduct a placebo-controlled treatment trial of an E. hirae probiotic in their foster kitten population. We expect to demonstrate that E. hirae will promote a healthier intestinal microbiota, decrease the infectious disease burden carried by this shelter population, and significantly decrease the number of kittens that develop and succumb to diarrhea. These results will have an immediate positive impact on survival of kittens at an individual and population level, decrease the burden of infectious disease at a shelter level, and will do this using a member of the kittens’ own intestinal microbiota.