Diarrhea is frequently a primary or contributing cause of death of foster-age kittens. Among the causes of diarrhea, bacterial culprits are perhaps the most problematic to identify. Enterococcus species are generally considered to be normal inhabitants of the intestine, but include members that are significant pathogens such as E. faecium and E. faecalis. Pathogenic enterococci are generally capable of forming extensive colonies called biofilm that adhere to host tissue and are very difficult to eradicate. In studies funded by the Winn Feline Foundation, the researchers discovered that a common enterococcus of the feline intestine called E. hirae can form an incredible biofilm along the lining of the intestine, and those kittens with this biofilm have a survival advantage. Conversely, kittens with E. faecium or E. faecalis were more likely to have died or been euthanized. They also discovered that a significant number of kittens had died with an E. coli rather than E. hirae biofilm lining the intestine. These observations led to the hypothesis that E. hirae is a natural probiotic of the kitten intestine, and that it is able to form a biofilm that can protect against a significant cause of kitten death – attaching and effacing E. coli infection. Also, researchers believe that E. faecium and E. faecalis are poorly able to out-compete E. coli and may be directly injurious. Researchers will examine these mechanisms in laboratories using 476 feline Enterococcus spp. isolates obtained from the prior study. Findings may have an enormous impact on selection of probiotics that directly promote the survival of kittens.