PROJECT SUMMARY Bacteriophages (phages) are viruses that specifically infect bacteria. Phages have been shown to play important roles in the life cycle of many bacterial pathogens, particularly water-borne pathogens that cause gastrointestinal tract infections. For example, the lysogenic phages, which integrate their DNA into the host genome, can play positive roles by providing virulence factors such as toxins that enhance the virulence of their host bacterium. Other phages known as lytic or virulent phages can play negative roles by infecting and killing their host bacterium, thus reducing the number of the pathogen below the threshold needed to cause disease. Vibrio cholerae is a water-borne pathogen that infects the small intestine to cause the severe diarrheal disease cholera. Although it is well known that a lysogenic phage called the CTX phage enhances virulence by providing the genes encoding cholera toxin, our recently published and preliminary data indicate a pervasive negative role for several virulent phages that are commonly found in cholera endemic areas around the Bay of Bengal. These virulent phages appear to reduce the virulence, transmission, and potentially dissemination of V. cholerae. However, there is much we still do not know about these phages in terms of their ability to kill V. cholerae in the environment, to inhibit fecal-oral transmission of cholera such as occurs frequently within households, and their impact on the emergence of new V. cholerae strains. In this project, we will investigate each of these areas. In addition, we will study the importance of another lysogenic phage called Kappa in the virulence of V. cholerae.