Use of bacteriophages as biocontrol agents to control Salmonella associated with seed sprouts

Two Salmonella bacteriophages (SSP5 and SSP6) were isolated and characterized based on their morphology and host range, and evaluated for their potential to control Salmonella Oranienburg in vitro and on experimentally contaminated alfalfa seeds. Phages SSP5 and SSP6 were classified as members of the Myoviridae and Siphoviridae families, respectively. Both phages had a broad host range of over 65% of the 41 Salmonella strains tested. During in vitro trials, the phages resulted in incomplete lysis of Salmonella cultures, in spite of high levels of phage remaining in the system. Phage SSP5 was more effective in reducing Salmonella populations. Addition of phage SSP6 to alfalfa seeds previously contaminated with S. Oranienburg caused an approximately 1 log(10) CFU g(-1) reduction of viable Salmonella, which was achieved 3 h after phage application. Thereafter the phage had no inhibitory effect on Salmonella population growth. A second addition of the same (SSP6) or different (SSP5) phage to a Salmonella culture treated with phage SSP6, did not affect Salmonella populations. It was further shown that development of Salmonella permanently resistant to phage was not evident in either seed or in vitro challenge trials, suggesting the existence of a temporary, acquired, non-specific phage resistance phenomenon. These factors may complicate the use of phages for biocontrol.