Submitted to: Journal of Microbiological Methods
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2006
Publication Date: 9/29/2006
Citation: McLaughlin, M.R. 2006. Factors affecting iron sulfide-enhanced bacteriophage plaque assays in Salmonella. Journal of Microbiological Methods. 67:611-615. Interpretive Summary: Special media used in detection and identification of Salmonella has been used to improve tests for Salmonella-killing viruses, called bacteriophages or phages. Additives in the media detect hydrogen sulfide, a gas produced by most strains of Salmonella. The result is production of a characteristic black color reaction in the media. Michael McLaughlin, an ARS research scientist in the Waste Management and Forage Research Unit at Mississippi State, MS developed procedures for measuring these reactions using very small volumes of test media. These microplate tests were used to characterize the effects of several variables on color development in the Salmonella media. The microplate tests showed that several factors influence the black color formation, including the type of Salmonella, nutrients, nutrient levels and nutrient placement in the media, temperature, exposure to air, and the ratio of media components. Optimization of these variables will allow scientists to improve diagnostic tests for Salmonella and aid in the search for Salmonella-killing phages.
Technical Abstract: Formation of iron sulfide from reaction of ferric ions in bacteriological media with hydrogen sulfide (H2S) produced by bacteria has been exploited to enhance contrast of bacteriophage plaques in sulfate-reducing strains of Salmonella. Ferric ammonium citrate (FAC) and sodium thiosulfate (ST) added to the top agar of agar layer plaque assays allows formation of black bacterial lawns in H2S+ strains. Contrast in FACST-amended agar, however, was diminished after 12 h in assays with S. enterica subsp. enterica serovar Enteritidis (ATCC 13076). The current study examined some of the variables involved in H2S production and detection with Salmonella. Black precipitate formation in the media was found to be influenced by amount and concentration of nutrients (peptones and glucose), FACST concentration and FAC:ST ratio, temperature, pH, exposure to air, and host strain.