Identifying and Characterizing Bacteriophages Capable of Inactivating Salmonella Isolated from Environmental Samples as a Pre-Harvest Antimicrobial Intervention

Abstract

Bacteriophages previously found in the feedlot environment may play a role in the ecology of Salmonella in the feedlot environment and also prove useful as a means of controlling this pathogen in beef. The ability of a phage to infect and lyse the target bacterial strain is generally agreed to be a basic requirement for successful phage therapy. The objective of this work was to 1) determine phage host range and virulence in a microtiter-plate based liquid assay using the features of an automated plate reader to monitor culture optical density over time in an incubating, aerated environment; 2) perform phage characterizations, including morphological identification, growth kinetics, genomic analysis and 3) conduct antimicrobial efficacy testing of phages in ex-vivo models and study relationship between phage efficiency in bacterial reduction and phage characterization. Host range scores obtained by two methods were compared to each other and to results from a study using phages to decontaminate cattle hides inoculated with S. Anatum, in order to determine the ability of the two host range methods to predict antimicrobial efficacy of phages in an ex vivo model. The host ranges of the tested phages were highly variable, ranging from infecting 10% to 85% of the tested Salmonella strains. Phage Melville was found to have the broadest host range, capable of infecting 85% (17/20) in both methods. Results obtained by the microtiter plate liquid method were found to have higher discriminatory power between bacterial strains. The ability of phages to reduce Salmonella loads on cattle hides were correlated with the results obtained by the microtiter plate method developed in this study but not with the traditional agar overlay method, implying that the microtiter method is a more sensitive predictor of antimicrobial capacity of phages compared to traditional agar overlay methods. The microtiter plate liquid assay could potentially serve as a more advanced alternative of characterizing phages that yields data on both host range and virulence. Bacteriophage capable of significantly reduce Salmonella population in cattle hide and soil proved the possible function as an intervention against Salmonella to prevent pathogen transmission in feedlot environment and colonization of cattle lymph nodes.