Detection of Escherichia coli O157:H7 in bovine rumen fluid and feces by flow cytometry

This study was designed to evaluate the efficacy of a flow cytometric method to detect and quantify Escherichia coli O157:H7 in the rumen fluid and feces. The rumen fluid and feces were collected from two ruminally cannulated beef steers consuming a grass-hay diet. Samples of rumen fluid and diluted (1:10) feces were filtered through cheesecloth before inoculation with E. coli O157:H7 (ATCC 43888) at various concentrations (103–107 cells/ml). A fluorescein (FITC) conjugated anti-O157:H7 monoclonal antibody was used to label the pathogen. Negative controls were samples of rumen fluid and feces with or without a strain of non-O157:H7 E. coli (ATCC 25922). Labeled-E. coli O157:H7 were accurately enumerated at concentrations ⩾104 cells/ml while concentrations ⩽103 were only detected after enrichment. This flow cytometric method can quantify or detect E. coli O157:H7 in bovine rumen fluid and feces at concentrations commonly shed by cattle.