Effects of a Saccharomyces cerevisiae feed supplement on Escherichia coli O157:H7 in ruminal fluid in vitro

A feed supplement, SCB, containing Saccharomyces cerevisiae subsp. boulardii, produced substances which were successful in inhibiting strains of E. coli O157:H7 in a spot agar assay. Growth and/or survival of E. coli O157:H7 in ruminal fluid in the presence of SCB was investigated in a batch culture incubation and in continuous culture using a Rumen Simulation Technique (Rusitec). In the batch culture incubation, E. coli O157:H7 (EC) was inoculated (104 CFU/ml final concentration) into 100 ml of clarified ruminal fluid amended with SCB at 0 (control), 5 (SCB5), 10 (SCB10), 15 (SCB15) or 20 (SCB20) mg/ml (n=3). Treatments were incubated at 39 °C, and EC was enumerated after 0, 12, 24 and 48 h by plating onto selective medium. Treatments SCB15 and SCB20 completely eradicated 104 CFU EC/ml within 48 h (P<0.05). There was no correlation between pH and the survival of E. coli O157:H7 among treatments (P>0.05). In the Rusitec incubation, fermentations were established in 12 Rusitec vessels using ruminal inoculum confirmed negative for E. coli O157. Each vessel was inoculated with EC (104 CFU/ml), then 8 h later, with stock solutions of SCB yielding initial concentrations of 0 (control), 5 (SCB5) or 20 (SCB20) mg/ml (n=4). Enumerations of EC were conducted 0, 4, 8, 12 and 24 h after EC inoculation, and daily thereafter for 6 days. Counts of EC did not differ (P>0.05) from the control with either level of SCB, at any of the time points assessed. The inhibitory effect of SCB on E. coli O157:H7 observed in batch culture at 48 h was not observed in continuous culture (Rusitec). Lack of an effect of SCB on E. coli O157:H7 in the Rusitec suggests that the production of inhibitory substances by S. cerevisiae may not occur in the mixed microflora of the rumen. Also, diffusion of inhibitory substances in the rumen or degradation of inhibitory substance by rumen microorganisms may have reduced their effectiveness against E. coli O157:H7.