1525 Inhibiting the growth of Escherichia coli O157:H7 in alfalfa silage with silage additives

This study examined if adding microbial inoculants or propionic acid to alfalfa silages contaminated with Escherichia coli O157:H7 would inhibit the growth of the pathogen during or after ensiling. Alfalfa forage was harvested at the early bloom stage, wilted to a DM of 54%, chopped to 19-mm lengths, and ensiled after treatment with one of the following: 1) distilled water (Control), 2) 1 × 105 cfu/g of E. coli O157:H7 (EC), 3) EC and 1 × 106 cfu/g of Lactobacillus plantarum (EC+LP), 4) EC and 1 × 106 cfu/g of Lactobacillus buchneri (EC+LB), and 5) EC and 2.2 g/kg of propionic acid (EC+ACID). Each treatment was ensiled in quadruplicate in laboratory silos for 0, 3, 7, 16, and 100 d and analyzed for EC counts, pH, and organic acids. In addition, samples from d 100 were analyzed for counts of yeasts and molds and aerobic stability. Data were analyzed using the GLIMMIX procedure of SAS. The pathogen was detected in all silages until d 7, but by d 16, it was not detected in those treated with EC+LB and EC+LP, although it was still detected in EC and EC+ACID silages. However, by d 100, the pathogen was not detected in any silage. The rate of pH decrease to 5.0 was fastest for the EC+LP silage (7 d) followed by the EC+LB silage (16 d). Nevertheless, all silages had attained a pH less than 5.0 by d 100. The rapid decrease in pH in EC+LP and EC+LB silages was associated with higher (P < 0.05) lactate and acetate concentrations, respectively, relative to the other silages during the early fermentation phase (d 3 to 16). Propionic acid was detected only in the EC+ACID silage. Yeast counts were lowest (P < 0.05) in EC+LB and EC+ACID silages. Subsamples of all d -100 silages were reinoculated with 1 × 105 cfu/g of EC immediately after silo opening. When the pathogen was subsequently enumerated after 168 h of aerobic exposure, it was not detected in silages treated with EC+ACID, EC+LB, or EC+LP, which all had pH values less than 5.0, whereas the EC silage had a pH value of 5.4 and 2.3 log cfu/g of the pathogen. Certain bacterial inoculants can hasten the inhibition of E. coli O157:H7 during ensiling, and like propionic acid, they can also prevent its growth on silage contaminated with the pathogen after ensiling.