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

Abstract Text: 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 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, though 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 only detected 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 re-inoculated 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.

Keywords: alfalfa, Escherichia coli O157:H7, microbial inoculants