Control of Escherichia coli O157:H7 in corn silage with or without various inoculants: Efficacy and mode of action

This study aimed to evaluate the effectiveness of 3 commercial bacterial inoculants at controlling Escherichia coli O157:H7 in corn silages during ensiling and feedout phases of silage production. A second objective was to determine whether the inoculants exhibited and transferred antibacterial activity against E. coli O157:H7 to the silages. Chopped corn forage was ensiled after treatment with the following: distilled water (control); 5 × 105 cfu/g of E. coli O157:H7 (EC); EC and 1 × 106 cfu/g of Pediococcus pentosaceus and Propionibacterium freudenreichii (EC+BII); EC and 1 × 106 cfu/g of Lactobacillus buchneri (EC+LB); and EC and 1 × 106 cfu/g of L. buchneri and P. pentosaceus (EC+B500). Each treatment was ensiled in triplicate in mini silos for 3, 7, 31, and 82 d and analyzed for pH and E. coli O157:H7 counts. Samples from d 82 were also analyzed for volatile fatty acids, lactate, and aerobic stability. Antibacterial activity of inoculants and silages was determined by the Kirby-Bauer disc diffusion test. The pH of silages from all treatments decreased below 4 within 3 d of ensiling and remained low until d 82. Therefore, E. coli O157:H7 was not detected in silages after any of the ensiling durations. Applying inoculants containing L. buchneri resulted in less lactate, more acetate, and greater aerobic stability compared with the control. Applying EC+BII containing P. freudenreichii did not increase propionate or aerobic stability. Subsamples of d 82 silages were reinoculated with 1 × 105 cfu/g of E. coli O157:H7 either immediately after silo opening on d 82 or after 144 h of aerobic exposure (d 88), and E. coli were enumerated 24 h later. All silages reinoculated with the pathogen on d 82 had similar, low pH values (<4) and no E. coli were detected 24 h later. Control, EC, and EC+BII silages reinoculated with the pathogen after 144 h of aerobic exposure had relatively greater pH values (4.71, 5.67, and 6.03, respectively) and E. coli counts (2.87, 6.73, and 6.87 log cfu/g, respectively) 24 h later, whereas those treated with L. buchneri had low pH values (<4) and undetectable (EC+B500) or 10-fold lower (1.97, cfu/g; EC+LB) E. coli counts. All pure cultures of commercial bacterial inoculants exhibited antibacterial activity independent of pH against E. coli O157:H7, but the pH-independent activity did not persist in the treated silages, suggesting that E. coli elimination from silages was mediated by pH reduction.