Evaluation of lactic acid bacteria for a direct-fed-microbial against shigatoxigenic Escherichia coli in food animals

Abstract

Shigatoxigenic Escherichia coli (STEC) have caused numerous foodborne outbreaks and recalls, with meat products and fresh produce as implicated vehicles. Food animals, especially ruminants, are well-known reservoirs. Appropriate measures to reduce this group of pathogens at the farm-level are warranted. Strains of lactic acid bacteria (LAB) may be utilized as direct-fed-microbials (DFM) in food animals, however, they must be carefully selected to maximize their inhibitory activity.

Purpose: To evaluate LAB strains for acid and bile tolerance and inhibition towards STEC for potential DFM.

Methods: Viability of several (n=205) strains of lactic acid bacteria (Genus: Lactobacillus, Lactococcus, Leuconostoc, or Streptococcus) were evaluated for inhibition against STEC, using agar-spot-test. Cocktails (5x107 CFU/ml) of 4 STEC strains per serotype (O157, O111, O103, O26, O121, O45, O145) were used. LAB that inhibited STEC significantly (inhibition zones >10.0 mm) were further tested for acid and bile tolerance. Each strain (1x108 CFU/ml) was added to MRS broth adjusted to pH (2, 4, 5) and bile (0.1, 0.3, 0.5%), along with controls, and incubated for 0, 1, 3, and 6 h at 37'C. Bacterial growth was determined by measuring absorbance at 620 nm and 660 nm for acid and bile tolerance, respectively.

Results: Fifty of the revived strains were further tested for inhibition against STEC serotypes. Among these, 15% showed excellent (>15 mm), 32% very-good (>10 mm), and 29% good (>5 mm) inhibition against STEC. Isolates showing >10 mm inhibition were further screened for acid and bile tolerance. Compared to 0 h (A620=0.222-0.736), all isolates showed stable growth up to 6 h (A620=0.243-0.986) at all pH values. For bile tolerance, 80% isolates at 0.1%, 40% at 0.3% and 30% at 0.5% showed increased growth over 6 h, indicating that they were better able to tolerate 0.1% compared to the 0.3 and 0.5% bile concentrations.

Significance: Lactic acid bacteria showed potential as DFM due to good survival capabilities for the GI-tract environment and significant inhibition against STEC.