A laboratory study and large-scale commercial wheat mill trial demonstrated that a bacteriophage cocktail can significantly reduce Shiga toxin-producing Escherichia coli (STEC) O157:H7 contamination throughout milling operations without affecting the baking qualities of flour.

Bacteriophages are bacterial viruses that exhibit high specificity toward their target bacteria, making them well-suited for foodborne pathogen control. They are also widespread in nature and can self-replicate only in the presence of a host. The use of phages for microbial control is well-established in poultry and meat processing; however, their application in wheat and grain milling operations has been largely unexplored.

Therefore, the study’s researchers sought to evaluate the efficacy of a commercially available phage cocktail for controlling STEC O157:H7 on wheat grains under laboratory conditions and flour in full-scale milling operations. A baking test using phage-treated flour was also conducted to determine the phage’s effect on baking qualities.

In the laboratory, the researchers found that the phage cocktail was effective, reducing STEC O157:H7 populations on wheat kernels by approximately 1.0 log colony forming units per gram (CFU/g) within 15 minutes at an application rate of 1 × 107 plaque-forming unit (PFU)/g.

Next, to determine whether the phage cocktail could be used as an effective pathogen control method in real-world applications, the researchers tempered wheat with three different phage concentrations in a large-scale milling operation. The researchers note that their findings likely underestimate the efficacy of the phage against STEC O157:H7, because the phage is highly specific and the researchers had to use a nonpathogenic E. coli strain as a surrogate for the trials. The phage cocktail is approximately 40 percent less effective against the surrogate strain than STEC O157:H7.

For inoculation in the mill trials, the researchers applied the phage cocktail with a 1.5–2 percent moisture addition, including moisture contributed by the inoculum in the laboratory studies. Results showed the limited amount of moisture was not a constraint for achieving significant reductions in E. coli.

Each of the three phage concentrations tested in the mill trials demonstrated significant reductions in E. coli populations throughout the milling process when compared to untreated control wheat. The efficacy of the phage cocktail was dose-dependent.

In several flour samples testing phage concentrations at 2.5 × 106 PFU/g and 1 × 106 PFU/g, E. coli concentrations fell below the enumerable limit. Therefore, all the flour samples were evaluated for the presence and absence of E. coli through an enrichment procedure, which showed that the phage-tempered flour had reduced percent positives by 15–29 percent compared to untreated flour.

Additionally, a reduction of ca. 1.7 logs was observed in mill feed following 1 × 107 phage application, suggesting that phage application was effective not only at the tempering step, but also during downstream operations. Mill feed is expected to contain most of the microbial contamination, so a significant reduction of E. coli on mill feed suggests that phage application is highly efficacious.

In contrast to other antimicrobial interventions in food processing, the baking tests showed that the phage cocktail did not affect flour functionality. It is also considered safe for its intended use, and does not harm workers or the environment.

To the best of the researchers’ knowledge, their study is the first to evaluate the efficacy of a phage cocktail for wheat grains in both a laboratory and a large-scale milling operation. The study can be read in the Journal of Food Protection.