Commercial bacteriophage preparations for the control of Listeria monocytogenes and Shiga toxin-producing Escherichia coli in raw and pasteurized milk

Listeria monocytogenes was the etiologic agent in nearly all recent outbreaks in North America attributed to pasteurized dairy products, whereas Escherichia coli O157 infections were responsible for most of the rare, yet serious complications from outbreaks involving unpasteurized dairy. This study determined the susceptibility of selected strains of L. monocytogenes and Shiga toxin-producing E. coli (STEC) to commercial phage preparations and their ability to control these pathogens in pasteurized and raw milk during 7-day storage at 7 °C. Both phage products demonstrated high lytic efficiency against 17 strains of L. monocytogenes whereas the efficiency of E. coli phages was more variable against 11 strains of O157 and non-O157 STEC. Broth microdilution assays identified effective endpoint multiplicities of infection (MOI) ranging from log 2.53 to 5.13, which differed between strains of L. monocytogenes and phage products. Mean log MOIs of E. coli phages against STEC also varied within and between products from 0.43 to 7.05. Despite these observations, the change in counts over time of three L. monocytogenes strains exposed to phage in pasteurized milk (log MOI 6) was similar with counts ∼ 4 log CFU/mL lower than control at day 7. Results for STEC O157 varied by strain but counts were lower than control in all cases by 72 h thorough day 7. Titers on isolates of both pathogens isolated from pasteurized milk indicated that the surviving populations were less susceptible to phage. The addition of a phage preparation to raw milk did not reduce populations of either pathogen or affect the change in counts of any strain over time when compared to control. Reduced efficacy in raw milk may be attributed to reduced phage binding as titers in raw milk decreased steadily (∼2 log PFU/mL) during storage. Commercial phage products may be a promising pathogen control intervention for pasteurized dairy products, warranting further investigation.