Research – Use of a phage cocktail to reduce the numbers of seven Escherichia coli strains belonging to different STEC serogroups applied to fresh produce and seeds

The aims of this research were to evaluate the effectiveness of a phage cocktail at reducing seven Shiga toxigenic Escherichia coli (STEC) serogroups on different food matrixes: mung bean sprouts (MBP), lettuce, and mung bean seeds (MBS) and to test the phage cocktail effectiveness to reduce E. coli O157 on Romaine and iceberg lettuce. To study the effect of the type of food matrix on the STEC phage cocktail effectiveness, a mixture of seven highly sensitive STEC strains designated as phage propagation strains (PPS) were used to adulterate Romaine lettuce, MBP, and MBS matrixes at a concentration of 10⁵ logs CFU/g. A subsample of the treated MBS was germinated to assess STEC survival. Recovered STEC strains were confirmed using latex agglutination and PCR. To test the phage cocktail effectiveness to reduce E. coli O157:H7 on Romaine and iceberg lettuce, a mixture of four STEC strains (different than phage propagation strains, non-PPS) at both low (10³ CFU/g) and high (10⁵ CFU/g) concentrations were used to spike the samples in scaled up trials for the purpose of potential commercialization. Phage treatments including a combination of STEC phage cocktail and chlorinated water treatment were then applied to lettuce in a simulated scaled-up trial. STEC was assessed on the treated samples at different storage time and temperatures (0, 24, 48, and 72 hr at 2, 10 and 25°C). In the food matrix trial, the combination of STEC phage cocktail and chlorinated water-reduced PPS (p < 0.001) STEC on lettuce by 2.1 log₁₀ CFU/g and on MBP by 2.2 log₁₀ CFU/g. However, isolates from all 7 STEC serogroups remained viable after phage treatment in both lettuce and MBP; particularly those associated with serogroup O111, O121, O103, and O145, while only a few colonies of serogroup O26, O45, and O157 were detected. Lettuce adulterated with low levels of 4 non-PPS E. coli O157:H7 (10³ CFU/g) achieved a reduction of 2.6–3.2 logs. While a reduction 1.7–2.3 logs was achieved by the phage cocktail when lettuce was inoculated with 10⁵ CFU/g. Overall phage performance was more effective at 2 and 10°C and improved over storage time up to 72 hr. However, for MBS, the phage cocktail was not able to kill any of the STEC populations as all of them recovered during germination.