Toward rational control of Escherichia coli O157:H7 by a phage cocktail.

Twenty six phages infected with Escherichia coli O157:H7 were screened from various sources. Among them, nine caused visible lysis of E. coli O157:H7 cells in LB liquid medium. However, prolonged incubation of E. coli cells and phage allowed the emergence of phage-resistant cells. The susceptibility of the phage-resistant cells to the nine phages was diverse. A rational procedure for selecting an effective cocktail of phage for controlling bacteria was investigated based on the mechanism of phage-resistant cell conversion. Deletion of OmpC from the E. coli cells facilitated the emergence of cells resistant to SP21 phage. After 8 h of incubation, SP21-resistant cells appeared. By contrast, alteration of the lipopolysaccharide (LPS) profile facilitated cell resistance to SP22 phage, which was observed following a 6-h incubation. When a cocktail of phages SP21 and SP22 was used to infect E. coli O157:H7 cells, 30 h was required for the emergence of cells (R-C) resistant to both phages. The R-C cells carried almost the same outer membrane and LPS components as the wild-type cells. However, the reduced binding ability of both phages to R-C cells suggested disturbance of phage adsorption to the R-C surface. Even though R-C cells resistant to both phages appeared, this work shows that rational selection of phages has the potential to at least delay the emergence of phage resistance.