The criterion for selecting effective phage for Escherichia coli O157:H7 control

Abstract

The bacteriophages SP15 and PP17, which showed strong virulency against Escherichia coli O157:H7, were isolated from activated sludge at a wastewater treatment plant and from pig feces, respectively. In liquid culture, the mixture of these phages delayed the appearance of phage-resistant E. coli O157:H7 cells longer than the addition of either phage alone. According to analysis of phage-resistant cells isolated from E. coli O157:H7 cultures infected with SP15 or PP17, it seemed that in the mixture, each phage attacked cells resistant to the other phage in complimentary manner. Phage-resistant cells existed in E. coli O157:H7 cultures at a certain ratio even before exposure to phage. This ratio, named R/ T ratio (CFU-resistant/CFU-total), was determined as (2.7±0.7)×10 −7 for SP15 and (3.6±0.8)×10 −5 for PP17. The R/ T ratio for the mixture of SP15 and PP17 was less than 2.0×10 −9, vastly reduced compared to administration of a single phage, thus quantitatively demonstrating the increased antibacterial potential of phage mixtures relative to single phages. For these reasons, the R/ T ratio represents a useful criterion in selecting effective phages for phage therapy.