Abstract
Escherichia coli O157 strains starved in sterile deionized water (SDW) and filter-sterilized natural river water (SRW) were investigated with specific reference to their culturability in selective and non-selective media. Growth of the strains starved in both SDW and SRW were markedly suppressed with time in selective liquid media such as modified trypticase soy broth supplemented with novobiocin (mTSB+n) and modified E. coli broth supplemented with novobiocin (mEC+n). This suppression was more pronounced when incubated at 42C than at 37C, especially with mEC+n. By contrast, such growth suppression was seldom observed when cultured at 37C in non-selective liquid media such as trypticase soy broth (TSB) and buffered peptone water. In mEC+n at 42C, the non-starved cells from overnight cultures with an initial density of less than 103 colony-forming units (CFU)/ml grew to the density of over 107CFU/ml after 24hr incubation, whereas those starved for 6 weeks in SRW were only to maintain their initial density or died off after 24hr incubation under the same culturing conditions. These results indicated that the isolation of starved cells of E. coli O157 from water samples would be most difficult with selective enrichment or direct plating on the selective plate media. It is thus highly recommended that a“resuscitation”of the cells with non-selective enrichment should be performed as a routine practice for maximum recovery of E. coli O157 from water systems.
