Assessment of transduction of Escherichia coli Stx2-encoding phage in dairy process conditions

In the environment, bacteriophages are regarded as natural vector for the transmission of Shiga-toxin genes among Shiga-toxin Escherichia coli strains. The possibility of transduction has been noticed in intestinal tract of various animals but experimental observations on this phenomenon in food processes are lacking. To investigate the transduction in milk at different temperature profiles and cell concentrations, an experimental plan including two different Stx(2)-phages (ϕGV2412 and ϕL34), induced respectively from E. coli O157:H7 181181/2 and E. coli O157:H7 EC34, and two recipient E. coli strains (CNCTC 6896, WG5) was performed. The donor strains were generated by lysogenization of CNCTC 6896 with ϕGV2412 and ϕL34 respectively. Spectinomycin resistance gene (aadA) was inserted into stx(2) operon in order to select transduced cells. Transductants were never observed at 4°C up to 24 h, whereas after a treatment at 37°C for 2 h and at 25°C for 22 h they were detected in 67% of the trials with a ratio of transduction varying from 1.13 10(-6) to 7.87 10(-8). A treatment at 48°C for 2 h followed by a second step at 25°C for 22 h showed an occurrence of transduction events in only 19% of cases with a ratio of transduction varying from 2.22 10(-7) to 2.67 10(-8). The generation of transductants and the spontaneous induction of phages in milk were not affected by initial or final concentration of the donor or recipient strains. The results show that transduction phenomenon occurs when the cells are metabolically active and it does not take place at low temperatures. Therefore, the maintenance of the chilling chain proved to be a main factor to prevent the spread of Stx-genes in dairy processes.