Preliminary investigation on two Stx2-carryng bacteriophaghes induced from STEC strains isolated from goat milk

The consumption of unpasteurized milk and raw milk cheeses is one of the factors involved in the transmission of Shiga-like Toxin-producing Escherichia coli (STEC) infection to humans. Bacteriophages are regarded as natural vector for the transmission of virulence genes in E. coli, leading to virulent strains. Phages carryng stx toxins genes (Stx-bacteriophages) have a significant role in the horizontal transfer of virulence factors through generalized transduction phenomena. Moreover, phages persist in the environment more than their host bacterial cells and they prove to be more resistant to sanitation operations in food equipment. Stx-bacteriophages studied up-to-date belong to Caudovirales order with long or short tail and double-strand DNA, and their common trait is the Stx-operon coding for the toxins.The milking-filters were more frequently found positive than the milk samples of the same farm for the presence of STEC; this is probably due to the accumulation of cells due to the sieving of the fluid. Only one strain has both the genes for cytotoxins, seven strains possess the Stx1 gene and eight the Stx2 gene. Among these last ones, five strains have also the eae gene (20%), whereas none of those with Stx1 gene showed this virulence marker. Most strains (80%) revealed the presence of the hlyA gene.After the induction of the nine strains carrying Stx2 gene, the potential presence of phages was discovered in 78% of the lysates: an halo of clarification was formed in spot test, when in contact with the cultures of both indicator strains. However, clear plaques of lysis were evident on E. coli WG5-RifR cells only for lysates from F1-1 and F80-2 strains with concentration of 3.1 x 104 UFP/ml and 1.5 x 103 UFP/ml, respectively. After isolation and purification two phages were examined with TEM: ΦF1-1 was ascribed to Podoviridae family, with a icosahedral head of 51 nm and a spike 14 nm long (Figure 1); ΦF80-2 was ascribed to Siphoviridae family, with isometric head of 50 nm in diameter and a flexible tail of approximately 150 nm.