Differential Induction of Shiga Toxin 2-encoding Bacteriophages in Shiga-toxin Producing Escherichia coli

Introduction: Shiga toxin-producing Escherichia coli (STEC) is an important foodborne pathogen that causes severe intestinal and systemic diseases. The main virulence factor is the Shiga toxin, whose genes are encoded in the genome of temperate phages (Stx phages). Stx phages activate their lytic cycle by various induction mechanisms. This increases the Stx expression and as a consequence the severity of the STEC infections. The process concludes with the lysis of the bacterial cell. The critical question is what is the advantage for STEC to harbor a Stx phage, if this could cause its final lysis and destruction? We speculate that differential rates of induction should happen within a single STEC population.

Purpose: To evaluate if there is a differential induction of Stx phages from a population of E. coli lysogenic for Stx phages.

Methods: A recombinant phage harboring a Green fluorescent protein inserted in the stx was constructed. Activation of phage lytic cycle results in an increase of fluorescence in the cells after activation and before its lysis. We evaluated this fluorescence through epifluorescence microscopy and flow cytometry.

Results: When inducing the lytic cycle of Stx phages in a STEC population, epifluorescence microscopy showed that not all the cells were fluorescent. Flow cytometry showed a fraction of the population that induced after 4 hours. This fraction increased after 6 and 24 hours. However, there was always a part of the STEC population that remained uninduced and never lysed.

Significance: A fraction of the STEC population lysed after Stx phage induction, causing higher Stx expression, thus higher virulence of the infection. This is the advantage conferred by the Stx phage. However, a small fraction of the population remained uninduced, and this will guarantee the continuity of the population.