Implications of free Shiga toxin-converting bacteriophages occurring outside bacteria for the evolution and the detection of Shiga toxin-producing Escherichia coli

Abstract

In this review we highlight recent work that has increased our understanding of the distribution of Shiga toxin-converting phages that can be detected as free phage particles, independently of Shiga toxin-producing bacteria (STEC). Stx phages are a quite diverse group of temperate phages that can be found in their prophage state inserted within the STEC chromosome, but can also be found as phages released from the cell after activation of their lytic cycle. They have been detected in extraintestinal environments such as water polluted with feces from humans or animals, food samples or even in stool samples of healthy individuals. The high persistence of phages to several inactivation conditions makes them suitable candidates for the successful mobilization of stx genes, possibly resulting in the genes reaching a new bacterial genomic background by means of transduction, where ultimately they may be expressed, leading to Stx production. Besides the obvious fact that Stx phages circulating between bacteria can be, and probably are, involved in the emergence of new STEC strains, we review here other possible ways in which free Stx phages could interfere with the detection of STEC in a given sample by current laboratory methods and how to avoid such interference.

The effect of antimicrobials on verocytotoxin bacteriophage transduction under bovine rumen fluid and broth conditions

“…This finding has been previously reported (Livny and Friedman, 2004;Iversen et al, 2015). Such a mechanism would ensure phage persistence even if bacterial inactivation processes were rapid and the VTEC hosts did not survive for sufficient time to allow phage release before being killed (Martinez-Castillo and Muniesa, 2014). Interestingly, cell-free phage populations peaked after 4-6 h in SIC-treated samples, at 2.0-2.6 log 10 pfu/mL, regardless of the medium.…”

“…Indeed, the low overall production and isolation of transductants may be attributed to the combination of donor and recipient cell concentrations being lower than the threshold value required for their generation, due to the decrease in bacterial cell numbers observed in the presence of the different antibiotic treatments (Imamovic et al, 2009). Moreover, the free phages observed in our study may have had reduced capacity to infect bacterial cells (Yue et al, 2012;Martinez-Castillo and Muniesa, 2014). The higher transduction rates in LB broth may be explained by the impact of the matrix on this process.…”

An Environmental Shiga Toxin-Producing Escherichia coli O145 Clonal Population Exhibits High-Level Phenotypic Variation That Includes Virulence Traits

“…Infective Shiga toxin-encoding phages have previously been detected in various ecosystems and in fecal samples from animals and humans (54)(55)(56)(57)(58)(59), and transfer of stx genes has been demonstrated in water and food samples (60). These phages persisted better in nonhost environments than in their bacterial hosts and retained their infectious activities in harsh environments (61,62), implying the potential of stx acquisition by phage transduction in natural environments (63,64).…”

Improving Detection of Shiga Toxin-Producing Escherichia coli by Molecular Methods by Reducing the Interference of Free Shiga Toxin-Encoding Bacteriophages

“…An important volume of work reports positive detection of stx and therefore STEC by PCR or qPCR techniques but then fails to isolate an STEC strain (33,34). This could be attributed to several causes, such as previous antibiotic treatment of the patient, a dis- coli lysogenic strain C600 (933Wgfp) (CFU/ml) (light gray) in mixtures containing different phage and bacteria densities with or without the application of the filtration procedure used to remove bacteriophage.…”