High Stability of Stx2 Phage in Food and under Food-Processing Conditions

Abstract

Bacteriophages (phages) carrying Shiga toxin genes constitute a major virulence attribute in enterohemorrhagic Escherichia coli (EHEC). Several EHEC outbreaks have been linked to food. The survival of such strains in different foods has received much attention, while the fate of the mobile Shiga toxin-converting phages (Stx phages) has been less studied. We have investigated the stability of an Stx phage in several food products and examined how storage, food processing, and disinfection influence the infectivity of phage particles. The study involved a recombinant Stx phage (⌬stx::cat) of an E. coli O103:H25 strain from a Norwegian outbreak in 2006. Temperature, matrix, and time were factors of major importance for the stability of phage particles. Phages stored at cooling temperatures (4°C) showed a dramatic reduction in stability compared to those stored at room temperature. The importance of the matrix was evident at higher temperatures (60°C). Phages in ground beef were below the detection level when heated to 60°C for more than 10 min, while phages in broth exposed to the same heating conditions showed a 5-log-higher stability. The phages tolerated desiccation poorly but were infective for a substantial period of time in solutions. Under moist conditions, they also had a high ability to tolerate exposure to several disinfectants. In a dry-fermented sausage model, phages were shown to infect E. coli in situ. The results show that Stx phage particles can maintain their infectivity in foods and under food-processing conditions.

Persistence of Infectious Shiga Toxin-Encoding Bacteriophages after Disinfection Treatments

“…Our assays, which were conducted under laboratory conditions, may or may not be the most optimal for lysogenization, since transduction under "in vivo" conditions seems to be very effective (25,26,28,29). The Stx phage induced from the Norwegian O103:H25 isolate was shown to be capable of lysogeny after storage in a dry-fermented sausage model (36).…”

“…Finally, both phages generate phage suspensions after induction from their lysogens with a constantly high number of phages, which allowed the replicates presented in our study. Other studies on the survival of Stx phages have been conducted using only one Stx phage of the same morphological type (36,37).…”

“…However, the success of this spread among the bacterial population depends on their ability to retain their infectious abilities under the conditions found in a given environment. Stx phages have been shown to retain their stability and infectivity under various food-related conditions (36) and, to some extent, in the presence of certain food preservatives (37).…”

Shiga toxins and stx phages: highly diverse entities

“…It has been shown that stx phages can persist longer than their host bacteria in an aquatic environment (Muniesa et al, 1999;Allué-Guardia et al, 2014). In addition, some stx phages have a high ability to tolerate exposure to certain disinfectants and can maintain their infectivity under food-processing conditions (Muniesa et al, 1999;Kajiura et al, 2001;Rode et al, 2011).…”

Induction of Shiga Toxin-Encoding Prophage by Abiotic Environmental Stress in Food

“…Induction of Stx2-prophages by 2.5% sodium citrate also induced Stx expression without formation of infective phage particles (39). Low pH interferes with phage infectivity by inhibition of the formation of Stx2-prophages or by inactivation of phages after correct assembly and release (40,41). Induction of Stx production without release of infectious phages may benefit E. coli in defense against predatory protozoa (35) but is not of concern for food safety.…”