Investigation of prevalence of free Shiga toxin-producing Escherichia coli (STEC)-specific bacteriophages and the correlation with STEC bacterial hosts in produce-growing area in Salinas, California

Authors: Liao, Yen-Te; QUINTELA, IRWIN - University Of Maine; Nguyen, Kimberly; Salvador, Alexandra; Cooley, Michael; Wu, Vivian

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2017
Publication Date: 1/4/2018
Citation: Liao, Y., Quintela, I., Nguyen, K.M., Salvador, A., Cooley, M.B., Wu, V.C. 2018. Investigation of prevalence of free Shiga toxin-producing Escherichia coli (STEC)-specific bacteriophages and the correlation with STEC bacterial hosts in produce-growing area in Salinas, California. PLoS One. 13(1):e0190534. https://doi.org/10.1371/journal.pone.0190534.
DOI: https://doi.org/10.1371/journal.pone.0190534

Interpretive Summary: Lytic bacteriophages that can mitigate their bacterial hosts are ubiquitous in the environment; however, a lot of studies isolate phages that target specifically against Shiga toxin-producing E. coli (STEC) from fecal-contaminated environment solely due to the natural reservoir of STEC strains in gastrointestinal tracts of ruminants or other animals. This study emphasized on prevalence of free STEC-specific phages at watershed sites around produce-growing area where fecal contamination levels were lower than the samples used in other studies. The results of this study show that the bacteriophages that can infect several STEC strains, including serogroups of O45, O145, O157 and O179, were isolated from several locations close to the city of Salinas where human activities were more prominent than agricultural activities. The prevalence of phage was not affected by either rainfall or solar radiation, indicating the resistance of these phages to environmental stresses. In addition, the findings of this study also reveal that sites where bacteriophages were present the bacterial host strains could not be found.

Technical Abstract: Shiga toxin-producing E. coli (STEC) strains, commensal to gastrointestinal tracts of ruminants or other animals, have been associated with serious human illnesses. Due to the natural habitats of STEC, the bacteriophages infectious against these bacteria are commonly isolated from fecal-contaminated environments such as feedlot or wastewater. However, little is known about the prevalence of free STEC-specific bacteriophages in pre-harvest produce growing area that has relatively lower level of fecal contamination. The objectives of this research were to study the correlation between STEC-specific bacteriophages and the bacterial hosts, and evaluate potential effects of environmental factors on the prevalence of the phages in produce-growing environment. Surface water samples collected from 20 different sites in produce-growing area were subjected to bacteriophage isolation using 4 generic E. coli and 29 STEC strains (O157, top six non-O157 and O179 strains) as host strains and bacterial isolation of O157 and non-O157 STEC by culture methods (immunomagnetic separation and selected media) combined with PCR-based confirmation. The information of weather conditions was collected from weather station. The results showed that the lytic STEC-specific (O45, O145, O157 and O179) bacteriophages were isolated from 8 different sites, while STEC isolates were isolated from 10 sites. The STEC-specific bacteriophages were commonly found in summer and early fall. STEC strains were isolated in spring and early summer. The environmental factors (rainfall and solar radiation) did not have significant impact (P>0.05) on the prevalence of bacteriophages. In addition, most bacteriophage-positive sites were close to city or the area where human activities were influential. However, there was a trend that the sites with isolation of bacteriophages were not shown with the isolation of their bacterial host strains of the specific serogroups. The findings of the study indicate that the prevalence of lytic STEC-specific bacteriophages is negatively correlated with STEC strains in produce pre-harvest environment.