Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2015
Publication Date: 1/19/2016
Citation: Ravva, S.V., Sarreal, C.Z. 2016. Persistence of F-specific RNA phages in surface waters from a produce production region along the central coast of California. PLoS One. PLOS One 11: e0146623.
Interpretive Summary: Major outbreaks associated with produce indicate that pre-harvest contamination has occurred in the field, so it is critical to identify and track sources of pathogens in the environment and interventions for minimizing them. Since contamination of ‘ready to eat’ produce cannot be washed off, on-site prevention of contamination is vital. Any such attempts require an understanding of the biological and environmental factors that regulate the proliferation of pathogens during their transport from animal reservoirs to produce grown in proximity to livestock operations. We have worked on developing improved detection methods for pathogens, and on understanding how pathogens survive, transport and re-grow in agricultural environments. Our long-term goals are to develop on-site prevention and control of pathogens at the source prior to contaminating produce we consume. Earlier we developed a molecular method to source track fecal contamination in irrigation waters through the detection of male specific coliphages as indicator organisms. In this study, we evaluated factors responsible for the prevalence of coliphages in surface waters in the produce production region of the central coast of California. Detection of members of genogroups GI and GIV indicates fecal contamination originated from animals and those of genogroups GII and GIII indicate human fecal contamination. Phages disappeared rapidly at 25oC and thus indicating source tracking attempts during summer would detect only recent fecal contamination. Presence of the bacterial host extended the survival of all phages and particularly the members of genogroups GIII and GI. If both detected, one can expect fecal contamination from both humans and animals.
Technical Abstract: F+ RNA coliphages (FRNA) are used to source-track fecal contamination and as surrogates for enteric pathogen persistence in the environment. However, the environmental persistence of FRNA is not clearly understood and thus we evaluated the survival of prototype and environmental isolates of FRNA representing all four genogroups in surface waters from the central coast of California. Temperature of surface water played a significant role in persistence and all prototype and environmental strains survived significantly longer at 10oC compared to 25oC. Similarly, availability of host bacterium was found to be critical in survival of FRNA. In the absence of the E. coli Famp, all prototype strains of FRNA disappeared rapidly with a D-value (days for one log decrease) of <1.2 d from surface waters incubated at 25oC and the longest surviving prototype was SP. In the presence of the host, the order of persistence at 25oC was QB>MS2>SP>GA and at 10oC it was QB=MS2>GA>SP. Thus, the persistence spectrum changed with the temperature of incubation and presence of the host. Significant differences in survival were observed between prototypes and environmental isolates of FRNA. While majority of environmental isolates disappeared rapidly in the absence of the host, members of genogroups GIII and GI persisted longer with the host compared to members of GII and GIV. Thus, FRNA based source tracking methods would detect phages from recent fecal contamination along with those that persist also longer in the environment as a result of cooler temperatures and increased host presence.