P3-219 Die-off Rates of Surrogate and Virulent EHEC-STEC Strains from the Surface of Strawberry Plants Vary with Time, Inoculum Dose and Chemical Interventions

Wednesday, July 12, 2017
Exhibit Hall (Tampa Convention Center)
Maria Albarracin , North Carolina State University , Raleigh , NC
Christopher Gunter , North Carolina State University , Raleigh , NC
Siddhartha Thakur , North Carolina State University , Raleigh , NC
Eduardo Gutierrez-Rodriguez , North Carolina State University , Raleigh , NC
Introduction: Agricultural water in contact with fresh produce during preharvest or postharvest operations may pose a risk to the microbial safety of produce. Current alternative provisions within FSMA allow bacterial in-field die-off rate of 0.5 log CFU/day between the last irrigation and harvest when water fails to meet microbial quality standards. 

Purpose: Determine validity of die-off rate of 0.5 log CFU/day between the last irrigation event and harvest in NC strawberry production system and field environment.

Methods: Strawberry plants were grown following standard commercial practices under BSL2 greenhouse conditions.  Generic E. coli-W778, Enterohemorrhagic E. coli (EHEC) and Shiga Toxin-Producing E. coli (STEC) strains were spot (100ul) inoculated to the surface of strawberry at log 6.0 and 3.0 CFU/ml. Bacterial recovery (4 replicates) was performed at 0, 2, 4 and 8 days post-inoculation (DPI); with or without chlorine or peroxyacetic acid (PAA) applications after 4-DPI at 100 and 60ppm concentrations, respectively. Environmental data was used for modeling die-off rates for each strain. 

Results: EHEC, STEC and E. coli-W778 die-off rates were strain, time and concentration dependent. However, all strains displayed bimodal die-off dynamics, with higher rates between 0 and 2-DPI and lower rates thereafter. Three out of the five EHEC-STEC strains inoculated at log 6.0 CFU/ml displayed die-off rates <0.5 log CFU/day throughout the experiment. At log 3.0 CFU/ml, average die-off rates were 0.31, 0.27 and 0.02 log CFU/day at 2, 4 and 8-DPI respectively. Similar die-off patterns were observed for E. coli-W778. Chlorine and PAA treatments marginally reduced the population (25%) of EHEC and STEC when inoculated at log 6.0 CFU/ml while at log 3.0 CFU/ml no conclusive effects were observed between strains. 

Significance: Irrespective of pathogen fitness and virulence, die-off rates over 8-DPI did not adjust to a linear model as proposed by the FSMA Produce Rule.