Purpose: In this work, inoculation experiments simulating real conditions were performed at laboratory scale in order to gain insight into the spread of contamination (i.e., pathogen distributions) when lettuce heads contaminated by E. coli O157:H7 enter a processing line during ordinary processing of fresh product.
Methods: A series of inoculation experiments at different levels of contamination were carried out in the pilot plant of National Center for Technology and Food Safety (high level, 6-7 log CFU/g; medium level, 4 log CFU/g; low level 1 log CFU/g). For each lot of production (3 kg of raw lettuce) one contaminated lettuce head was introduced, previously inoculated with a marked E. coli O157:H7 strain resistant to nalidixic acid. The presence and numbers of pathogen were tested in the lettuce packaged bags and washing water. Outcomes were statistically analyzed and probability distributions were fitted to count data.
Results: The study showed that E. coli O157:H7 is homogenously distributed on fresh-cut leafy vegetables as a result of processing (mainly washing) at both high-medium and low simulated contamination levels and this increases prevalence and spread of the pathogen over other product units. In addition, Negative Binomial, Poisson-lognormal and lognormal distributions were suitable to describe pathogen distribution at the high and medium levels, while Poisson distribution was poorly fitted to counts in spite of the Coefficient of Variation (CV) indicated no overdispersion (i.e., clustering).
Significance: These findings can have relevant implications in the development of either effective sampling plans intended to detect the pathogen at factory or other mitigation strategies to mitigate risk by E. coli O157:H7.