Fate of Shigella sonnei on parsley and methods of disinfection

Abstract

Outbreaks of shigellosis associated with chopped parsley used as a garnish for foods occurred in four states in the United States and in two Canadian provinces in 1998. This prompted a study to determine survival and growth characteristics of Shigella sonnei inoculated onto raw parsley. Two inoculum levels (~103 and 106 CFU/g) were applied to parsley leaves, portions of which were then chopped. Inoculated whole and chopped parsley leaves were held at 4°C or 21°C for up to 14 days. Initial populations of the organism on chopped parsley receiving high or low levels of inoculum increased by approximately 3 log10 CFU/g, within 1 day at 21°C. Populations of S. sonnei on inoculated chopped or whole parsley leaves held at 4°C decreased by 2.5 to 3.0 log10 CFU/g during a 14-day storage period. The pathogen multiplied, without a lag phase, on inoculated (2.72 log10 CFU/g) chopped parsley held at 21°C, exceeding 6 log10 CFU/g within 24 h. Treatment of inoculated whole parsley leaves with vinegar containing 5.2% (vol/vol) acetic acid or 200 ppm free chlorine for 5 min at 21°C reduced the population of S. sonnei by more than 6 log10 CFU/g, whereas treatment with vinegar containing 7.6% acetic acid or 250 ppm free chlorine reduced initial populations of 7.07 and 7.26 log10 CFU/g, respectively, to undetectable levels (<0.6 log110 CFU/g). These studies revealed that S. sonnei can grow rapidly on chopped parsley held at ambient temperature and remain viable for at least 14 days at 4°C. Treatment of contaminated parsley with vinegar or chlorinated water offers a simple method to reduce markedly or eliminate the pathogen in food-service or home settings.