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 log10 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.

Six human isolates of Escherichia coli O157:H7 and E. coli (ATCC 11229) were used to determine the concentrations of free chlorine and exposure times required for inactivation. Free chlorine concentrations of 0.25, 0.5, 1.0, and 2.0 ppm at 23 degrees C were evaluated, with sampling times at 0, 0.5, 1.0, and 2.0 min. Results revealed that five of six E. coli O157:H7 isolates and the E. coli control strain were highly susceptible to chlorine, with >7 log10 CFU/ml reduction of each of these strains by 0.25 ppm free chlorine within 1 min. However, comparatively, one of the seven strains was unusually tolerant to chlorine at 23 degrees C for 1 min, with a 4-, 5.5-, 5.8-, and >5.8-log CFU/ml reduction at free chlorine concentrations (ppm) of 0.25, 0.5, 1.0, and 2.0. respectively. Based on these studies most isolates of E. coli O157:H7 have no unusual tolerance to chlorine; however, one strain was exceptional in being recovered after 1-min of exposure of 10(7) CFU/ml to 2.0 ppm of free chlorine. This isolate may be a useful reference strain for future studies on chlorine tolerance of E. coli O157:H7.

No abstract

An outbreak of Escherichia coli O157:H7 infection associated with the consumption of coleslaw in several units of a restaurant chain prompted a study to determine the fate of the pathogen in two commercial coleslaw preparations (pH 4.3 and 4.5) held at 4, 11, and 21 degrees C for 3 days. At an initial population of 5.3 log10 CFU/g of coleslaw, E. coli O157:H7 did not grow in either coleslaw stored at the three temperatures. Rather, the population of E. coli O157:H7 decreased by 0.1 to 0.5 log10 CFU/g within 3 days. The greatest reduction (0.4 and 0.5 log10 CFU/g) in population occurred at 21 degrees C, whereas only slight decreases (0.1 to 0.2 log10 CFU/g) occurred at 4 and 11 degrees C. A pH of 4.3 to 4.5 of coleslaw had little effect on reducing E. coli O157:H7 populations. Results suggest that the tolerance of E. coli O157:H7 to acid pH, not temperature abuse, is a major factor influencing the pathogen's fate in restaurant-prepared coleslaw.