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 c
haracteristics of Shigella sonnei inoculated onto raw parsley. Two inoculum
levels (similar to 10(3) and 10(6) CFU/g) were applied to parsley leaves,
portions of which were then chopped. Inoculated whole and chopped parsley l
eaves were held at 4 degrees C or 21 degrees C for up to 14 days. Initial p
opulations of the organism on chopped parsley receiving high or low levels
of inoculum increased by approximately 3 log(10) CFU/g, within 1 day at 21
degrees C. Populations of S. sonnei on inoculated chopped or whole parsley
leaves held at 4 degrees C decreased by 2.5 to 3.0 log(10) CFU/g during a 1
4-day storage period. The pathogen multiplied, without a lag phase, on inoc
ulated (2.72 log(10) CFU/g) chopped parsley held at 21 degrees C, exceeding
6 log(10) 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 degrees C reduced the population of S. sonnei by more than
6 log(10) CFU/g, whereas treatment with vinegar containing 7.6% acetic aci
d or 250 ppm free chlorine reduced initial populations of 7.07 and 7.26 log
(10) CFU/g, respectively, to undetectable levels (<0.6 log(10) CFU/g). Thes
e 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 degrees
C. Treatment of contaminated parsley with vinegar or chlorinated water offe
rs a simple method to reduce markedly or eliminate the pathogen in food-ser
vice or home settings.