Typically, enrichment protocols for Shigella in food involve
growing produce rinsates in Shigella broth in the presence of 0.5e
3 mg/ml of novobiocin at 38e44 C for about 20 h (FDA, 2001;
Mehlman et al., 1985; Warren et al., 2009; International
Organization for Standardization, 2004). To lower the variability
between samples, the indigenous flora fromvarious produce types
was first enriched overnight in Shigella broth containing 0.5 mg/ml
of novobiocin and titered on non-selective TSA plates, yielding from
0.5 109
CFU/ml in parsley to 1.6 1010
CFU/ml in green onions
(with bulbs, leaves and some roots). For the other four produce
types tested, post-enrichment bacterial counts were from 1.4 to
2.6 109
CFU/ml. One ml of each enrichment culture was then
spiked with various 10-fold dilutions of a pure Shigella culture
(Table 3), prior to DNA isolation.