Purpose: To compare the survival of Escherichia coli O157:H7 on cilantro plants inoculated in the evening (cooler temperature, higher RH) and morning.
Methods: Mature cilantro plants (4 and 6 weeks post seeding) were spray inoculated (7 log CFU/ml) with rifampicin-resistant attenuated E. coli O157:H7 ATCC 700728 in the evening (9 or 10 pm) or morning (7 am) in a commercial field (Salinas, California). Field temperatures, RH and bacterial survival were monitored until plants were 8 weeks post seeding. Harvested plants (n = 5 to 10) were stomached in 0.1% peptone. Sample preparations were plated onto tryptic soy and CHROM O157 agars; the remaining sample was enriched in tryptic soy broth. All media contained rifampicin.
Results: Survival of E. coli was consistently greater on plants inoculated in the evening than those inoculated in the morning during the first 2-days after inoculation. Populations decreased from 5 log CFU/g at the time of inoculation to 1 log CFU/g at 48 h. However, E. coli O157:H7 was isolated in 100% of plants by enrichment of 70 to 100 g samples of cilantro for up to 4 weeks after inoculation. Average field temperature and relative humidity (RH) during the 9 h after morning and evening inoculation were 14 °C and 93% RH, and 17 °C and 71% RH, respectively.
Significance: Environmental conditions at the time of inoculation may impact E. coli O157:H7 survival on cilantro plants for a short period after contamination.