P3-91 The Impact of Inoculation Time of Day on the Survival of Attenuated Escherichia coli O157:H7 and Generic Escherichia coli on Field-grown Cilantro

Wednesday, July 31, 2013
Exhibit Hall (Charlotte Convention Center)
Tyann Blessington, Oak Ridge Institute for Science and Education, Davis, CA
Anne-laure Moyne, University of California-Davis, Davis, CA
Irene Zhao, University of California-Davis, Davis, CA
Linda Harris, University of California-Davis, Davis, CA
Introduction: Cool temperatures and high relative humidity (RH) are known to improve the survival of enteric foodborne pathogens on produce.

Purpose: To determine the impact of temperature and RH on the survival of attenuated Escherichia coli O157:H7 and generic E. coli on field-grown cilantro.

Methods: In two Salinas, California field trials (summer and fall) mature cilantro plants (4 to 7 weeks post-seeding) were spray inoculated (6 to 8 log CFU/ml) with rifampicin-resistant attenuated E. coli O157:H7 ATCC 700728 and generic E. coli in the evening (8 to 10 pm), morning (7 am), and afternoon (12 to 3 pm). Temperature, RH and bacterial survival were monitored through commercial maturity. Harvested plants (n = 6 to 10; 8 to 99 g) were homogenized and diluted in 0.1% peptone and plated onto rifampicin containing tryptic soy and CHROMagar O157 or CHROMagar ECC; the remaining sample was enriched in tryptic soy broth with rifampicin.

Results: Within 24 h after inoculation, bacterial levels declined from an initial level of 5 to 7 log CFU/g to as low as <1 log CFU/g; 2-log CFU/g greater declines were observed for E. coli O157:H7. Both inoculated bacteria could be isolated by enrichment from most cilantro samples 1 to 6 weeks post-inoculation. The temperature and RH during the morning (12-13°C; 88-95%), afternoon (19-23°C; 62-66%), and evening (13-17°C; 76-91%) were similar for three inoculation times but were hotter and dryer during the fourth inoculation (morning, 16-18°C; 48-52%; afternoon, 32-35°C; 29-30%; and evening, 18-20°C; 52-63%). Greater reductions (by 1 to 4 log CFU/g) during the first 24 h after inoculation were observed for E. coliO157:H7, in the morning and afternoon, and during the fourth trial.

Significance: The survival of E. coli on cilantro plants is impacted for a short time by temperature and RH at the time of contamination.