P1-126 Survival of Escherichia coli in Field-inoculated Basil

Monday, August 4, 2014
Exhibit Hall D (Indiana Convention Center)
Anne-Laure Moyne, University of California-Davis, Davis, CA
Linda Harris, University of California-Davis, Davis, CA
Introduction: Leafy herbs including basil have been associated with foodborne illness. The persistence of human pathogenic Escherichia coli on plants varies with plant species and growing conditions. Few studies have investigated the preharvest survival of E. coli inoculated on field-grown basil plants

Purpose: To evaluate the survival of two environmental isolates of E. coli as surrogates of pathogenic E. coli after inoculation onto field inoculated-basil. 

Methods: Lawns of rifampicin-resistant E. coli PTV 354 or LJH1614 were harvested from tryptic soy (TS) agar plates and diluted to a concentration of 7 log CFU/ml in milli-Q water or milli-Q water supplemented with fungicide (Quadris, Syngenta) at the recommended dose. Basil plants growing in a UC Davis research field were inoculated with a backpack sprayer. Plants were harvested at 2, 4, 6, 8, and 24h and at 1, 2, 3 and 4 weeks after inoculation. E. coli was recovered by stomaching and enumerated by plating onto TS agar or filtration and plating onto CHROMagar ECC both with 50 μg/ml of rifampicin. When counts were below the limit of detection, samples were enriched in TS broth with 50 μg/ml of rifampicin followed by plating on CHROMagar ECC.

Results: E. coli levels applied to basil plants were 5.5 and 6.5 log CFU/g in two successive trials. Populations of both E. coli strains declined by 2 log CFU/g within 2h of inoculation and by 3.5 to 4 log CFU after 24h. One week after inoculation, E. coli was below the limit of detection by plating (less than 10 CFU/g) but continued to be detected by enrichment throughout the duration of the field trial. Similar patterns of survival were observed for both strains regardless of the presence of fungicide.

Significance: E. coli surrogates inoculated on basil plants survived at low level for an extended period of time.