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.