Purpose: To evaluate the persistence of E. coli as surrogate of pathogenic E. coli on field-inoculated basil in two distinct tropical environments.
Methods: Basil field trials were conducted at Waimanalo and Poamoho, Hawaii, representing coastal and inland tropical environments, respectively. Rifampicin (rif)-resistant E. coli was inoculated at concentrations of 4.5 (low) or 7.1 (high) log CFU/ml in irrigation water with or without Quadris® fungicide and Latron*B-1956 surfactant. E. coli was recovered from plant samples by stomaching and enumerated by plating on tryptic soy agar (TSA) + 50 μg/ml rif or filtration and plating on CHROMagar ECC+ 50 μg/ml rif. When counts fell below the limit of detection, samples were enriched in TSB + 50 μg/ml rif, followed by plating on CHROMagar ECC + 50 μg/ml rif.
Results: At Waimanalo, E. coli inoculated at the high level was recovered at concentrations of 5.3-5.4 log CFU/g immediately after inoculation, 4.2-4.8 log CFU/g after 3h, and 3.4-3.9 log CFU/g after 24h. E. coli could still be enumerated by filtration in some samples 4 weeks post-inoculation. At Poamoho, E. coli inoculated at low and high concentrations with fungicide and surfactant was recovered at concentrations of 2.4 and 4.2 log CFU/g immediately after inoculation, 0.75 and 2.3 log CFU/g after 3h, and 0.03 and 1.9 log CFU/g after 24h, respectively. After 4 weeks, E. coli was detected by enrichment in 7% (1/15) and 13% (2/15) of the samples taken from basil plants inoculated at low and high concentration, respectively.
Significance: E. coli populations declined more rapidly at Poamoho than Waimanalo. The slower initial decline in E. coli populations recorded in Waimanalo could be the result of climactic variation between the two sites.