Purpose: This study examined the attachment and die-off rate of microorganisms on watermelon surface in an agricultural setting.
Methods: Replicated watermelon plots (18 plots, three plots per treatment, plot size 12’ by 30’, 30 plants per plot) were treated with five varieties of herbicides that resulted in different weed levels in each plot. At the time of harvesting, a cocktail of generic E. coli (ATCC 23716, 25922, and 11775) was inoculated on the outer surface (around 50 cm2) of the watermelon fruits (n=162). The attachment strength (SR) and die-off rate of generic E. coli was examined up to 120 hours. Samples (0 and 120 hours) were treated with a chlorine solution (150 ppm) to evaluate the effect of sanitizer against the firmly and loosely attached generic E. coli.
Results: The attachment strength (SR value) and die-off rate of inoculated generic E. coli on watermelon surface increased significantly (P<0.05) within 24 hours. The initial SR value of 0.04 reached 0.99 after 120 h. There was a significant increase in firmly attached generic E. coli levels (from 3.93 log CFU/cm2 to 4.47 log CFU/cm2) after 72 h; however, within 24 h the loosely attached bacteria reduced to at or below the detectable limit of the test. After 96 h, the total generic E. coli levels on the watermelon surface was reduced 5.68 log CFU/cm2. The chlorine treatment resulted in the reduction of generic E. coli level by 4.22 log CFU/cm2 and 0.61 log CFU/cm2on the 0 h and 120 h samples, respectively.
Significance: The finding suggests that preharvest environmental conditions can induce microbial attachment and biofilm formation on produce surface which may affect subsequent postharvest processing and sanitizing treatments.