Attachment strength and on-farm die-off rate of Escherichia coli on watermelon surfaces

Abstract

Pre-harvest contamination of produce has been a major food safety focus. Insight into the behavior of enteric pathogens on produce in pre-harvest conditions will aid in developing pre-harvest and post-harvest risk management strategies. In this study, the attachment strength (SR) and die-off rate of E. coli on the surface of watermelon fruits and the efficacy of aqueous chlorine treatment against strongly attached E. coli population were investigated. Watermelon seedlings were transplanted into eighteen plots. Prior to harvesting, a cocktail of generic E. coli (ATCC 23716, 25922 and 11775) was inoculated on the surface of the watermelon fruits (n = 162) and the attachment strength (SR) values and the daily die-off rates were examined up to 6 days by attachment assay. After 120 h, watermelon samples were treated with aqueous chlorine (150 ppm free chlorine for 3 min). The SR value of the E. coli cells on watermelon surfaces significantly increased (P<0.05) from 0.04 to 0.99 in the first 24 h, which was primarily due to the decrease in loosely attached population, given that the population of strongly attached cells was constant. Thereafter, there was no significant change in SR values, up to 120 h. The daily die-off rate of E. coli ranged from -0.12 to 1.3 log CFU/cm2. The chlorine treatment reduced the E. coli level by 4.2 log CFU/cm2 (initial level 5.6 log CFU/cm2) and 0.62 log CFU/cm2 (initial level 1.8 log CFU/cm2), on the watermelons that had an attachment time of 30 min and 120 h respectively. Overall, our findings revealed that the population of E. coli on watermelon surfaces declined over time in an agricultural environment. Microbial contamination during pre-harvest stages may promote the formation of strongly attached cells on the produce surfaces, which could influence the efficacy of post-harvest washing and sanitation techniques.

Fig 1. Die-off and attachment of E. coli on watermelon surfaces.

Average E.coli count on the watermelon surfaces and FSMA- PSR predicted count on produce surfaces based on the die-off rate of 0.5 log CFU/day, up to 120 h (Day 6). The population of loosely attached and strongly attached cells is also shown for up to 120 h. S_R Value (Attachment Strength) = Strongly Attached Cells/ Total Cells (strongly attached+ loosely attached). Data are shown as mean values ± standard deviation.

Fig 2. Scanning electron micrographs of E. coli inoculated watermelons surfaces.

SEM images (SEI 10 KV, 5,000x & 10,000x) of watermelon surfaces after inoculating with E. coli and leaving them in an agriculture field for 0 h (A), 24 h (B), 48 h (C), 72 h (D), 96 h (E) and 120 h (F).

Fig 3. Efficacy of chlorine (150 ppm for 3 min) on E. coli inoculated watermelon surfaces.

30 min: Inoculated 30 min prior to the treatment; 120 h: Inoculated 120 h prior to the treatment. The bar diagrams with different lowercase letters on the top are significantly different (P < 0.05) from each other. The detection limit of the test was 0.28 log CFU/cm².