Evaluation of an Attachment Assay on Lettuce Leaves with Temperature- and Starvation-Stressed Escherichia coli O157:H7 MB3885

Attachment of enteric pathogens such as Escherichia coli O157:H7 to fresh produce is a crucial first step for contamination to occur, and irrigation water (IW) is considered a potentially important preharvest introduction route. In a natural situation, E. coli O157:H7 may be present in the irrigation water for some time and may, therefore, be starved. Most research, however, is performed with freshly cultured strains. The aim of this study was to examine the behavior of E. coli O157:H7 MB3885 under starvation stress in water used for overhead irrigation in the greenhouse and the consequence on its subsequent ability to attach to butterhead lettuce leaves. E. coli O157:H7 MB3885 was starvation stressed by introducing it at +/- 7.5 log CFU/ml into phosphate-buffered saline (PBS), sterile distilled water (SDW), or IW. The suspensions were stored at 4 or 20 degrees C and were used after 0, 2, and 6 days for an attachment assay on butterhead lettuce. E. coli O157:H7 MB3885 levels were determined by plating method and live and dead quantitative PCR technique. A decrease in plate counts, an indicator of stress, was observed for most of the conditions, whereas a die-off, as revealed by the live and dead quantitative PCR data, was only observed in IW stored at 20 degrees C. Overall, stress appeared to be highest in IW and lowest in PBS. The stressed cells were still able to recover, even at 4 degrees C, and to attach to the lettuce. Furthermore, our results show that standard laboratory solutions such as PBS and SDW may not be the best to simulate stressed cells in IW, in which the bacteria may behave significantly differently.