Purpose: To determine how indicator microorganisms behave during chlorine inactivation and washing when compared with E. coli O157:H7.
Methods: Inactivation of various indicator groups [total viable count (TVC), Enterobacteriaceae, total coliforms] and E. coli in wash water after exposure to chlorine (0 - 30 ppm) with added organic load (0.1% lettuce juice powder) was determined. Cross-contamination of E.coli O157:H7 and the indicators to water was evaluated in 50-ml lettuce washing. Indicator microorganisms enriched from lettuce were co-inoculated with E. coli O157:H7 (~5 log CFU) on lettuce, which was washed in 30 ml of water or 1 ppm chlorine. Bacterial survival or transfer to wash water was enumerated by plating on appropriate media.
Results: E. coli and total coliforms were nearly eliminated by exposure to 10 ppm chlorine, whereas the TVC was only reduced by ~3.5 logs after exposure to 30 ppm chlorine. Enterobacteriaceae, total coliforms and E. coli decreased similarly as the chlorine concentration increased from 0 to 5 ppm. Results suggest that the indicator groups, except TVC, showed correlation in chlorine inactivation. In lettuce washing, approximately 3 log CFU/ml was transferred into the water for all indicator groups in the absence of chlorine. With 1 ppm chlorine, less than 30 total CFU were detected for Enterobacteriaceae, total coliforms and E. coli O157:H7; however, higher levels of TVC (~ 1 log CFU/ml) were detected in some replicates.
Significance: Microbial indicator tests might be useful in assessing effectiveness of chlorine washing for fresh produce.