Growth characteristics of Shiga toxin-producing Escherichia coli (STEC) stressed by chlorine, sodium chloride, acid, and starvation on lettuce and cantaloupe

Authors: Yoo, Byong; Liu, Yanhong; Juneja, Vijay; Huang, Lihan; Hwang, Cheng An

Submitted to: Food Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2015
Publication Date: 3/3/2015
Publication URL: http://handle.nal.usda.gov/10113/61147
Citation: Yoo, B.K., Liu, Y., Juneja, V.K., Huang, L., Hwang, C. 2015. Growth characteristics of Shiga toxin-producing Escherichia coli (STEC) stressed by chlorine, sodium chloride, acid, and starvation on lettuce and cantaloupe. Food Control. 55:97-102.

Interpretive Summary: Fresh produce is commonly consumed raw for health benefit despite the recent increase in occurrence of relevant outbreaks. Since several intervention strategies are used in food manufacturing process to prevent the growth of Shiga toxin producing Escherichia coli in fresh produce, it is highly likely for pathogenic E. coli to be exposed to sub-lethal level of environmental stresses prior to consumption. Results showed that non-O157 E. coli exposed to sub-lethal stress conditions including sodium hypochlorite, sodium chloride, mild acid, and starvation prior to inoculation on fresh produce had higher growth at low temperatures than unstressed control strains, indicating that they adjust and grow faster in new environment to survive. This study extends our understanding of the potential mechanisms used by pathogenic E. coli to adapt, survive and grow under environmental stresses.

Technical Abstract: Shiga toxin producing Escherichia coli (STEC) is one of the major foodborne pathogens causing serious illnesses, leading to hospitalizations in the United States. Bacteria that are exposed to environmental stresses during food processing may exhibit different growth patterns in subsequent growth environment. The purpose of this study was to examine the effect of environmental stresses on growth of non-O157 STEC in fresh produce. Approximately 8 log CFU/ml of each strain of O157:H7 and four non-O157 STEC (O26:H11, O103:H1, O104:H4, and O145:NM) were subjected to four stresses: chlorine (1, 2, and 5 ppm) at 22C for 24 h or starvation (free of nutrients) at 22C for 15 d. Approximately 3 log of each strain was exposed to water activity (aw 0.95, 0.96, 0.97, and 0.98) or pH (4, 5, 6, and 7) at 22C for 24 h. Stressed cells were collected, non-O157 or O157 cocktail of 3 log CFU/ml (control or stressed) was inoculated on cantaloupe or lettuce and incubated at three different temperatures (4, 10, and 22C) for four weeks, followed by enumeration and statistical analysis using Student’s t test. There were significant differences (p less than 0.05) in the cell numbers between stressed and unstressed cells of STEC during the 4-week growth period. Compared to control non-O157, significantly higher populations were observed in stressed non-O157 cocktail mix. Survived cells of chlorine-stressed non-O157 were 5.6 log CFU/ml, while control non-O157 was about 3.8 log CFU/ml after 1 week at 4C in lettuce. Osmotic-stressed non-O157 STEC showed significantly higher cell populations than control with 2 log difference (9.0 vs. 6.8 log CFU/ml) after 4 weeks at 10C in lettuce. Acid-stressed non-O157 STEC had significantly higher cell populations than control at 10C after 4 weeks with over 1 log difference (7.7 vs. 6.3 log CFU/ml) in cantaloupe. Starvation-stressed non-O157 showed higher cell populations than control after 4 weeks of incubation, with approximately 1.7 log difference (6.4 vs. 4.8 log CFU/ml) at 4C in lettuce. Non-O157 STEC which were stressed by chlorine or starvation had better growth when they were incubated at 4C, whereas non-O157 STEC which were osmotic- or acid-stressed grew better when they were incubated at 10C compared to unstressed controls. This information provides insight into the growth characteristics of non-O157 STEC in fresh produce exposed to sub-lethal stresses prior to contamination of food with the pathogen.