Environmental inactivation and irrigation-mediated regrowth of Escherichia coli O157:H7 on romaine lettuce when inoculated in a fecal slurry matrix

• Published in: PeerJ (San Francisco, CA) Vol. 7; p. e6591
• Main Authors: Chase, Jennifer A, Partyka, Melissa L, Bond, Ronald F, Atwill, Edward R
• Format: Journal Article
• Language: English
• Published: United States PeerJ. Ltd 08-03-2019; PeerJ, Inc; PeerJ Inc
• Subjects: Agricultural Science; Bacteria; Climate effects; Contamination; Crop diseases; Crops; Disease control; E coli; E. coli O157:H7; Epidemiology; Escherichia coli; Feces; Field trial; Food contamination & poisoning; Food processing industry; Food safety; Harvesting; Infectious Diseases; Irrigation; Leaves; Lettuce; Manures; Microbiology; Produce; Public Health; Rain; Romaine lettuce; Slurries; Slurry; Studies; Summer; Survival; Survival analysis; Swine; Wildlife conservation; Salinas Valley; United States > US; California; Fecal contamination; Enumeration; Produce; Slurry; Foliar irrigation; E. coli O157:H7; Field trial; Food safety; Romaine lettuce; Survival
• ISSN: 2167-8359; 2167-8359
• DOI: 10.7717/peerj.6591

Field trials were conducted in July-August and October 2012 to quantify the inactivation rate of
O157:H7 when mixed with fecal slurry and applied to romaine lettuce leaves. Lettuce was grown under commercial conditions in Salinas Valley, California. One-half milliliter of rabbit, chicken, or pig fecal slurry, containing an average of 4.05 × 10
CFU
O157:H7 (C
), was inoculated onto the upper (adaxial) surface of a lower leaf on 288 heads of lettuce per trial immediately following a 2.5 h irrigation event. To estimate the bacterial inactivation rate as a function of time, fecal matrix, irrigation and seasonal climate effects, sets of lettuce heads (
= 28) were sampled each day over 10 days and the concentration of
O157:H7 (C
) determined.
O157:H7 was detected on 100% of heads during the 10-day duration, with concentrations ranging from ≤340 MPN/head (∼5-log reduction) to >3.45 × 10
MPN/head (∼5-log growth). Relative to C
, on day 10 (C
) we observed an overall 2.6-log and 3.2-log mean reduction of
O157:H7 in July and October, respectively. However, we observed relative maximum concentrations due to bacterial growth on day 6 (maximum C
) apparently stimulated by foliar irrigation on day 5. From this maximum there was a mean 5.3-log and 5.1-log reduction by day 10 (C
) for the July and October trials, respectively. This study provides insight into the inactivation and growth kinetics of
O157:H7 on romaine lettuce leaves under natural field conditions. This study provides evidence that harvesting within 24 h post irrigation has the potential to increase the concentration of
O157:H7 contamination, if present on heads of romaine lettuce; foliar irrigation can temporarily stimulate substantial regrowth of
O157:H7.