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

Field trials were conducted in July-August and October 2012 to quantify the inactivation rate of Escherichia coli 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 x 10⁷ CFU E. coli O157:H7 (C₀), 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 (n = 28) were sampled each day over 10 days and the concentration of E. coli O157:H7 (C_t) determined. E. coli 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 x 10¹² MPN/head (~5-log growth). Relative to C₀, on day 10 (C_t=12 ) we observed an overall 2.6-log and 3.2-log mean reduction of E. coli O157:H7 in July and October, respectively. However, we observed relative maximum concentrations due to bacterial growth on day 6 (maximum C_t = 8) 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_{t = 12}) for the July and October trials, respectively. This study provides insight into the inactivation and growth kinetics of E. coli 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 E. coli O157:H7 contamination, if present on heads of romaine lettuce; foliar irrigation can temporarily stimulate substantial regrowth of E. coli O157:H7. [ABSTRACT FROM AUTHOR]