Inactivation of Escherichia coli O157:H7 on Romaine Lettuce When Inoculated in a Fecal Slurry Matrix

Abstract

A field trial was conducted in July 2011 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, CA. One-half milliliter of rabbit fecal slurry, containing 6.3 × 10 CFU of E. coli O157:H7, was inoculated onto the upper (adaxial) surface of a lower leaf on 240 heads of lettuce within 30 min after a 2.5-h irrigation event. Forty-eight romaine lettuce heads were collected per event at 2.5 h (day 0.1), 19.75 h (day 0.8), 43.25 h (day 1.8), 67.25 h (day 2.8), and 91.75 h (day 3.8) postinoculation and were analyzed for the concentration of E. coli O157:H7 (C). E. coli O157:H7 was detected on 100% of collected heads in concentrations ranging from 340 to 3.40 × 10 most probable number (MPN) per head. Enumeration data indicate substantial growth of E. coli O157:H7 postinoculation (2.5 h), leading to elevated concentrations, 1 to 3 log above the starting inoculum concentration (C). By the end of the 92-h trial, we observed a net 0.8-log mean reduction of E. coli O157:H7 compared with C; however, after accounting for the substantial bacterial growth, there was an overall 2.3-log reduction by the final sampling event (92 h). On the basis of two different regression models that used either the raw data for C or log-transformed values of C/C during the period 2.5 to 91.75 h postinoculation, there was an estimated 76 to 80% reduction per day in bacterial counts; however, more accurate predictions of MPN per head of lettuce were generated by using non-log-transformed values of C. This study provides insight into the survival of E. coli O157:H7 transferred via splash from a contaminated fecal source onto produce during irrigation. Moreover, these findings can help generate inactivation times following a potential contamination incident.

Spatiotemporal Variability in Microbial Quality of Western US Agricultural Water Supplies: A Multistate Study

“…While there have been an abundance of studies to model bacterial survival kinetics (Chase et al, 2017, Salazar et al, 2017), transfer dynamics (Atwill et al, 2015), and preharvest risk factors (Pagadala et al, 2015) linked to raw produce consumption (Ibekwe et al, 2005; Pachepsky et al, 2016; Tian et al, 2012), relatively little is known about the potential risk inherent in agricultural water supplies.…”

Peer Review #2 of "Environmental inactivation and irrigation-mediated regrowth of Escherichia coli O157:H7 on romaine lettuce when inoculated in a fecal slurry matrix (v0.1)"

“…Manuscript to be reviewed to experience high levels of log-reduction under summer field conditions (McKellar et al 2014;Moyne et al 2011). However, a recent study under California summer conditions found that E. coli O157:H7 mixed with a fecal slurry to simulate fecal splash during foliar irrigation exhibited reduced rates of inactivation in contrast to this earlier work (Chase et al 2017). In addition, Weller et al (2017b) replicated these findings of extended survival of E. coli when the bacteria were mixed in a fecal slurry under growing conditions typical of upstate New York.…”

“…var. longifolia) was grown under commercial conditions in the Salinas Valley, California with plant thinning, weed removal, and irrigation being performed as needed up to field inoculation described by Chase et al (2017). The lettuce was planted on June 12 and August 14 for the summer and fall trials, respectively.…”

“…We used a rifampicin resistant (50 μg/mL) E. coli O157:H7 strain ATCC 700728 that lacked Stx 1 and 2 genes (Atwill et al 2015;Chase et al 2017;Moyne et al 2011). Bacterial cultures were grown in Tryptic Soy Broth (TSB) (Difco, BD, Sparks, MD) supplemented with 50 μg/mL rifampicin (+R) (Gold Biotechnology, St. Louis, Missouri USA) and incubated at 37 °C for 6 h with orbital rotation (100 rpm).…”

“…Research studies have been conducted to estimate the prevalence of pathogens in irrigation water supplies (Falardeau et al 2017;Gorski et al 2014;Partyka et al 2018b;Ssemanda et al 2018), survival kinetics of these pathogens in agricultural water (Steele & Odumeru 2004;Wang et al 2018), and how well pathogens survive once transferred onto crops via irrigation (Atwill et al 2015;Chase et al 2017;Moyne et al 2011;Van Der Linden et al 2014;Weller et al 2017b). For example, numerous studies have shown the occurrence of bacterial pathogens in US agricultural water supplies, especially for surface water sources (Falardeau et al 2017;Partyka et al 2018a;Partyka et al 2018b;Partyka et al 2016).…”