Chase Jennifer A, Partyka Melissa L, Bond Ronald F, Atwill Edward R
Western Center for Food Safety, University of California, Davis, Davis, CA, USA.
School of Fisheries, Aquaculture and Aquatic Sciences, Auburn University, Auburn, AL, USA.
PeerJ. 2019 Mar 8;7:e6591. doi: 10.7717/peerj.6591. eCollection 2019.
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.
2012年7 - 8月以及10月进行了田间试验，以量化O157:H7与粪便浆液混合并施用于生菜叶时的失活率。生菜在加利福尼亚州萨利纳斯山谷的商业条件下种植。每次试验时，在2.5小时灌溉事件后，立即将0.5毫升平均含有4.05×10 CFU O157:H7（C）的兔、鸡或猪粪便浆液接种到288颗生菜下部叶片的上（近轴）表面。为了估计细菌失活率作为时间、粪便基质、灌溉和季节气候影响的函数，在10天内每天对生菜头（ = 28）进行采样，并测定O157:H7（C）的浓度。在10天期间，100%的生菜头上都检测到了O157:H7，浓度范围从≤340 MPN/头（约5个对数减少）到>3.45×10 MPN/头（约5个对数增长）。相对于C，在第10天（C）时，我们分别在7月和10月观察到O157:H7的平均减少量为2.6个对数和3.2个对数。然而，由于第5天的叶面灌溉显然刺激了细菌生长，我们在第6天（最大C）观察到了相对最大浓度。从这个最大值开始，7月和10月试验分别在第10天（C）时平均减少了5.3个对数和5.1个对数。本研究深入了解了自然田间条件下O157:H7在生菜叶上的失活和生长动力学。本研究提供了证据，表明如果生菜头上存在O157:H7污染，灌溉后24小时内收获有可能增加O157:H7的污染浓度；叶面灌溉可暂时刺激O157:H7大量重新生长。
