The effects of irrigation on the survival of Clostridium sporogenes in the phyllosphere and soil environments of lettuce

Abstract

This study aimed to address the gap in knowledge regarding the fate of foodborne pathogens within agro-ecosystems. It specifically focused on the surrogate microorganism Clostridium sporogenes, which was introduced into lettuce-producing environments via surface and spray irrigation methods, respectively. The concentration of C. sporogenes in the rhizosphere, phyllosphere, and non-rhizosphere soil was quantified by quantitative polymerase chain reaction (qPCR) over a 42-day trial. The surface irrigation method exhibited a more noticeable contamination effect on the soil environments, compared to the phyllosphere. The results indicated a noticeable increase in C. sporogenes concentrations during the initial 22 days, with a 10.4-fold rise (0.39–4.05 log copy numbers/g soil) in the rhizosphere and 1.9-fold increase (2.97–5.59 log copy numbers/g soil) in the non-rhizosphere. However, concentrations in both soil environments subsequently decreased, falling below the initial inoculum concentration by the end of the trial. In contrast, the spray irrigation method resulted in most of the contamination being localised on the lettuce phyllosphere, with a high C. sporogenes concentration of 9.09 log copy numbers/g leaves on day 0. This concentration exponentially decreased to a minimal 0.019 log copy numbers/g leaves by day 32. Although concentrations in both soil environments decreased over time, trace concentrations of C. sporogenes were detectable at the end of the trial, posing a potential hazard to the microbiological safety of postharvest produce. These findings shed light on the dynamics of C. sporogenes in agro-ecosystems and underscore the importance of irrigation practices that ensure the safety of those who consume fresh produce.