Abstract
Fresh leafy produce, such as lettuce and coriander, are subject to post-harvest microbial contamination and decay. Because of increasing pesticide resistance and consumer pressures, alternative residue-free treatments, such as ozone, are being actively explored and encouraged to reduce microbial loads and curb spoilage of crops in storage/transit. However, several researchers have reported that a component of the bacterial population on leaf surfaces is resistant to ozone treatment. To investigate the potential reasons for this bacterial survival, confocal microscopy was used to visualise microbes on leaf surfaces before and after ozone treatment. Direct observation (live/dead cell staining) of cells after ozone exposure showed that some cells were still alive; this included cells in small colonies as well as individual cells. We hypothesised that cell (colony) age and prior stress (cold) contributes to, or is responsible for, the ozone resistance observed. Interestingly, cells derived from older agar-grown colonies (7–12-day-old) and cold stressed cells of a Pseudomonas sp. (isolated from coriander) showed higher ozone resistance than that of control cells (4-day-old colonies). These findings suggest that a range of factors are responsible for ozone resistance and further work to improve our understanding of the mechanisms of ozone resistance may lead to improved methods to reduce microbial spoilage of fresh produce.
Highlights
Over the past decade, there have been increasing concerns about food quality and microbial safety, especially with regard to leafy salads, herbs, seed and vegetables which may be minimally processed and are often consumed raw (Losio et al, 2015)
Our results suggest that pre-exposure of bacteria (Pseudomonas sp.) to cold stress enhanced ozone resistance in vitro
This work focused on visualising microbes on leaf surfaces after ozone treatment by using confocal scanning microscopy and investigating potential reasons for ozone resistance in leaf surface bacteria
Summary
There have been increasing concerns about food quality and microbial safety, especially with regard to leafy salads, herbs, seed and vegetables which may be minimally processed and are often consumed raw (Losio et al, 2015). It appears that microbial contamination can occur at any stage from production to consumer handling, and may arise from animal, environmental or human sources or by simple multiplication of surface biofilms to create slime and off odours (Olaimat and Holley, 2012). The use of ozone is already permitted in many Asian and European countries, and the gas holds Generally
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