School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemBacterial persistence on injured vegetable plants: implications for food safety( 2012)Vegetables can become contaminated with human pathogenic bacteria from various sources including manure fertiliser, animals, people, and faecally contaminated irrigation and floodwaters. Moreover, that contamination may not be totally removed via washing and treatment with disinfectant. Fortunately, a recent discovery suggested the level of contamination, and therefore risk of disease, could be reduced by avoiding shoot injury to vegetable plants. Escherichia coli was found to survive at much higher levels on Cos lettuce, celery and chives with minor shoot damage. That phenomenon was termed shoot injury-induced increased persistence (SIIIP). In that context, this thesis aimed to produce guidelines to reduce the risk of foodborne illness from raw vegetables contaminated prior to harvest with human-pathogenic bacteria. Factors that may affect SIIIP were investigated via the colony-count technique. Salmonella enterica serovar Sofia persisted at higher levels on injured versus uninjured plants in the glasshouse. The initial counts on the injured and uninjured Cos lettuce were in the order of 6 log10 colony forming units per gram (CFU/g). On the injured plants that count fell significantly (P < 0.05) to 4.9 and 4.8 log10 CFU/g after the first and third days, respectively. However on the uninjured plants, these corresponding counts were significantly (P < 0.05) lower, being 4.4 and 3.4 log10 CFU/g. After 2 and 3 weeks the mean count from the injured plants fell to 3.4 and 2.9 log10 CFU/g respectively, whilst for the uninjured plants, the counts for the same times were 0.5 log10 CFU/g or less. A similar pattern of SIIIP was also found with Listeria innocua on Cos lettuce, and for S. enterica serovar Sofia on chive plants in the glasshouse. Also, irrigation with nutrient-rich versus deionised water significantly (P < 0.05) elevated E. coli counts on uninjured plants, but only on the second day of the experiment. Additionally, confocal microscopy revealed that E. coli expressing the green fluorescent protein became internalised by injured leaves of Cos lettuce. Moreover, a field trial showed that delaying bacterial contamination of injured lettuce for 3 or more days negated SIIIP. That negation was not prevented by cool temperatures in a growth-cabinet study. Finally, these findings demonstrated that SIIIP was actually due to plant injury and not because of the failure of bacteria to grow on selective media. Prior to the work reported here, it was possible to argue that the importance of SIIIP was restricted to E. coli. However, the demonstration of SIIIP with Salmonella and Listeria has strengthened the justification for avoiding that phenomenon during vegetable production. Therefore, growers should not use untreated animal manure or faecally polluted water on leafy vegetable plants that have been injured by harsh weather, pests, diseases or cropping practices. Alternatively, growers should separate injury and contamination by at least 3 days. That last point was proven by the work presented here, to be applicable to the field and not to be compromised by cool weather. Furthermore, consideration should be given to withdrawing irrigation with nutrient-rich water, several days before harvest. Also injured shoots should be removed because any internalised bacteria would be protected from sanitisation. However, injured shoots should be included in bacteriological testing, to ensure the detection of potentially dangerous contamination from bacteria undergoing SIIIP. Finally, the mechanism responsible for SIIIP, probably does not involve nutrients from injured plant tissue.