טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentFrechtel-Gerzi Roni Hai
SubjectMicrobial Differences Between Organically and Conventionally
Grown Lettuce
DepartmentDepartment of Biotechnology and Food Engineering
Supervisor Professor Sima Yaron


Abstract

The increase in consumption of fruit and vegetables intensifies the importance of examination of the plants’ microbial population as a part of monitoring the quality and safety of these products. The study was driven under the hypothesis that microbiota of plants is affected by the agricultural growth conditions (organic and conventional). In this study, microbial analyses of lettuce plants grown under organic and conventional conditions were applied in order to assess the differences in the microbiota composition while focusing on human pathogens and antibiotic-resistant bacteria. Lettuce seedlings were planted and grown in a greenhouse under organic (N=45) and conventional (N=45) agriculture conditions along 28 days. The organic plants were planted in soil supplemented with commercial organic compost (manure), and the conventional plants were planted in soil supplemented with synthetic nitrogen-based fertilizer and sprayed with fungicides after 7 days. During 28 days of growth and after 7 days of storage of harvested plants at 4°C microbial analysis was conducted in two methods: (1) viable counts of specific bacteria including total heterotrophic organisms, Enterobactericeae, yeast and molds, lactic acid bacteria and Pseudomonas; (2) taxonomic comparison of 16S DNA sequences from total microbial DNA extracted from organic and conventional lettuce leaves. Both methods showed a similar tendency in which no significant quantitative differences between the organic and conventional lettuce were observed in any of the tested time points. Still, while all plants carry similar core microbial populations, a few bacterial strains like Sphingomononas and Agrobacterium were found at different prevalence. This tendency also supports the result of the mineral profile comparison that indicated differences in the amounts of sodium (Na) and manganese (Mn) only. Apart from the contamination of foodborne pathogens, the presence of antibiotic resistant bacteria is another major problem related to fresh produce. Resistance to the antibiotic drugs ampicillin, ciprofloxacin, tetracycline, trimethoprim-sulfamethoxazole, and amikacin was tested on Enterobactericeae that were isolated from organic and conventional lettuce seedlings (N=316). The majority of the tested Enterobactericeae (>94%) were found to be resistant to ampicillin and trimethoprim-sulfamethoxazole, with no significant difference between organic and conventional lettuce. However, Enterobactericeae originated from organic lettuce demonstrated higher levels of minimal inhibitory concentration (MIC) compared to Enterobactericeae from conventional lettuce. Next, survival of the enteric pathogen Salmonella Typhimurium (S. Typhimurium) on artificially inoculated organic and conventional lettuce was tested along the plants’ growth. Results revealed that S. Typhimurium was able to survive better on organic lettuce plants and that Ridomil gold MZ 68 fungicide, which is used in conventional farming, inhibited S. Typhimurium as well as other bacteria at the concentration that is applied by the farmers (9 g•L-1). This research indicates that the influence of the agricultural cultivation method (organic or conventional) on the microbiota composition of lettuce leaves is relatively small, however the observation regarding the antibiotic resistant bacteria and the survival of enteric pathogens highlights the risks involved in fresh organic produce.