Ready-to-eat vegetable salads containing lettuce as a main ingredient have become popular food items in recent years. Microorganisms associated with these products determine their shelf-life, sensory appeal and safety. This thesis investigates the bacterial ecology of lettuce, aspects of their pre-harvest contamination with microorganisms, and the presence of antimicrobial constituents in such produce. Commercial pesticides (insecticides, herbicides, fungicides), used during lettuce cultivation were examined as potential sources of microbial contaminants. None of the pesticide concentrates contained viable microorganisms. After reconstitution in water, two of the pesticides supported growth of inoculated species of Pseudomonas, Salmonella and Escherichia coli. Pesticides reconstituted in agricultural waters (bore, dam and river) supported the growth of microorganisms (e.g. Pseudomonas, Acinetobacter, Aeromonas spp. and coliforms) naturally present in these waters. Unless properly managed, pesticide application could contribute microbial contaminants to vegetable produce, thereby affecting their quality. Bacterial species associated with retail samples of lettuce were examined by plate culture on Tryptone Soy Agar and PCR-DGGE analysis. Macerates and rinses of lettuce sub-samples with and without addition of Tween 80 were examined to maximize bacterial recoveries. Predominant bacteria isolated by agar culture included species of Pseudomonas, Agrobacterium, Curtobacterium and Burkholderia, at populations of 103-106 cfu/g. PCR-DGGE was unable to recover the same incidence of species as agar culture and failed to detect bacteria in many samples. In some samples, PCR-DGGE detected species of Bacillus, Pseudomonas, Serratia and Acinetobacter, not found by culture. Failure of the PCR-DGGE analyses was attributed to interference by plant chloroplast DNA. Preparative agarose gel electrophoresis of lettuce macerates was necessary to remove chloroplast DNA before application of PCR-DGGE analysis. Thirty percent of lettuce samples contained Acinetobacter species at 101-104 cfu/g when examined after culture on minimal salts agar or enrichment in Baumann enrichment medium. Other Acinetobacter media failed to give reliable isolation of these species from lettuce and salad vegetables. Lettuce could be an environmental source of Acinetobacter nosocomial infections. Juices, solvent extracts and supercritical fluid carbon dioxide extracts of lettuce and capsicum samples did not exhibit antimicrobial action against a range of food spoilage and pathogenic bacteria.

Ready-to-eat vegetable salads containing lettuce as a main ingredient have become popular food items in recent years. Microorganisms associated with these products determine their shelf-life, sensory appeal and safety. This thesis investigates the bacterial ecology of lettuce, aspects of their pre-harvest contamination with microorganisms, and the presence of antimicrobial constituents in such produce. Commercial pesticides (insecticides, herbicides, fungicides), used during lettuce cultivation were examined as potential sources of microbial contaminants. None of the pesticide concentrates contained viable microorganisms. After reconstitution in water, two of the pesticides supported growth of inoculated species of Pseudomonas, Salmonella and Escherichia coli. Pesticides reconstituted in agricultural waters (bore, dam and river) supported the growth of microorganisms (e.g. Pseudomonas, Acinetobacter, Aeromonas spp. and coliforms) naturally present in these waters. Unless properly managed, pesticide application could contribute microbial contaminants to vegetable produce, thereby affecting their quality. Bacterial species associated with retail samples of lettuce were examined by plate culture on Tryptone Soy Agar and PCR-DGGE analysis. Macerates and rinses of lettuce sub-samples with and without addition of Tween 80 were examined to maximize bacterial recoveries. Predominant bacteria isolated by agar culture included species of Pseudomonas, Agrobacterium, Curtobacterium and Burkholderia, at populations of 103-106 cfu/g. PCR-DGGE was unable to recover the same incidence of species as agar culture and failed to detect bacteria in many samples. In some samples, PCR-DGGE detected species of Bacillus, Pseudomonas, Serratia and Acinetobacter, not found by culture. Failure of the PCR-DGGE analyses was attributed to interference by plant chloroplast DNA. Preparative agarose gel electrophoresis of lettuce macerates was necessary to remove chloroplast DNA before application of PCR-DGGE analysis. Thirty percent of lettuce samples contained Acinetobacter species at 101-104 cfu/g when examined after culture on minimal salts agar or enrichment in Baumann enrichment medium. Other Acinetobacter media failed to give reliable isolation of these species from lettuce and salad vegetables. Lettuce could be an environmental source of Acinetobacter nosocomial infections. Juices, solvent extracts and supercritical fluid carbon dioxide extracts of lettuce and capsicum samples did not exhibit antimicrobial action against a range of food spoilage and pathogenic bacteria.