Villarreal Silva, Mariana (2016-08). Role of Epiphytic Bacteria in the Colonization of Fruits and Leafy Greens by Foodborne Bacterial Pathogens. Doctoral Dissertation.
Thesis
The epiphytic bacteria content in fruits and leafy greens and their effect toward the colonization of foodborne bacterial pathogens was studied. Populations of mesophilic, lactic acid, coliform, and psychrotrophic bacteria were recovered from cantaloupe, tomato, pepper, spinach, endives, and parsley, and the effect of environmental and agricultural conditions toward epiphytic bacteria content was evaluated. The epiphytic bacteria content was variable by commodity, with cantaloupes and spinach being the most populated commodities. The environmental temperature and the irrigation method also affected the epiphytic bacteria content. To determine the inhibitory effect of epiphytic bacteria toward Escherichia coli O157:H7 and Salmonella enterica serovar Saintpaul, 9,307 isolates were evaluated in vitro. In total, 2.6, 0.7 and 6.4% of the isolates were antagonistic toward E. coli O157:H7, S. Saintpaul, or both pathogens, respectively. Most antagonistic isolates were psychrotrophs and lactic acid bacteria. Overall, more antagonistic isolates from fruits were found in samples collected in the fall than the summer. Further biochemical identification revealed that most of the antagonistic psychrotrophs were Alcaligenes faecalis sbsp. faecalis. In fruits, most of the antagonistic isolates were Leuconostoc, Enterococcus, and Streptococcus species. Furthermore, the effect of epiphytic bacteria toward S. Saintpaul growth in fruits and toward E. coli O157:H7growth in leafy green leaves was studied in the plant surface. Enterococcus kobei and Enterococcus casseliflavus from cantaloupe, and of Staphylococcus hominis subsp. hominis from tomato inhibited S. Saintpaul on cantaloupe rind, and tomato skin, respectively. Similarly, Enterococcus faecalis affected S. Saintpaul on peppers and Gemella morbillorum, Enterococcus gallinarum, and Bacillus mycoides affected E. coli O157:H7 growth on parsley. The effect of Streptococcus alactolyticus, Bacillus licheniformis, Gemella bergeri, Staphylococcus sciuri, and Enterococcus gallinarum toward E. coli O157:H7 growth and stomata invasion in endives was observed using confocal microscopy. After 24 h, E. coli O157:H7 growth was moderately inhibited by all epiphytic isolates tested. However, after three days, treated and control samples presented similar pathogen growth. The results from this study demonstrated that some epiphytic bacteria from fruits and leafy greens are potential biocontrol agents, able to reduce the proliferation of E. coli O157:H7 and S. enterica in fruits and vegetables.
