Purpose: To determine the bacterial loads on leafy green surfaces and identify epiphytic bacteria antagonistic to the enteric pathogens Escherichia coli O157:H7 and Salmonella enterica serovar Saintpaul.
Methods: Spinach, lettuce, and parsley (50 samples each) were collected from different fields in Texas. Mesophiles, lactic acid bacteria (LAB), coliforms and psychrotrophs were enumerated. One to 10 colonies per sample and bacterial group were tested for antagonistic effects against E. coli O157:H7 and Salmonella Saintpaul using the agar spot method. Antagonistic isolates were then identified using a Vitek-2 system.
Results: Mean bacterial populations on parsley, lettuce, and spinach were 3.0 ± 0.7, 5.2 ± 0.6 and 6.0 ± 0.6 log CFU/g for coliforms, 3.0 ± 0.4, 4.6 ± 0.5 and 5.2 ± 0.8 log CFU/g for LAB, 4.6 ± 0.4, 6.1 ± 0.4, and 6.9 ± 0.5 log CFU/g for mesophiles and 4.2 ± 0.8, 5.0 ± 0.5, and 6.4 ± 0.6 log CFU/g for psychrotrophs, respectively. Spinach presented the highest overall bacterial populations, while lowest populations were found on parsley (P < 0.05). Antagonists of enteric pathogens from lettuce were Bacillus licheniformis and pumilus, Gemella bergeri, Leuconostoc mesenteroides, Listeria grayi, Pantoea spp., Pediococcus pentosaceus, Staphylococcus sciuri, Streptococcus sanguini, mutans and alactolyticus, and Vagococcus fluvialis. Antagonistic isolates from spinach included Cupriavidus pauculus, Enterococcus cecorum, and Kocuria kristinae. In parsley, pathogen antagonists identified were Gemella morbillorum, Staphylococcus intermedius and gallinarum, Pseudomonas paucimobilis, Serratia plymuthica, and Providencia rettgeri. Antagonistic isolates recovered from multiple commodities included Enterococcus casseliflavus, and Pseudomonas pseudoalcaligenes (spinach, parsley), Enterococcus gallinarum, Myroides spp. and Staphylococcus lentus (parsley, lettuce), Aerococcus viridans and Alcaligenes faecalis (all commodities).
Significance: The populations of epiphytic bacteria on leafy greens were affected by commodity type, and included bacteria showing antagonism against Salmonella and E. coli O157:H7. These bacteria may be useful biocontrol agents for controlling enteric pathogens.