Survival of Biocontrol Pseudomonas Strains and Human Pathogens in Fruit Juices

Abstract

Rifampicin resistant Pseudomonas syringae 1.1S and rifampicin-tetracycline resistant P. fluorescens A506 were analysed for survival ability in nutrient broth and fruit juices, in different incubation condition individually and in co-inoculation with Salmonella sp. and Listeria sp. strains. Persistence of bacteria was monitored by using conventional plating methods and enrichment procedures, and identity of recovered bacteria was verified through antibiotic resistance. All strains, inoculated at a level of approximately 106 CFU ml−1, were able to growth in nutrient broth at 28°C in dynamic culture reaching population of 108 ÷ 1010 CFU ml−1 2 days after inoculation. P. syringae, P. fluorescens and Listeria sp. strains, when individually inoculated and co-inoculated in apple, orange and tomato juices, failed to survive 9 h after incubation at 28°C, whereas Salmonella sp. was recovered at population densities of 105 ÷ 107 CFU ml−1 48 h after inoculation. When incubated at 4°C in static culture, P. syringae was detected in orange and peach juices 24 h after inoculation, and in apple and tomato juices, respectively, 3 and 10 days after inoculation. P. fluorescens was recovered in peach juice 3 days after inoculation, in orange and apple juices 7 days after inoculation, and in tomato juice 55 days after inoculation. Salmonella sp. and Listeria sp. survived longer than Pseudomonads. They were detectable (102 CFU m1−1) in tomato juice 70 days after the inoculation, even if at low concentration in enrichment medium. Salmonella sp. and Listeria sp. survived 24 and 34 days respectively in orange juice, 24 days in peach juice, and 14 and 34 days respectively in apple juice. In co-inoculated juices, P. syringae and P. fluorescens populations were higher than in individually inoculated media, but their populations never influenced the growth of Salmonella sp. and Listeria sp..