Skip to main content
ARS Home » Research » Publications at this Location » Publication #171613


item Fett, William

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2005
Publication Date: 4/1/2006
Citation: Fett, W.F. 2006. Inhibition of salmonella enterica by plant-associated pseudomonads in vitro and on sprouting alfalfa seed. Journal of Food Protection. 69(4):719-728.

Interpretive Summary: Foodborne illness due to consumption of alfalfa and other types of sprouts contaminated with bacterial pathogens such as Salmonella continues to be a concern in the U.S. and elsewhere. New, more effective intervention methods are required that can be used by both conventional and organic sprout growers. In this study we determined the ability of several plant-associated benign (not pathogenic towards humans, animals or plants) bacteria to inhibit the growth of Salmonella on various microbiological media as well as on growing alfalfa sprouts. Results indicated that one strain of the bacterium Pseudomonas fluorescens was very inhibitory towards Salmonella. Application of this bacterium to alfalfa seed inoculated in the laboratory with Salmonella reduced growth of the pathogen by greater than 99.9% during sprouting. The use of this antagonistic bacterium as a biological control agent against Salmonella has great promise for helping to ensure the microbial safety of alfalfa and other types of sprouts for the consumer.

Technical Abstract: Foodborne illness due to consumption of contaminated seed sprouts and other types of raw produce continues to be a concern. In contrast to a large number of studies on potential chemical and physical antimicrobial interventions, biological interventions for ensuring microbial safety have not received a great deal of attention. In this study we tested several antibiotic-producing bacteria representative of genera known for biological control of soilborne fungal diseases of plants for their ability to inhibit growth of five serovars of Salmonella enterica. A deferred agar spot bioassay method was employed using four different agar media. Three bacteria (Burkholderia cepacia ATCC 25416, Pseudomonas aeruginosa PAO1 and P. fluorescens 2-79) produced clear zones of inhibition dependent on media and indicator strain. Antibiosis by mutant strains of P. fluorescens 2-79 defective in production of the antibiotic phenazine-1-carboxylic acid and iron-binding siderophore was not reduced. Studies also indicated that media acidification, the production of bacteriocins and bacteriophage were not responsible for antibiosis. However, mutants defective in GacA (global antibiotic and cyanide control), the response regulator of the two-component global regulatory system GacA/GacS, were severely reduced in inhibitory activity. Growth of a five-strain cocktail of S. enterica was reduced by 5.6 log10 cfu/ml by 24 h of co-culture with P. fluorescens 2-79 in trypticase soy broth. Addition of P. fluorescens 2-79 to the seed soak water prior to germination of Salmonella-inoculated seed led to the greatest control of pathogen growth with an average reduction of greater than 3 log10 cfu/g at 6 days of sprouting. These results indicate that competitive exclusion strategies using fluorescent pseudomonads may be useful as an intervention strategy to help ensure the microbial safety of sprouts and possibly other types of produce.