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Inhibition of
Salmonella enterica by plant-associated pseudomonads in vitro and on sprouting
alfalfa seed.
J Food Prot. 2006 Apr;69(4):719-28.
Fett WF.
Food Safety Intervention Technologies Research Unit, Eastern Regional Research
Center, Agricultural Research Service, US Department of Agriculture, Wyndmoor,
Pennsylvania 19038, USA.
Foodborne illness due to the consumption of contaminated raw or lightly cooked
sprouts is a continuing food safety concern. In this study, we tested several
plant-associated pseudomonads for their ability to inhibit the growth of
Salmonella enterica both in vitro and in situ. An agar spot bioassay method was
used with three different media. Only Pseudomonas fluorescens 2-79 produced
clear zones of inhibition when tested against five serovars of S. enterica, and
activity was dependent on media type and serovar. The antibiosis by derivative
strains of P. fluorescens 2-79 defective in the production of
phenazine-1-carboxylic acid and fluorescent siderophore was not reduced,
indicating that these known antimicrobial metabolites were not responsible for
the inhibition observed in our studies. However, mutants defective in the
regulatory gene gacS (global antibiotic and cyanide control) were severely
reduced in inhibitory activity. In tryptic soy broth, the control cultures of a
cocktail of S. enterica strains reached approximately 10 log CFU/ml by 24 h but,
when coinoculated with P. fluorescens 2-79, reached only approximately 5 log
CFU/ml. The addition of P. fluorescens 2-79 to the seed soak water prior to
the germination of alfalfa seed previously inoculated with a cocktail of S.
enterica strains led to an average reduction of 5 log CFU/g at 6 days of
sprouting without an adverse effect on sprout yield or appearance. Time
course studies indicated that S. enterica outgrowth was controlled on days 1
through 6 of sprouting. Competitive exclusion as a potential food safety
intervention for seed sprouts merits further study.
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