ARS | Publication request: Growth of salmonellae on sprouting alfalfa seeds as affected by the inoculum size, native microbial load, and Pseudomonas fluorescens 2-79

Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 8, 2007
Publication Date: January 1, 2008
Citation: Liao, C. 2008. Growth of salmonella on sprouting alfalfa seeds as affected by the inoculum size, native microbial load, and Pseudomonas fluorescens 2-79. Letters in Applied Microbiology. 46:232-236.

Interpretive Summary: Consumption of contaminated fresh alfalfa, clover and mung bean sprouts has been implicated in more than 27 foodborne illness outbreaks between 1990 and 2005. The pathogens (or disease-causing bacteria) associated with most of these outbreaks such as salmonellae have been found to be originated from seeds used in sprouting. Development of an improved method to suppress the growth of pathogens on sprouting seeds is needed to ensure the safety of fresh sprout. This study was undertaken to investigate the growth dynamics of salmonellae on sprouting alfalfa seeds as affected by the initial pathogen number, the level of native microorganisms on alfalfa seeds, and by the application of an antagonistic bacterium ¿Pseudomonas fluorescens 2-79¿. The results presented in this report showed that the growth of salmonellae on alfalfa seeds usually reached the maximum 2 to 3 days after sprouting and at the same time frame when total microbial density on the surfaces of sprouting seeds also reached the maximum. This study also showed that treatment of alfalfa seeds with ¿Pseudomonas fluorescens 2-79¿ could reduce the growth of salmonellae on sprouting seeds by 99 to 99.9%. The potential of this antagonistic bacterium as a biological control agent for use in control of disease-causing bacteria on alfalfa sprouts looks promising and warrants further investigation.

Technical Abstract: The incidence of human illness associated with the consumption of fresh sprouts has increased very sharply during the past decade. The objective of this study was to investigate the growth dynamics of salmonellae on sprouting alfalfa seeds as affected by the inoculum size, native microbial load, and a strain of antagonistic bacterium Pseudomonas fluorescens 2-79. Alfalfa seeds pre-inoculated with different concentrations of salmonellae (' 101 to 103 CFU g-1) were sprouted in glass jars and the change in the number of salmonellae and total bacterial count was determined daily for up to 6 days. The population of salmonellae on alfalfa seeds treated with or without Ps. fluorescens 2-79 reached the maximum after 2 to 3 days of sprouting when total bacterial count also reached the maximum (109 CFU g-1). The population of salmonellae on seeds not treated with Ps. fluorescens 2-79 showed a net increase of 3 to 4 log units and was independent from the initial number of pathogens on seeds. However, the maximal population of salmonellae on seeds treated with Ps. fluorescens 2-79 showed a net increase of only 1 to 2 log units. Treatment of alfalfa seeds with calcium hypochlorite reduced viable cells of all microflora. During sprouting, salmonellae were able to able to regrow to levels higher than in the untreated control, possibly due to the reduction in competing microflora. The potential of applying Ps. fluorescens 2-79 or competitive exclusion as a biological-based intervention for use in control of foodborne pathogens on fresh sprouts warrants further investigation.