|Liao, Ching Hsing|
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2001
Publication Date: 4/12/2001
Citation: Liao, C., Fett, W.F. 2001. Analysis of active microflora and selection of strains against human pathogens on fresh produce. Journal of Food Protection V.64 (No. 8) p.1110-1115.
Interpretive Summary: The incidence of human illness associated with the consumption of fresh produce and sprouts has increased very dramatically during the past two decades. Disease-causing bacteria such as Salmonella and E. coli have been implicated as the cause of several large and well-publicized outbreaks. Very little is presently known about how these disease-causing bacteria survive, grow, and compete with the resident microorganisms naturally present on the surface of fresh produce and sprouting seeds. In this study, the authors analyzed the microorganisms naturally present on three types of produce (Romaine lettuce, green pepper, and pre-peeled baby carrots) and two types of sprouting seeds (alfalfa and clover). Several groups of bacteria and yeasts that may cause spoilage of these produce were identified. More than six strains of native bacteria and yeast from fresh produce and sprouting seeds were isolated, which inhibited the growth of food-borne human pathogens including Salmonella, E. coli, and Listeria monocytogenes on culture media and on fresh-cut green bell pepper slices. When green pepper slices were treated with two of these antagonistic strains, the growth of L. monocytogenes and Salmonella was reduced by approximately 99% and 90%, respectively. The feasibility of using these two antagonistic strains as biopreservatives to suppress the growth of human pathogens and spoilage-causing bacteria on fresh produce and seed sprouts is now under investigation. This study will lead to the develop- ment of a new treatment method for this fresh produce industry.
Technical Abstract: The native microflora of three types of produce (green bell peppers, Romaine lettuce, and pre-peeled baby carrots) and two types of sprouting seeds (alfalfa and clover) were investigated. Total aerobic count (TAC) for each produce or seed type as determined on Pseudomonas agar F (PAF) with incubation at 28oC was in the range of 4 to 7 log cfu per gram of tissue or seed. Fluorescent pseudomonads accounted for 23 to 73% of TAC and 6 to 18% of TAC recovered from carrots, pepper, and lettuce were pectolytic. Forty-eight strains of pectolytic bacteria were randomly isolated and identified respectively as members of the genera of Pseudomonas, Erwinia, Bacillus, Xanthomonas, or Flavobacterium. Lactic acid bacteria and/or yeast were consistently isolated from baby carrots, lettuce, and sprouting seeds (alfalfa or clover) but not from green bell peppers. Approximately 120 strains of indigenous microflora were tested for their ability to inhibit the growth of Salmonella Chester, Listeria monocytogenes, Escherichia coli, Erwinia carotovora subsp. carotovora on PAF. Six isolates capable of inhibiting the growth of at least one pathogen were isolated and identified respectively as Bacillus spp. (3 strains), Pseudomonas aeruginosa (1 strain ), P. fluorescens (strain A3), and yeast (strain D1). When green pepper disks were inoculated with strains A3 and D1, the growth of S.Chester and L. monocytogenes on the disks was reduced by 1 and 2 logs, respectively, over a period of 3 days.