Analysis of native microflora and selection of strains antagonistic to human pathogens on fresh produce

The native microflora of three types of produce (green bell peppers, Romain e lettuce, and prepeeled baby carrots) and two types of sprouting seeds (al falfa and clover) were investigated. Aerobic plate count (APC) for each pro duce or seed type as determined on Pseudomonas agar F (PAF) with incubation at 28 degreesC was in the range of 4 to 7 log CFU per g of tissue or seed, There was no significant difference (P greater than or equal to 0.05) in A PC when the determinations were made with three agar media including PAF, b rain heart infusion agar, and plate count agar. However, the APC as determi ned from plates that were incubated at 28 degreesC was significantly (P : 0 .05) higher than with incubation at 37 degreesC. Fluorescent pseudomonads a ccounted for 23 to 73% of APC and 6 to 18% of A-PC recovered from carrots, pepper, and lettuce were pectolytic. Forty-eight strains of pectolytic bact eria 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 carr ots, lettuce, and sprouting seeds (alfalfa or clover) but not from green be ll peppers. Approximately 120 strains of indigenous microflora were tested for their ability to inhibit the growth of Salmonella Chester, Listeria mon ocytogenes, Escherichia coli, or Erwinia carotovora subsp. carotovora on PA F Six isolates capable of inhibiting the growth of at least one pathogen we re isolated and identified, respectively, as Bacillus spp. (three strains), Pseudomonas aeruginosa (one strain), Pseudomonasfluorescens (strain A3), a nd yeast (strain DI). When green pepper disks were inoculated with strains A3 and DI, the growth of Salmonella Chester and L. monocytogenes on the dis ks was reduced by 1 and 2 logs, respectively, over a period of 3 days. Appl ication of strains A3 and Dl as potential biopreservatives for enhancing th e quality and safety of fresh produce is discussed.