Analysis of Native Microflora and Selection of Strains Antagonistic ToHuman Pathogens On Fresh Produce

Analysisof Native Microflora and Selection of Strains Antagonistic to Human Pathogens onFresh Produce

Journalof Food Protection: Vol. 64, No. 8, pp. 1110-1115.

Ching-HsingLiao and William F. Fett
USDA, ARS, Eastern Regional Research Center

Abstract-The native microflora of three types of produce (green bell peppers,Romaine lettuce, and prepeeled baby carrots) and two types of sprouting seeds(alfalfa and clover) were investigated. Aerobic plate count (APC) for eachproduce or seed type as determined on Pseudomonas agar F (PAF) with incubationat 28°C was in the range of 4 to 7 log CFU per g of tissue or seed. There wasno significant difference (P >0.05) in APC when thedeterminations were made with three agar media including PAF, brain heartinfusion agar, and plate count agar. However, the APC as determined from platesthat were incubated at 28°C was significantly (P <0.05) higher than with incubationat 37°C. Fluorescent pseudomonads accounted for 23 to 73% of APC and 6 to 18%of APC recovered from carrots, pepper, and lettuce were pectolytic. Forty-eightstrains 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 wereconsistently isolated from baby carrots, lettuce, and sprouting seeds (alfalfaor clover) but not from green bell peppers. Approximately 120 strains ofindigenous microflora were tested for their ability to inhibit the growth ofSalmonella Chester, Listeria monocytogenes, Escherichia coli, or Erwiniacarotovora subsp. carotovora on PAF. Six isolates capable of inhibiting thegrowth of at least one pathogen were isolated and identified, respectively, asBacillus spp. (three strains), Pseudomonas aeruginosa (one strain), Pseudomonasfluorescens (strain A3), and yeast (strain D1). When green pepper disks wereinoculated with strains A3 and D1, the growth of Salmonella Chester and L.monocytogenes on the disks was reduced by 1 and 2 logs, respectively, over aperiod of 3 days. Application of strains A3 and D1 as potential biopreservativesfor enhancing the quality and safety of fresh produce is discussed.