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Analysis
of Native Microflora and Selection of Strains Antagonistic to Human Pathogens on
Fresh Produce
Journal
of Food Protection: Vol. 64, No. 8, pp. 1110–1115.
Ching-Hsing
Liao 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 each
produce or seed type as determined on Pseudomonas agar F (PAF) with incubation
at 28°C was in the range of 4 to 7 log CFU per g of tissue or seed. There was
no significant difference (P >0.05) in APC when the
determinations were made with three agar media including PAF, brain heart
infusion agar, and plate count agar. However, the APC as determined from plates
that were incubated at 28°C was significantly (P <0.05) higher than with incubation
at 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-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, or 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. (three strains), Pseudomonas aeruginosa (one strain), Pseudomonas
fluorescens (strain A3), and yeast (strain D1). When green pepper disks were
inoculated 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 a
period of 3 days. Application of strains A3 and D1 as potential biopreservatives
for enhancing the quality and safety of fresh produce is discussed.
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