Control of foodborne pathogens and soft-rot bacteria on bell pepper by three strains of bacterial antagonists

Forty-two representative strains of native bacteria associated with fresh peeled baby carrots were isolated and characterized. Two of these strains, identified as Pseudomonas fluorescens AG3A (Pf AG3A) and Bacillus YD1, were evaluated in conjunction with another known antagonist, P. fluorescens 2-79 (Pf 2-79), for their potential as biocontrol agents of human pathogens (Listeria monocytogenes, Yersinia enterocolitica, Salmonella enterica, and Escherichia coli O157:H7) and soft-rot bacteria (Erwinia carotovora subsp. carotovora, Pseudomonas marginalis, and Pseudomonas viridiflava). When grown on iron-deficient agar media, all three antagonists produced inhibition zones up to 25 mm in diameter against the growth of human pathogens and soft-rot bacteria. However, when grown on iron-rich agar media, only Pf 2-79 and Bacillus YD1 exhibited antimicrobial activity. Treatment of bell pepper disks with Pf 2-79 or Bacillus YD1 reduced the growth of pathogen by 1.4 to 4.1 log units, depending upon the ratio of the number of antagonist cells to pathogen cells (1:1, 10:1, 100:1, or 1,000:1). The greatest reduction was observed when 10- to 100-fold higher number of antagonists than pathogens was applied. Pf AG3A and Bacillus YD1 reduced the growth of pathogens on pepper disks at 20 degrees C but not at 10 degrees C. However, Pf 2-79 reduced the growth of L. monocytogenes and Y. enterocolitica by up to 4 log units at either 20 or 10 degrees C. Treatment of pepper disks with Pf 2-79 also reduced the incidence of soft rot induced by soft-rot bacteria by 40 to 70%. Pf 2-79 is the most effective of the three antagonists tested for control of spoilage bacteria and human pathogens on bell pepper.