Purpose: The aim of this work was to evaluate the ability of Salmonella to adhere and to colonize on the surface of apple varieties Rayada, Red Delicious and Golden Delicious cultivated in Queretaro, Mexico.
Methods: Salmonella strains resistant to rifampicin were used: one strain of S. Typhimurium, another strain of Salmonella spp. isolated from composta and three strains of Salmonella spp. isolated from an apple orchard (soil, fecal materia and apple). Rayada Red Delicious and Golden Delicious apples were inoculated (with mixture and individual strains) by partial immersion (3 h at 22 °C) in Salmonella suspension (7 log CFU/ml). The apples were washed with peptone diluent. Washed apples were exposed to soil suspension (10% w/v) and stored at 22 °C for 48 h. Periodically, three units of each variety were taken off and washed. Attached cells were removed and quantified in tripticase soy agar (rifampicin 200 ppm). In apples inoculated with individual strains, attached cells were only quantified at 0 and 48 h of storage. Salmonella adhesion and/or biofilm formation were investigated by scanning electron microscopy.
Results: Adhesion ability of Salmonella on the three apple varieties was similar. Adhesion percentage observed were 0.88, 0.95 and 1.27% for Rayada, Red Delicious and Golden Delicious, respectively. Maximum population reached during Salmonella colonization was similar among the apple varieties (5.1-5.8 log CFU/apple). The Salmonella strain isolated from soil and growing on Rayada apple showed the biggest population increase after 48 h (2.2 log CFU). Scanning electron microscopy observations showed a structured biofilm.
Significance: Salmonella showed an ability to colonize apple surface with soil solids as nutrient. Biofilm formation could improve the pathogen survival on the friut.