Response of vegetable pads of two cactus cultivars to the presence of Salmonella strains

Tender cactus pads (cladodes) or nopalitos (Opuntia ficus-indica) are an important vegetable in Mexico. They are often pre-trimmed, cut and packaged, and while usually consumed cooked, they may also be eaten raw in salads. Salmonella is an enteropathogenic bacterium that can adapt to adverse environmental conditions. Although it has been reported that Salmonella-infected plants are asymptomatic, wilting, yellowing, biomass loss and a hypersensitive response (HR) have been demonstrated in plant tissues. The aim of this study was to investigate plant tissue response to different strains of Salmonella enterica in two cultivars of cactus as well as the persistence of Salmonella strains in the tissue. The top of pads (20-25 cm) of greenhouse-grown (20 degrees C, 51.2% RH) 'Milpa Alta' and 'Atlixco' cultivars were inoculated with water (control) or 1 mL 8.0 log CFU of strains Salmonella typhimurium (N4), Salmonella javiana (N7) isolated from nopalitos, and laboratory strain S. typhimurium ATCC 23564 (Sal 4). Cladodes (n=20) from each cactus cultivar were sampled and evaluated every 24 h for 4 days starting 6 h after inoculation for visual symptoms, color change and Salmonella persistence in the tissue. Data were subjected to analysis of variance with mean separation by T test (alpha=0.05). Color and appearance changed in longitudinal sections of pads of both cultivars only in the area where the bacteria were infiltrated and this could be considered to be a hypersensitive response. All three Salmonella strains persisted 48 h or longer.