Effect of the Normal Microflora on Survival of Salmonella typhimurium Inoculated into a Hydroponic Nutrient Solution

Salmonella typhimurium (ATCC 14028) was inoculated into nutrient solution collected from a hydroponic lettuce farm. The objective was to determine the effect of the presence or absence of the normal microflora of the nutrient solution on growth and persistence of Salmonella . In the unsterilized nutrient solution, Salmonella did not exceed 10⁴ CFU ml^-1, even when introduced at that concentration in the presence of a starting total aerobic count of 10³ CFU ml^-1 for the normal flora. Growth of Salmonella appeared to be suppressed, while that of the normal flora was unaffected and reached the usual level of 10⁵ - 10⁶ CFU ml^-1 by 24 h. The normal microflora apparently restricted growth of Salmonella , and by 48 h after its introduction, Salmonella counts were decreasing. Salmonella inoculated into filter-sterilized nutrient solution grew rapidly to as high as 10⁸ CFU ml^-1, demonstrating that the nutrient solution contained the elements necessary to promote exponential growth of the bacterium. However, the fact that these levels were not achieved in the presence of other organisms, strongly suggests that Salmonella could not compete favorably with the normal flora of the hydroponic system.