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.

Microbiology of Hydroponically-Grown Lettuce.

The microbial quality of lettuce ( Lactuca sativa L. var. "Ostinata") cultivated in a hydroponic system was evaluated. Over a 3-month study period, samples of lettuce, nutrient solution, and peat-vermiculite growing mixture from the greenhouse were analyzed for total aerobic bacteria, yeasts, molds, and coliforms. There was a consistent amount of each type of organism occurring within each sample group for a given month, and the numbers of aerobic bacteria and coliforms present were generally similar to those reported for lettuce and leafy vegetable crops propagated by the conventional method of agriculture. Over the study period, the modal values for each type of organism in lettuce were: aerobic bacteria, 7.9 × 106 CFU/g (range of 3.8 × 104 to 2.3 × 108); coliforms, 1.5 × 104 CFU/g (range of none detected to greater than or equal to 5.3 × 107); molds, 2.9 × 103 CFU/g (range of 1.2 × 102 to 5.3 × 104); and yeasts, 2.4 × 104 CFU/g (range of 6.9 × 102 to 2.3 × 106). The primary organisms associated with the growing system were Citrobacter freundii , Enterobacter cloacae , and Enterobacter agglomerans . No organisms of human health concern (i.e. Salmonella spp., Clostrium botulinum , Escherichia coli , or Staphylococcus aureus ) were detected in the samples. The bacteriology of lettuce produced for market by this type of hydroponic farming and packaging appears to be generally comparable to that of field-grown lettuce and to present no unique microbiological hazards to consumers.