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.