Culture-Independent Analysis of Microbial Communities Associated with Hydroponically Grown "Living Lettuce"

The bacteria associated on and within a plant make up that plant’s microbiome. Given recent interest in how microbes can affect human health, as well as developments in DNA sequencing that allow us to examine microbial communities without culture techniques, a number of recent studies have investigated the composition of the microbial communities on edible plants. Leafy green salad vegetables pose an added interest, since they are consumed without sterilization or cooking. This study investigated the bacterial composition of endophytic and epiphytic communities of hydroponically grown green leaf lettuce that is sold with the roots intact and referred to as “living lettuce.” In addition to examining the composition of the bacterial communities associated with living lettuce, comparisons were made to conventionally grown and packaged lettuce, both at the time of purchase and following refrigerator storage to reflect the time of consumption. The structure of bacterial communities was determined using next generation sequencing of the 16S rRNA gene. Bacterial communities on living lettuce were significantly different from those observed on conventional lettuce. While differences between the dominant phyla and subphyla of communities of the two types of lettuce were observed on the day of purchase, community composition became more similar following two weeks of refrigeration. As expected, community diversity decreased across both types of lettuce following refrigerator storage. Major taxa observed were consistent with results from previous studies of the bacterial communities of salad greens, and there were no significant populations of known human pathogens such as Salmonella spp. and Escherichia coli. This study demonstrates that the bacteria associated with hydroponically grown living lettuce, while different than that of traditionally grown and packaged lettuce, presents no apparent risk to the consumer in terms of its associated bacteria.