Sharing of bacteria between water and plants in wastewater treatment and hydroponic production

To assess bacterial sharing between plants and their water environments by identifying bacteria present both in surrounding water and as plant endophytes in soft rush (Juncus effusus) grown in a full-scale floating wastewater treatment wetland, lettuce (Lactuca sativa) or cress (Lepidium sativum) in commercial hydroponic production systems, and lettuce seeds germinated in tap water inoculated with a blaKPC-positive Escherichia coli strain.

The microbiological analysis included enumeration of coliforms and total heterotrophic bacteria, quantification of the genes 16S rRNA, intI1, and uidA, and bacterial community analysis based on 16S rRNA gene metabarcoding. Of the bacterial genera identified in wastewater (n = 2220) and water (n = 1631 cress; n = 1170 lettuce), at least a quarter were also detected as endophytes in roots, and mainly belonged to the phylum Pseudomonadota. The genes uidA and intI1 were significantly more prevalent (∼1 log-unit/16S rRNA gene) in wastewater than in soft rush roots. In the hydroponic systems, the gene uidA was not detected and the gene intI1, detected in cress but not in lettuce, was significantly more prevalent in water (∼1 log-unit/16S rRNA gene) than in the roots. In lettuce seeds germinated in tap water inoculated with E. coli, uptake values of 11%-48% of the gene blaKPC were observed.

These results suggest that plants share a significant proportion of the bacteria thriving in their external environment, highlighting the importance of microbiological water quality. Antibiotic-resistant bacteria or pathogens can be taken up alongside with natural microbiota, a potentially critical human health hazard for edible crops.