Phylogenetic Analysis of Triclosan-Sensitive and Triclosan-Tolerant Onion Rhizobacteria.

Triclosan (5-chloro-2-(2,4-dichlorophenoxy)-phenol), an antimicrobial agent, is a contaminant of emerging concern that is commonly found in surface streams recharged with treated wastewater. Effects of recycled waters used for agricultural irrigation on rhizobacteria is currently limited. In a previous study, onions were irrigated with triclosan at 0.00, 0.015, 0.15, and 1.5 µg/L concentrations. The isolated rhizobacteria (67 individual isolates) were screened for triclosan sensitivity using a disk diffusion assay at 0.00, 0.15 and 1.5, and 1,500 µg/L triclosan concentrations. A 1,500-bp sequence of the 16S rRNA gene was used to identify the isolates and compare phylotypes. Preliminary data shows that Proteobacteria and Actinobacteria are the only phyla determined to be triclosan sensitive among triclosan pre-exposed groups, whereas Actinobacteria, Bacterodetes, Firmicutes and Proteobacteria are found to be tolerant within the highest pre-exposure level. Of the pre-exposed group, 30% exhibit sensitivity to triclosan, while 18% of the unexposed group are resistant at the 1500µg/L triclosan concentration. Based on this data, pre-exposure largely determines triclosan resistance of rhizobacteria. Identification of triclosan-sensitive and triclosan-tolerant rhizobacteria and their phylogenetic relationship will allow us to better understand the impact triclosan may have on beneficial rhizobacteria for plant growth within agricultural ecosystems using recycled water.