Effect of Triclosan-Tolerant Plant Growth-Promoting Rhizobacteria on Triclosan Degradation in Soils
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Triclosan (TCS), an antimicrobial found in commercial products, can also be found in freshwater systems used for crop irrigation. Thus, TCS may accumulate over time in soils, potentially affecting plant growth-promoting rhizobacteria (PGPRs). Bacterial isolates from a previous study which demonstrated PGPR activity and TCS tolerance (TPGPRs) were chosen to evaluate the use of triclosan as a sole carbon source on a minimal salts medium (MSMT). TPGPRs, which demonstrated TCS utilization, were chosen for inoculation into soil microcosm for triclosan degradation (TD-PGPRs). Soil microcosms were arranged in a 2 x 5 factorial randomized block design and irrigated with triclosan treatments (0 µg or 1.5 µg of total TCS) and five inoculation treatments (no inoculation [CTRL], individual rhizobacteria isolates [OT1-11C2, OT2-03A, OT2-17], and a microbial consortium of all selected isolates [OT-MIX]). Soils were subsampled every three days during a 30-day period for triclosan and methyl TCS extraction, followed by GCMS analysis. A two-way repeated measures ANOVA determined that inoculum and inoculum x concentration was significant across all treatments and time points for mean heterotrophic bacterial counts (p < 0.0200), with the exception of day 0 R2AT 15 ng g-1. There was an observed increase in mean heterotrophic bacterial counts for total (R2A) and triclosan tolerant populations (R2AT 1.50 µg L-1 TCS and R2AT 15 µg L-1 TCS) in OTMIX and OT1-11C2 for days 15 and 30. The data suggests that triclosan is still accumulating even with the presence of TD-PGPRs. However, by day 30 samples inoculated with OTMIX (14.22 ng g-1) demonstrated a higher triclosan reduction when compared to the CTRL (26.03 ng g-1). Methyl triclosan (initial biodegradation by product of bacteria) was observed to increase overtime for all treatments, indicating that TCS degradation is still occurring even in uninoculated soils (CTRL). However, TCS breakdown was less efficient in the CTRL treatment with 21.89 ng g-1 of MeTCS to 28.19 ng g-1 in the OTMIX soils. This study was the first to confirm the TCS degradative capabilities of the isolated TD-PGPRs. The use of these TD-PGPRs assisted in the elucidation of the fate of triclosan and by-product formation in irrigated soils.