Micropollutants in closed life-support systems: the case of triclosan, a biocide excreted via urine
Abstract
OBJECTIVES: The impact of triclosan on the growth and physiology of the bacterium Rhodospirillum rubrum was studied in the frame of the regenerative life-support system, Micro- Ecological Life Support System Alternative (MELiSSA). A wide range of compounds, such as steroid hormones, pharmaceuticals and personal care products, might enter the life support system via the excrements that are to be treated and recycled. Triclosan was chosen as the first compound to be tested because MELiSSA is a closed system, which is consequently particularly sensitive to compounds inhibiting the microbial metabolism. Because triclosan is increasingly used as an antimicrobial biocide in hygienic formulations (such as toothpaste, mouthwash, deodorants, etc.) and due to its chemical stability, it is considered an emerging pollutant in terrestrial ecosystems. METHODS: In a first phase, the triclosan concentration expected in the life-support system was estimated, the Minimal Inhibitory Concentration (MIC) was determined via plating, and the effect on growth kinetics was assessed by comparing growth parameters in the Gompertz model. In a second phase, the secondary effects of triclosan on cell physiology and gene expression were studied through flow-cytometry and microarray analyses, respectively. RESULTS: Based on the pharmacokinetic data from literature, the predicted concentration range is estimated to be 6-25µg/L triclosan in the Rhodospirillum rubrum compartment of the MELiSSA. The minimal inhibitory concentration of triclosan was determined to be 71 µg/L after 7 days of exposure on Sistrom medium. Upon exposure to 50-200µg/L triclosan, triclosan-resistant mutants of Rhodospirillum rubrum arose spontaneously at high frequency (3.1 ∗ 10 - 4). Analysis of the growth kinetics of the wild-type revealed that triclosan causes an important elongation of the lag-phase and a decrease in growth rate. At concentrations higher than 75mg/L(LD = 500mg/L), triclosan is bactericidal to wild-type cells, which coincides with increased membrane permeability. Yet, triclosan depolarises the bacterial membrane by significantly reducing the membrane potential prior to being lethal. Therefore, the lysis of wild-type cells appears not to be directly associated with membrane depolarisation and is probably the result of a disturbance in the cellular envelope by triclosan. CONCLUSIONS: The triclosan concentrations expected in the MELiSSA are within the 'Predicted No Effect Concentration' range. In addition, the effect of triclosan on growth rate is minimal; even at sub-inhibitory effect concentrations, where triclosan is mainly influencing the lag-phase instead of the growth rate. Efficient reactor operation will therefore remain unchanged with slight modification of the operating parameters. However, since there are no clear indication that triclosan might be degraded in the system, except for the nitrifying compartment, triclosan might accumulate in the loop. Therefore, a thorough study of the effect of triclosan on the other compartments in the MELiSSA loop is desired, as well as potential countermeasures. Keywords: triclosan, Rhodospirillum rubrum, MELiSSA, microarray analysis, flow cytometry, chlorinated biphenylether, minimal inhibitory concentration.
- Publication:
-
37th COSPAR Scientific Assembly
- Pub Date:
- 2008
- Bibcode:
- 2008cosp...37.1945M