Effect of antimicrobial washout from anaerobic digesters on microbial community composition†
The antimicrobials triclosan (TCS) and triclocarban (TCC) were banned from over-the-counter hand soaps by the U.S. Food and Drug Administration. The objective of this research was to determine the impact of removing these antimicrobials on microbial communities in anaerobic digesters. For this, lab-scale digester feed was amended with either TCS or TCC along with synthetic primary sludge for greater than three solid retention time (SRT) values. Subsequently, TCS and TCC feed amendment were eliminated (washout). Microbial communities before and after antimicrobial washout in the digesters were analyzed via Illumina sequencing of 16S rRNA genes. Interestingly, methane production was greater after washout in digesters that were previously fed triclosan or triclocarban. The bacterial community structures based on non-metric multidimensional analysis (nMDS) in antimicrobial-fed digesters prior to washout were different from those in digesters not fed antimicrobials. However, all digester communities were more similar on day 180, after washout, and the similarity was comparable to that observed prior to antimicrobial addition. The return of the bacterial community composition in amended digesters to that of control digesters after antimicrobial amendment was stopped suggests that, if the loading of TCC or TCS is reduced or stopped, the bacterial community structure will return to that observed before antimicrobials were fed. A persistent difference in abundance of genera key to methanogenesis and anaerobic digestion was observed though after washout. Gene copies of methyl-coenzyme M reductase, a key enzyme for methanogenesis, was higher in triclocarban-amended digesters after washout. It is possible that functional performance of digesters exposed to these antimicrobials could improve after chemical washout.