Occurrence, impacts, and microbial transformation of 3-methylindole (skatole): A critical review

Odor pollution is one of the seven public nuisances prevalent worldwide causing continuously increasing public complaints over the last decades. 3-Methylindole (skatole), produced from tryptophan by anaerobic bacteria in the animal intestinal tract and environment, is one of the most notorious compounds with fecal odor in animal wastes, wastewater treatment plants, and public latrines. The environmental concentrations of skatole can even reach mg/L or mg/kg level, especially in animal wastes. In addition to the odor nuisance, evidence has shown that skatole contributes to the pneumotoxicity to rumen animals and humans, boar taint, and bacteriostatic effects, while our understanding of its ecological and physiological significance is still in infancy. The skatole production pathways and key enzymes have been revealed, however, the underlying mechanisms affecting skatole production remain elusive. Biodegradation is regarded as a feasible approach to remove skatole, yet the comprehensive understanding of the metabolic pathways under both aerobic and anaerobic conditions is still lacking, and the functional genes responsible for skatole biodegradation are still a black box. This review provides comprehensive information of the occurrence, environmental and biological impacts, and microbial transformation of skatole. Some important questions worthy of further investigation are also outlined.