Date Published: January 28, 2019
Publisher: Public Library of Science
Author(s): Tujuba Ayele Tesso, Aijuan Zheng, Huiyi Cai, Guohua Liu, Pankaj Kumar Arora.
3-Methylindole (3MI) or Skatole is a volatile lipophilic organic compound produced by anoxic metabolism of L-tryptophan and associated with animal farming and industrial processing wastes. Pure cultures of bacteria capable of utilizing 3MI were isolated from chicken manure using enrichment culture techniques. The bacteria were identified as Acinetobacter toweneri NTA1-2A and Acinetobacter guillouiae TAT1-6A, based on 16S rDNA gene amplicon sequence data. The optimal temperature and pH for degradation of 3MI were established using single factor experiments. Strain tolerance was assessed over a range of initial concentrations of 3MI, and the effects of initial concentration on subsequent microbial 3MI degradation were also measured. During the degradation experiment, concentrations of 3MI were quantified by reverse-phase high-performance liquid chromatography (HPLC). The strains were capable of degrade initial concentrations of 3MI ranging from 65–200 mg/L. The degradation efficiency was >85% in 6 days for both strains when the initial concentration is less than 200 mg/L. The strains were tested for enzymatic activity using 65 mg/L 3MI. The enzyme extracts of NTA1-2A and TAT1-6A from the 3MI medium degraded 71.46% and 60.71% of 3MI respectively, but no appreciable change in 3MI concentration in the control group was witnessed. Our experiment revealed betaine and choline were identified as 3MI degradation metabolites by both strains while nitroso-pyrrolidine and beta-alaninebetaine formed by NTA1-2A and TAT1-6A strains respectively. The NTA1-2A and TAT1-6A strains removed 84.32% and 81.39% 3MI respectively from chicken manure during fermentation in 8 days and showed a statistically significant difference (P < 0.05) compared with the control group. The optimum temperature and pH were 31°C and 6 respectively, for 3MI degradation by A. toweneri NTA1-2A and A. guillouiae TAT1-6A. We concluded that A. toweneri NTA1-2A and A. guillouiae TAT1-6A are potential strains of interest to degrade 3MI and control odorant in poultry and other livestock industries.
3-Methylindole (3MI) is a common metabolite found in sewage and mammalian and avian feces, and is a well-known, foul-smelling fecal odorant [1–3]. The production and appearance of this compound has practical consequences for farm animal production, human health and environmental pollution [4, 5]. It is the third most harmful odorant in livestock farming, following ammonia and H2S. In high concentrations, 3MI not only pollutes the air in barns and around the farm, but also increases the risk of boar taint owing to animal’s absorption of 3MI from the air. Moreover, due to their methyl derivatives, 3MI and other indolic compounds have mutagenic properties . For instance, 3MI has been reported to cause severe pulmonary edema, emphysema, lung disease , hemoglobinuria, and hemolysis  in livestock such as cattle and goats. Due to the increasing demands of livestock production, 3MI is an environmental and public health issue contributing to the release of noxious odors on animal farms .
Screening indigenous microorganisms is an ideal approach to find effective methods to biodegrade pollutants. In the last two decades, many pollutant-degrading microbes have been isolated from the environments, including strains capable of heterotrophic ammonia removal  and degradation of long-chain hydrocarbons  and aromatic compounds such as phenol and chlorophenol [19–21]. Because 3-methylindole is hazardous to the environment and human and animal welfare, and is difficult to degrade in nature, numerous studies have isolated and evaluated bacterial strains capable of degrading 3MI from different environments [9, 13–16, 28–31]. Our paper is the first report that the genus Acinetobacter isolated from chicken manure is capable of degrading 3MI. It is well-known that Acinetobacter are ubiquitous and important soil organisms, where they contribute to the mineralization of different organic compounds. They are strictly aerobic, with oxygen as terminal electron acceptor. Along with our results, these studies imply that Acinetobacter spp. may be promising candidates for pollution abatement in livestock husbandry and other industries, such as the petrochemical industry.
This study is the first to identify 3MI degradation in Acinetobacter species. Two Acinetobacter strains capable of utilizing 3MI were isolated from chicken manure and identified as Acinetobacter towneri NTA1-2A and Acinetobacter guillouiae TAT1-6A based on 16S rDNA gene amplicon sequence analysis. The optimum conditions for the two strains are pH 6 and 31°C with 3MI as source of carbon. The strains can degrade initial concentration of 3MI ranging from 65 to 200 mg/L. The degradation efficiency is >85% in 6 days in both strains when the initial concentration is less than 200 mg/L. Our experiments revealed that betaine and choline were identified as 3MI degradation metabolites by both strains while nitroso-pyrrolidine and beta-alaninebetaine formed by NTA1-2A and TAT1-6A strains respectively. NTA1-2A and TAT1-6A have exhibited the capability of removing 3MI from chicken manure. We conclude that the two strains have the potential to minimize noxious fecal odorants due to indolic compounds in the poultry and other livestock industries. Hence, future studies should focus on the application of strains, and investigate the biochemical and genetic basis of degradation of 3MI by the two strains.