# Indole Degradation in a Model System and in Poultry Manure by _Acinetobacter_ spp.
## 4 Discussion  
Indole biodegradation was studied in the last two decades using bacteria from different sources; marine sediments [13; 20], sewage plant [49; 50], and evaluated their degradation potential. For instance, [PERSON] and [PERSON] [1] studied _Desulfobacterium indolicum_ gen. nov., sp. nov. anaerobically degrading \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Indole Removal Efficiency from Chicken Manure (\%)**} \\ \hline
**Time (d)** & **NTA1-2A** & **TAT1-6A** & **Mix of Strains** & **Control** & \(p\)**-Value** \\ \hline
2 & \(58.73\pm 5.81\)  & \(69.55\pm 1.57\)  & \(0.025\) \\
4 & \(72.38\pm 3.66\)  & \(85.64\pm 1.59\)  & \(0.001\) \\
6 & \(74.49\pm 2.63\)  & \(78.62\pm 3.10\)  & \(0.002\) \\
8 & \(78.67\pm 1.70\)  & \(83.70\pm 1.02\)  & \(0.000\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Indole removal efficiency of NTA1-2A, TAT1-6A and the mix of strains from poultry manure. Data values are mean \(\pm\) SD of triplicate assays. Figure 7: Indole removal test by extract or supernatant fluid obtained from medium cultures of NTA1-2A and TAT1-6A. Error bars: Mean \(\pm\) SE of three replicates. 1.5 mM of indole added to sulfate-rich marine mud or sulfate-free sewage digester sludge within one week. [PERSON] et al. [20] reported _Pseudomonas aeruginosa_ Gs degrading indole completely in four days when the concentration was 3.0 mM. Recently, _Acinetobacter_ strains named NTA1-2A and TAT1-6A obtained from 3-methylindole acclimated samples [47] were used to degrade indole. The strains also utilized indole as their sole carbon source and efficiently removed \(<\)300 mg/L indole within six days when cultured in simple mineral salt medium at 31\({}^{\circ}\)C and pH 6. _Cupiravidus_ strains named IDO and SHE also utilized indole as their sole carbon source and efficiently removed 100 mg/L indole within a day [9; 51]. [PERSON] et al. [52] reported that strain _Acinetobacter baumannii_ ATCC1960 could detoxify indole by converting it to non-toxic indigo. [PERSON] et al. [41] also investigated _Acinetobacter_ sp. O153 which degraded indole via indole-2,3-dihydrodiol, 3-hydroxyindolin-2-one. In another study, _Acinetobacter_ sp. JW and _Burkholderia_ sp. IDO3 exhibited degradation ability of 100 mg/L indole within 14 and 20 h, respectively [42; 43]. [PERSON] et al. [44] reported that _Acinetobacter pittii_ L1 can completely remove 400 mg/L indole within 48 h. There are also other indole-degrading strains such as _Arthobacter_ sp., _Alcaligenes_ sp., _Burkholderia unamae_, _Pseudomonas aeruginosa_, and _Agrobacteriumium tunefaciens_ reported [53; 54; 55; 56]. The indole biodegradation results obtained in the current study by strain TAT1-6A is similar with _Pseudomonas aeruginosa_ (300 mg/L) reported by [PERSON] et al. [20]. NTA1-2A indole removal ability was low when compared with TAT1-6A, however better than _Pseudomonas_ sp. ST-200 (100 mg/L) [28] and _Asprigillus niger_ (180 mg/L) [2]. Different environmental factors are responsible for the metabolism of indole. [PERSON] et al. [24] reported that pH, temperature, and concentration of sewage sludge influence indole metabolism under anaerobic conditions. [PERSON] et al. [20] discovered pH and salinity of media solution influences the degradation activity and proliferation of _Pseudomonas aeruginosa_ Gs isolated from mangrove sediment. In our experiment, temperature and pH highly affected the degradation of indole (Figures 4 and 5) and our two strains showed better indole removal ability at 28-31 \({}^{\circ}\)C and pH 6-7 which is similar with _Acinetobacter_ sp. JW [43]. In _Burkholderia_ sp. IDO3, [PERSON] et al. [42] reported indole degradation performance maintained under conditions of pH 4.0-9.0 and temperature 25-35 \({}^{\circ}\)C which is a broad range compared with NTA1-2A and TAT1-6A strains. Studies revealed that indole has microbial specific inhibitory effect on their growth [57; 58; 59]. [PERSON] et al. [60] reported indole and its derivatives inhibit the growth of microorganisms and biofilm formation.