Background: In Escherichia coli, the enzyme tryptophanase (TnaA) is responsible for the production of indole from tryptophan. However, the factors governing the production and excretion levels of indole by Escherichia coliremain uncertain, complicating the interpretation of experiments exploring the biological effects of indole at high concentrations.
The objective of the study:Most investigations into indole signaling focused on concentrations within the range of 0.5-0.6 mM, like those found in the supernatant of Escherichia colistationary phase. While reports on indole concentrations in human stool samples exhibit variability, the average concentration hovers around 2.6 mM. As a result, indole concentrations ranging from 0.5 to 1 mM are commonly considered physiologically relevant and are frequently used as experimental additives. Methods: We investigated the indole production capacity of three Escherichia coli strains under different conditions, including, medium composition, temperature, and pHto understand how these factors influenced the indole yield. Results: we demonstrated a direct relationship between tryptophan concentration in the growth medium and indole production, emphasizing the importance of this amino acid in metabolic pathways associated with indole synthesis. Moreover, we found that higher temperature and pH levels were linked to increased levels of indole production. Conclusion: This research underscores the intricate interplay between microbial metabolism and   environmental cues influencing the indole production in three Escherichia coli strains. These insights are crucial for optimizing conditions in laboratory-scale experiments focused on indole production processes, particularly in understanding its impact as a signal molecule on biological activity at various levels of interspecies and interkingdom interactions.