|Indole molecule (C8H7N). Source: wikimedia commons|
When a bacterial infection is treated with antibiotics, bacteria that are in a so-called dormant, inactive state may escape death – this because antibiotics only kill growing bacteria. It becomes a serious problem when these sleeping beauties start to grow again, in particular when they do so after the period of antibiotic treatment has ended… Thus, an infection that was apparently cured could be followed by a secondary infection days or weeks later. This problematic phenomenon is called bacterial persistence, and it should not be confused with bacterial resistance, in which growing bacteria are immune to one or several antibiotics.
Now what about indole? (The molecule displayed on top of this post.) Actually indole is present in very common and important biomolecules, such as the amino acid tryptophan, the animal hormone serotonin and the plant growth hormone auxin. We have known for more than a century that E. coli produces indole in stationary phase (Lee, 2010), and it does so thanks to an enzyme called tryptophanase, which cleaves tryptophane into indole, pyruvate and ammonia.
But E. coli is not the only bacterium capable of that: more than 85 species (both Gram-negative and Gram-positive) can synthesize indole (Lee, 2010). For a long time the biological functions of indole were overlooked, but now we know that indole can act as an extracellular signal and can for instance increase antibiotic resistance and control biofilm formation in E. coli.