Identification of Indole Derivatives as Self-Growth Inhibitors of Symbiobacterium thermophilum , a Unique Bacterium Whose Growth Depends on Coculture with a Bacillus sp

Abstract

Symbiobacterium thermophilum is a syntrophic bacterium whose growth depends on coculture with a Bacillus sp. Recently, we discovered that CO 2 generated by Bacillus is the major inducer for the growth of S. thermophilum; however, the evidence suggested that an additional element is required for its full growth. Here, we studied the self-growth-inhibitory substances produced by S. thermophilum. We succeeded in purifying two substances from an ether extract of the culture supernatant of S. thermophilum by multiple steps of reverse-phase chromatography. Electron ionization mass spectrometry and nuclear magnetic resonance analyses of the purified preparation identified the substances as 2,2-bis(3-indolyl)indoxyl (BII) and 1,1-bis(3-indolyl)ethane (BIE). The pure growth of S. thermophilum was inhibited by authentic BII and BIE with MICs of 12 and 7 g/ml, respectively; however, its growth in coculture with Bacillus was not inhibited by BII at the saturation concentration and was inhibited by BIE with an MIC of 14 g/ml. Both BII and BIE inhibited the growth of other microorganisms. Unexpectedly, the accumulation levels of both BII and BIE in the pure culture of S. thermophilum were far lower than the MICs (<0.1 g/ml) while a marked amount of BIE (6 to 7 g/ml) equivalent to the MIC had accumulated in the coculture. An exogenous supply of surfactin alleviated the sensitivities of several BIE-sensitive bacteria against BIE. The results suggest that Bacillus benefits S. thermophilum by detoxifying BII and BIE in the coculture. A similar mechanism may underlie mutualistic relationships between different microorganisms.Symbiobacterium thermophilum is a thermophilic bacterium that was originally isolated by its ability to produce a thermostable tryptophanase from compost collected at Hiroshima, Japan (19). This bacterium is characterized by a syntrophic property that is reproduced under a laboratory culture condition; although it does not grow or shows impaired growth with a very low cellular yield in pure culture, it effectively propagates, achieving a density of approximately 5 ϫ 10 8 cells/ml, when it is cocultured with the cognate Bacillus sp. strain S, which was isolated together with S. thermophilum (19). The molecular phylogenetic and genomic sequencing studies (13, 22) uncovered unique genetic and taxonomic features of S. thermophilum. S. thermophilum exhibits a number of genetic features associated with Firmicutes (low-GϩC, gram-positive bacteria represented by Bacillus and Clostridium), despite its high GϩC content (68.5%). This unusual property has now become a new topic in bacterial systematics (2, 4) and suggests that this kind of bacterium has been left uncharacterized due to its "unculturable nature," despite its wide distribution in the natural environment (20). We are interested in the fundamental details of the syntrophic feature of S. thermophilum and expect that this information will provide us with new knowledge regarding not only the microbial physiology but also the issue of unculturability of environme...

Growing Unculturable Bacteria

“…This isolate was found to be dependent on a Bacillus thermophile, and in 2006, the original group determined that this helper was providing carbon dioxide, a likely component of its natural environment (85). While this growth-inducing effect may be restricted to laboratory culture, the Bacillus species helper also appears to ameliorate the effects of toxic metabolites produced by S. thermophilum, leaving open the possibility of significant interactions in the environment (84).…”

The Effect of a Cell-to-Cell Communication Molecule, Pseudomonas Quinolone Signal (PQS), Produced by P. aeruginosa on Other Bacterial Species

“…For example, indole derivatives were isolated as growth inhibitors in Symbiobacterium thermophilum (35). Antibiotic reactions are another example of an interaction where bacterial growth is inhibited by extracellular bacterial metabolites.…”

Antibiotics in microbial coculture

“…21 The latter is a syntrophic bacterium whose growth depends on the coculture with G. stearothermophilus. Originally, the coculture of the two bacterial organisms was obtained from a compost sample because of the presence of a thermostable tryptophanase.…”

“…The concentration of 1,1-bis (3′-indolyl)ethane in the coculture is higher than that completely inhibiting pure growth of S. thermophilum. 21 Thus, it appears likely to be that not only the formation of but also resistance to the …”