For decades the Streptomyces lifecycle was considered to be well established, consisting of three developmental stages: vegetative hyphae, aerial hyphae, and spores. After the co-inoculation of Streptomyces venezuelae and the yeast Saccharomyces cerevisiae by Jones et al (1), a new form of Streptomyces growth was discovered and termed “Streptomyces Exploration”. In contrast to the canonically described stationary lifecycle, exploring cells are capable of passive rapid traversal across solid surfaces through the formation of non-branching vegetative hyphae. While the mechanisms behind exploration remain unknown, it is established that either depletion of glucose or the induction of alkaline conditions by volatile organic compounds can induce exploration.
The incident of exploration amongst Streptomyces remains to be investigated, however it was found that 19 of 200 temperate species were capable of this novel form of growth. We found that of 54 Antarctic Streptomyces spp. 22 were capable of exploration, suggesting that this could be a survival strategy adopted by microbes under extreme conditions. While it is known that glucose can repress exploration, the role of glucose concentration on exploration had not been investigated. We observed a clear interaction between glucose concentration and the distance explored by two polar Streptomyces, S. lavandulae and S. polyantibiotics, suggesting there may be a critical minimal concentration of glucose that induces exploratory growth.
TMT-based proteomics analysis was conducted on exploring and stationary Streptomyces sp. INR7 colonies to examine the pathways associated with exploratory growth. Through comparison of relative abundances of proteins, we identified those associated with carbon catabolite repression (CCR) and the alkaline stress response and found a significant increase in expression of proteins involved in these processes. These include the CCR regulatory ROK family proteins and glucose kinases, and an alkaline associated increase in cytochrome c complex proteins.
Given the similarities between sliding motility and exploration, future work will be focused on identifying proteins associated with surfactant biosynthesis and transport to determine if surfactants play a role.