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Microbial motility is a survival strategy. Bacteria utilise motility in growth and reproduction to protect against desiccation and antibiotics, defend against competitors, colonise new habitats, and find nutrient-rich environments. Antarctica is the most extreme and pristine region on Earth. Here, soils are exposed to low temperatures, considerable levels of UV radiation, strong winds, extreme drought, and low availability of nutrients. Currently, there are no reports of motility in polar soil bacteria. Due to the extreme conditions in the Antarctic continent, we used these polar desert soils as a study model. We hypothesise that Antarctic soils are an ideal environment for bacteria to perform motility to find more favourable conditions. In this thesis, we describe for the first-time mechanisms of motility by Antarctic soil bacteria. As model microorganisms, we selected bacteria from the genera Streptomyces and Arthrobacter, belonging to the phylum Actinomycetota, the most abundant in East Antarctic soils. The first aim was to investigate the incidence and behaviour of Antarctic Streptomyces performing exploration. The genus Streptomyces had been described as a group of sessile bacteria. In 2017, the phenomenon of Streptomyces exploration was described for the first-time as a mechanism of translocation performed by Streptomyces cells through biotic and abiotic surfaces to facilitate access to distant nutrients. It was reported that the incidence of wild-type Streptomyces strains exhibiting exploration was ~10%. In addition, two critical conditions involved in the exploration process were described, glucose depletion and alkaline pH in the medium produced by the synthesis of volatile organic compounds (VOCs). Here, we screened 57 Streptomyces strains previously isolated from four sites in Eastern Antarctica. We found that 22.8% did not exhibit exploration capabilities, and 19.4% showed exploration behaviour in glucose depletion and alkaline environments. In contrast, 57.8% exhibited exploration at neutral pH or in the presence of glucose. These findings about the incidence of glucose-independent exploration suggest that the exploratory behaviour might be encouraged by searching for alternative carbon sources due to the low nutrient availability in East Antarctic soils. The second aim was to provide insight into potential compounds involved in Streptomyces exploration. Here, we obtained a fraction with emulsifying, antimicrobial, and haemolytic activities from the secretome of Streptomyces INR7 explorer cells. LC-MS analysis of the fraction and bioactive predictors revealed that a LysM-containing peptide with amphipathic properties is the responsible for the antibacterial and haemolytic activities. Furthermore, genomic annotation and sequence alignment indicate that the peptide is part of a protein belonging to the resuscitation-promoting factors family, contributing to physiological processes of cell wall remodelling and metabolic activation of dormant cells. The third aim was to describe whether Antarctic Arthrobacter isolates can use microbial hitchhiking as a motility strategy to migrate across the fungal hyphae of Antarctomyces psychrotrophicus. So far, Arthrobacter bacteria have been associated with active motility, also known as flagella-dependent motility. Flagella-assisted displacement is typical in aqueous environments; however, East Antarctic soils are exposed to arid and hyperarid conditions, suggesting that the motility of Arthrobacter isolates from these polar desert soils might be challenging. Here, we found that microbial hitchhiking facilitates the migration of Arthrobacter strains previously isolated from Antarctic soils. Hitchhiking is a form of transportation where microorganisms with null or limited motility, known as 'hitchhikers' or 'riders', use the motility of nearby motile organisms known as 'carriers' or 'host' to move. In soils, fungi facilitate the migration of bacteria over short and long distances by forming 'networks' or 'highways' on which bacteria can travel actively, passively or by direct attachment to the fungal hyphae. By using SEM, motility screening, flagella identification and hitchhiking assays, we found that of 17 Arthrobacter strains examined, 14 migrated through the fungus A. psychrotrophicus. The results suggest that Arthrobacter isolates migrated by three mechanisms: two strains travelled flagella-dependent motility, seven isolates translocated by flagella-independent motility and five strains migrated by direct attachment to the fungal surface. These findings provide evidence that Antarctic soil bacteria, specifically members of the genera Streptomyces and Arthrobacter, exhibit a high incidence of motility. We propose that motility could be a survival strategy used by bacteria to cope with the extreme conditions in Antarctica. |
