The article was recently accepted for publication in the prestigious journal Nature Communications (A22, IF)12). B. subtilis is a surface-dwelling bacterium competent for natural transformation. Genetically distinct B. subtilis swarms form a boundary upon encounter, mediated by a rapidly evolving kin-discrimination (KD) system consisting of different genes encoding cellular attack and defense mechanisms, resulting in killing of the antagonized strain. Here, we show that these swarm antagonisms promote transformation-mediated horizontal gene transfer between strains with low relatedness. Gene transfer is largely unidirectional and competence induction in the recipient cell is associated with activation of a stress response in the donor cell mediated by SigW, a sigma factor known to respond to cell wall stress. Closer related strains, which theoretically would undergo more efficient recombination due to greater sequence homology, do not upregulate transformation when encountered. This result demonstrates that social interactions can override mechanistic barriers to horizontal gene transfer. We hypothesize that KD-mediated competence in response to the encounter of different neighboring strains may maximize the likelihood of efficient incorporation of novel alleles and genes that have been shown to be effective in a genomically and ecologically similar context.
COBISS.SI-ID: 62369539
In this recently published work (A'), Bacillus subtilis was used as a model bacterium to study the role of surfactin in genetic transformation for the uptake and integration of extracellular DNA. We have shown that surfactin, a potent biosurfactant and antimicrobial lipopeptide, contributes to horizontal gene transfer between two nearly identical populations of B. subtilis by inducing cell lysis in a portion of the cells. The eDNA released from lysed cells then serves as a source of new genetic traits for nearby competent cells or for DNA repair when exchange occurs within a clonal population. Therefore, surfactin is linked to the horizontal gene as a pore-forming lipopeptide that contributes to the eDNA pool. These results provide evidence for a fundamental mechanism involved in HGT and greatly expand our understanding of DNA transfer in clonal B. subtilis populations and the spread of antibiotic resistance genes and diversification of microbial communities in the environment.
COBISS.SI-ID: 58756355
Bacillus subtilis is a cosmopolitan bacterium found virtually everywhere in the world. Bacterial biogeography is an interesting field that examines how bacteria differ not only in terms of geographic distance, but also in terms of the environment from which they were isolated. In this study, we showed that desert bacterial strains isolated from soil in the US differed from isolates from Slovenian soil. Based on genetic evidence, the Slovenian isolates were similar regardless of whether they were isolated from the rhizosphere of tomato or from riverbank soils, suggesting that environment plays a critical role in the evolution of this species. Furthermore, we have shown that the relatedness of B. subtilis strains does not decrease with distance within the same habitat, which, together with the diversity data, suggests that differential environmental selection pressure plays a fundamental role in the evolution of this species. The results of this project, published in A' journal are consistent with our observations on agar plates, where we found strain pairs from the riverbank and rhizosphere that fused despite being 200 km apart. Additionally, this justifies the use of riverbank strains alongside rhizosphere tomato strains in our project.
COBISS.SI-ID: 5134712
These results were presented in an invited lecture by the project leader dr. Štefanič at the conference of the Slovenian Microbiological Society in the section Microbes and Environment in Bled in 2017. The lecture included the results relevant to this project, which addresses the kin discrimination of B. subtilis and the consequences of this behavior. She showed that B. subtilis strains isolated from the soil microscale are capable of kin discrimination during swarming on a semi-solid medium. Swarms of kin strains fused on semisolid medium while a border line was formed between two non-kin swarms. Biofilm formation on plant roots showed mixed biofilms of kin strains, while only one of the strains predominated in the biofilm when two non-kin strains were inoculated. Similarly, a common swarm was observed, when two kin strains were inoculated on the agar surface, while in the mixture of two non-kin swarms on the semi-solid medium, one dominated and spread over the entire surface of the plate. Electron microscopy showed damaged cells between two non-kin swarms, while the cells at the site of the kin strain mixture were undamaged and intact. An increased frequency of transformation was observed at the border line, indicating that the exchange of DNA between non-kin strains was more frequent than between kin strains. These discoveries reveal the interactions of highly related B. subtilis bacteria, their sociality, and the consequences of such interactions on the survival and evolution of this species.
COBISS.SI-ID: 4824184
Project leader Dr Štefanič presented her work as a selected lecture at the Nanjing International Congress, "Plant-Microbe Interactions in the Rhizosphere", which hosted the most renowned experts in the field of rhizospheric plant growth promoters. During the talk, she presented past and recent results in the field of kin discrimination and the hypothesis that antagonism between strains plays a key role in this phenomenon. This hypothesis was supported by several experimental results on 1) competition of kin and non-kin strains on plant roots, 2) study of boundary and cell progression with electron microscope, 3) testing the ability of bacterial sporulation in the boundary, 4) DNA transfer and transcription of competence genes. Experiments have shown that swarms of mixed Kin strains form a mixed biofilm on the root, while the biofilm consists of one strain when two non-Kin strains are inoculated simultaneously. Electron microscopy at the boundary showed lysed and damaged cells, while cells in the swarms were healthy. Sporulation as a survival strategy at the boundary showed that sporulation of a non-kin strain was significantly lower compared to sporulation of the same strain in combination with the Kin strain. Increased transfer of DNA between non-kin strains was observed at boundary.
COBISS.SI-ID: 4992888