It is generally accepted that planktonic bacteria in dilute suspensions are not mechanically coupled as are cells imbedded in the biofilm. Here we employ optical tweezers to show that bacteria in dilute suspensions are mechanically coupled and show long-range correlated motion. The strength of the coupling increases with the growth of liquid bacterial culture. The matrix responsible for the mechanical coupling is composed of cell debris and extracellular polymer material. The fragile network connecting cells behaves as viscoelastic liquid of entangled extracellular polymers. Our findings point to physical connections between bacteria in dilute bacterial suspensions that may provide a mechanistic framework for understanding of biofilm formation, osmotic flow of nutrients, diffusion of signal molecules in quorum sensing, or different efficacy of antibiotic treatments at low and high bacterial densities. The work was assigned among Excellent in Science and awarded as one of the best works published at University of Ljubljana; the senior author has also presented the results to the public during an interview.
The work addresses aims of the J4-7637 project and was published in the prestigious journal Current Biology, IF=8.9, for the first time describes the molecular mechanisms behind a fascinating ability of bacteria to discriminate between and respond differentially to kin and non-kin; phenomenon identified as bacterial kin discrimination. By using transposon mutagenesis, reverse genetics, transcriptomics, comparative genomics and swarming compatibility assay we showed for the first time in collaboration with Harvard that KD relays on combinatorial system involving several genetic loci. These function in 1) attack and defense, 2) synthesis of bacterial cell wall, 3) production of the major extracellular polysaccharide Eps and as 4) regulatory factors involved in stress response. The number of different attack and defense functions increases with phylogenetic distance between strains. The KD in B. subtilis is of combinatorial nature as it relies on more than one genetic determinant which are present in natural strains in different combinations and provide a dynamic system for assortment of strains based on their relatedness which are expected to have overlapping ecological niches. This KD system may therefore stabilize cooperation between kin. The paper has been already cited 29 times (google scholar 39 times). The work was also recognized by ARRS and chosen among Excellent in science in 2016.
In this review article we discuss the multiple mechanisms that the Gram-positive bacterium Bacillus subtilis uses to allow it to communicate between cells and establish community structures. The work therefore connects tightly to the aims of J4-7637. The modes of action that are used by bacteria are highly varied and include routes that sense pheromone levels during quorum sensing and control gene regulation, the intimate coupling of cells via nanotubes to share cytoplasmic contents, and long range electrical signaling to couple metabolic processes both within and between biofilms. We explored the ability of B. subtilis to detect “kin” (and “cheater cells”) by looking at the mechanisms used to potentially ensure beneficial sharing (or limit exploitation) of extracellular “public goods” and mechanisms that govern biofilm development. Finally reflecting on the array of methods that a single bacterium has at its disposal to ensure maximal benefit for its progeny, we highlighted that a large future challenge will be integrating how these systems interact in mixed species communities.
In the scope of the J4-7637 grant, we are studying Streptomyces-Bacillus interactions and their influence on production of secondary metabolites. For this purpose, as described in this paper, we develop suitable gene tools for rapid gene manipulation of S. rimosus strains,which produce antibiotic oxytetracycline (OTC). This efforts relate to the development of suitable vectors, promoters and resistance markers needed to express target genes, as described in this publication. Tools developed in the scope of this publication have also been used to delete OTC gene cluster and to identify and study gene clusters, potentially encoding metabolites produced by S. rimosus, whose production is induced in co-culture with B. subtilis.
At the occasion of »World Food Day« celebration in the year of 2018 we have presented with the lecture and written article our projects in the field of food safety management – control of food-borne Campylobacter spp. In this frame also microbial interactions between pathogenic Campylobacter jejuni in potentially probiotic Bacillus subtilis were discussed in the context of the results we obtained during this project. The article comprised also the aims of the bilateral project between Slovenia and USA (2018-2019, coord. Prof. Zhang, Prof. Smole Možina), which are tightly connected to the broad aims of the project J4-7637. Bacillus strains and Campylobacter strains investigated in our project are essential components of the collaboration and the partner Prof. Zhang kindly provides his expertise on animal model and probiotic testing. The article thus presents the present understanding of Bacillus_ Campylobacter interactions and discusses the potential for improvements in its practical use in management of food-borne bacteria and food safety were exposed.