Transfer of mobile genetic elements from one bacterium to another is the principal cause of the spread of antibiotic resistance. However, the dissemination of these elements in environmental contexts is poorly understood. In clinical and environmental settings, bacteria are often found living in multicellular communities encased in a matrix, a structure known as a biofilm. In this study, we examined how forming a biofilm influences the transmission of an integrative and conjugative element (ICE). Using the model Gram-positive bacterium B. subtilis, we observed that biofilm formation highly favors ICE transfer. This increase in conjugative transfer is due to the production of extracellular matrix, which creates an ideal biophysical context. Our study provides important insights into the role of the biofilm structure in driving conjugative transfer, which is of major importance since biofilm is a widely preponderant bacterial lifestyle for clinically relevant bacterial strains.

No abstract

Siderophores are soluble or membrane-embedded molecules that bind the oxidized form of iron, Fe(III), and play roles in iron acquisition by microorganisms. Fe(III)-bound siderophores bind to specific receptors that allow microbes to acquire iron. However, certain soil microbes release a compound (pulcherriminic acid, PA) that, upon binding to Fe(III), forms a precipitate (pulcherrimin) that apparently functions by reducing iron availability rather than contributing to iron acquisition. Here, we use Bacillus subtilis (PA producer) and Pseudomonas protegens as a competition model to show that PA is involved in a peculiar iron-managing system. The presence of the competitor induces PA production, leading to precipitation of Fe(III) as pulcherrimin, which prevents oxidative stress in B. subtilis by restricting the Fenton reaction and deleterious ROS formation. In addition, B. subtilis uses its known siderophore bacillibactin to retrieve Fe(III) from pulcherrimin. Our findings indicate that PA plays multiple roles by modulating iron availability and conferring protection against oxidative stress during inter-species competition.

Bacillus subtilis is a Gram-positive plant-growth-promoting rhizobacterium exerting many beneficial effects on plant health. Because they secrete antimicrobial compounds and elicit induced systemic resistance, B. subtilis and phylogenetically related species are of particular interest as antifungals in organic agriculture. These bacteria are also known for their capacity to differentiate phenotypically into endospores able to withstand many environmental stresses. However, although B. subtilis is often inoculated on plants as spores, dynamics of germination and sporulation on roots remain unexplored. Using a hydroponic culture system and a soil system for Arabidopsis thaliana, we observed that B. subtilis spores germinate rapidly on contact with plants. However, the vegetative cells are abundant on roots for only a few days before reversing back to spores. We observed that the germinant receptor GerK and sporulation kinases KinA and KinB identified in vitro control sporulation dynamics on plants. Surprisingly, when plants are inoculated with B. subtilis, free-living cells sporulate more rapidly than plant-associated cells. However, direct contact between plant and bacteria is required for the induction of sporulation in the surrounding B. subtilis. This study has fundamental implications for our understanding of interactions between Bacillus spp. and plants, and particularly for a more efficient usage of B. subtilis as a biofertilizer or biofungicide.

20Secondary metabolites have an important impact on the biocontrol potential of soil-21 derived microbes. In addition, various microbe-produced chemicals have been 22 suggested to impact the development and phenotypic differentiation of bacteria, 23including biofilms. The non-ribosomal synthesized lipopeptide of Bacillus subtilis, 24 surfactin, has been described to impact the plant promoting capacity of the bacterium. 25Here, we investigated the impact of surfactin production on biofilm formation of B. 26 subtilis using the laboratory model systems; pellicle formation at the air-medium 27 interface and architecturally complex colony development, in addition to plant root-28 associated biofilms. We found that the production of surfactin by B. subtilis is not 29 essential for pellicle biofilm formation neither in the well-studied strain, NCIB 3610, nor 30 in the newly isolated environmental strains, but lack of surfactin reduces colony 31 expansion. Further, plant root colonization was comparable both in the presence or 32 absence of surfactin synthesis. Our results suggest that surfactin-related biocontrol and 33 plant promotion in B. subtilis strains are independent of biofilm formation. 34 35 Keywords: Bacillus subtilis, biofilm, surfactin, plant root colonization, pellicle 36 39 Several species from the "Bacillus subtilis complex" are well-characterized plant growth-40 promoting rhizobacteria (PGPRs), providing various beneficial activities for plants and 41 inhibiting fungal and bacterial pathogens [1]. Many strains of Bacillus subtilis, Bacillus 42 amyloliquefaciens and Bacillus velezensis are currently used in organic and traditional 43 agriculture to prevent infection and/or increase yields of various crops [2-4]. These 44 species are of particular interest because they can form stress-resistant endospores, a 45 cell-type ideal for product formulation. Most PGPR Bacillus spp. also produce a wide 46 range of bioactive molecules, such as lipopeptides, which directly influences plant growth 47 and defence [5]. 48 49 Many of these molecules are synthesized by multienzyme-complexes called non-50 ribosomal peptide synthetases (NRPS) [6]. B. subtilis NCIB3610 possesses 3 NRPS 51 clusters and one NRPS/polyketide synthetase (PKS) cluster, which is few compared to 52 the bioactive molecule synthesis capacity of B. velezensis strains [1]. Bacillaene, a broad 53 spectrum antibiotic, is synthesized by proteins encoded in 80 kB pksA-S cluster [7]. The 54ppsA-E encodes for the peptide synthetase responsible for the synthesis of plipastatin 55 (fengycin family), a strong antifungal molecule [5,8], while the siderophore bacillibactin is 56 synthesized by the product of the dhbA-F operon [9]. Finally, SrfAA-AD produces versatile 57 molecules from the surfactin family [10]. 58 59Surfactin molecules are composed of a heptapeptide, i.e. two acidic and five nonpolar 60 amino acids, interlinked with a β-hydroxy fatty acid, and condensed in a cyclic lactone 61 right structure [10,11]. The amino acid sequence, the length, and the ...