Cet article présente les thèmes abordés lors de la conférence de clôture d'une session d'études internationales qui a porté sur l'analyse du travail des formateurs de stagiaires dans les programmes de formations supérieures professionnalisantes. Il s'est agi de présenter notre regard sur le développement professionnel en stage, les postures et des gestes dans l'accompagnement et l'évaluation de stagiaires, les mesures d'accompagnement et d'accommodement des stagiaires en situation de handicap, les enjeux et les défis du travail de formateurs de stagiaires.

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.

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Iron (Fe) is one of the most important micronutrients for most life forms on earth. While abundant in soil, Fe bioavailability in oxic soil is very low. In environmental conditions, bacteria need to acquire sufficient Fe to sustain growth while limiting the energy cost of siderophore synthesis. Biofilm formation might mitigate this Fe stress, since it was shown to accumulate Fe in certain Gram-negative bacteria and that this Fe could be mobilized for uptake. However, it is still unclear if and to what extent the amount of Fe accumulated in the biofilm can sustain growth, and if the mobilization of this local Fe pool is modulated by the availability of environmental Fe (i.e., Fe outside the biofilm matrix). Here we use a non-domesticated strain of the ubiquitous biofilm-forming soil bacterium, Bacillus subtilis, and stable Fe isotopes to precisely evaluate the origin of Fe during growth in the presence of tannic acid and hydroxides, used as proxies for different environmental conditions. We report that this B. subtilis strain can accumulate a large quantity of Fe in the biofilm, largely exceeding Fe associated with cells. We also report that only a fraction of biofilm-bound Fe is available for uptake in the absence of other sources of Fe in the vicinity of the biofilm. We observed that availability of environmental Fe modulates the usage of this pool of biofilm-bound Fe. Finally, our data suggest that consumption of biofilm-bound Fe relates to the efficacy of B. subtilis to transport Fe from the environment to the biofilm, possibly through siderophores. Importance Recent evidences suggest that Fe bound to the biofilm could assume at least two important functions; a local source of Fe for uptake and a support to extracellular metabolism such as extracellular electron transfer. Our results show that B. subtilis can use biofilm-bound Fe for uptake only if it does not compromise Fe homeostasis of the biofilm, i.e., maintaining a minimum Fe concentration in the biofilm for extracellular purposes. We propose a theoretical framework based on our results and recent literature to explain how B. subtilis manages biofilm-bound Fe and Fe uptake in response to environmental Fe availability. These results provide important insights into the management of biofilm-bound and environmental Fe by B. subtilis in response to Fe stress.