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International audience; The interactions between microbes and their hosts are among the most complex biological phenomena known today. The interaction may reach from overall beneficial interaction, as observed for most microbiome/microbiota related interactions to interaction with virulent pathogens, against which host cells have evolved sophisticated defence strategies. Among the latter, the confinement of invading pathogens in a phagosome plays a key role, which often results in the destruction of the invader, whereas some pathogens may counteract phagosomal arrest and survive by gaining access to the cytosol of the host cell. In the current review, we will discuss recent insights into this dynamic process of host-pathogen interaction, using Mycobacterium tuberculosis and related pathogenic mycobacteria as main examples.

Bacterial evolution drove the emergence of various specific and aspecific strategies to cope with hostile environments. Several transition metals (TMs) such as copper, zinc, iron, manganese, and cobalt are essential for most living organisms owing to their implication in a large number of cellular processes. Yet, they turn toxic when their cellular concentration exceeds a specific threshold. It is therefore crucial to maintain an adequate metal homeostasis to ensure a good fitness. In this chapter, we reviewed the main defense mechanisms described in the literature in the context of bacterial exposure to toxic concentrations of TMs. On the one hand, we considered metal dedicated systems that are engaged upon metal stress in distinct bacterial compartments relative to the metal species, its localization, and its cellular targets. On the other, metal resistance was addressed in the context of metabolic and developmental changes at both cellular and population levels. Finally, a case study was devoted to the original bimodal response of the aquatic alphaproteobacterium Caulobacter crescentus to copper stress, illustrating how phenotypic diversity can increase bacterial ability to adapt to changing environments.

This month’s Under the Lens discusses electron cryotomography (ECT) as a powerful tool for investigating host–pathogen interactions. This month’s Under the Lens discusses electron cryotomography (ECT) as a powerful tool for investigating host–pathogen interactions.

Stress response to fluctuating environments often implies a time-consuming reprogramming of gene expression. In bacteria, the so-called bet hedging strategy, which promotes phenotypic stochasticity within a cell population, is the only fast stress response described so far1. Here, we show that Caulobacter crescentus asymmetrical cell division allows an immediate bimodal response to a toxic metals-rich environment by allocating specific defence strategies to morphologically and functionally distinct siblings. In this context, a motile swarmer cell favours negative chemotaxis to flee from a copper source, whereas a sessile stalked sibling engages a ready-to-use PcoAB copper homeostasis system, providing evidence of a prompt stress response through intrinsic bacterial dimorphism. Depending on its dimorphic life cycle, Caulobacter crescentus differentially initiates an efflux or chemotactic response to avoid copper toxicity.

Accurate assessment of the antibacterial activity of graphene requires consideration of both the graphene fabrication method and, for supported films, the properties of the substrate. Large-area graphene films produced by chemical vapor deposition were grown directly on copper substrates or transferred on a gold substrate and their effect on the viability and proliferation of the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli were assessed. The viability and the proliferation of both bacterial species were not affected when they were grown on a graphene film entirely covering the gold substrate, indicating that conductivity plays no role on bacterial viability and graphene has no antibacterial activity against S. aureus and E. coli. On the other hand, antibacterial activity was observed when graphene coated the copper substrates, resulting from the release of bactericidal cupric ions in inverse proportion to the graphene surface coverage., Comment: 24 pages, 7 figures (paper+supplementary material), accepted for publication in Carbon