Summary: There is a growing interest in the bacterial pellicle, a biofilm floating at the air–liquid interface. Pellicles have been well studied in the G ram‐positive bacterium B acillus subtilis, but far less in G ram‐negative bacteria, where pellicle studies have mostly focused on matrix components rather than on the regulatory cascades involved. Several G ram‐negative bacteria, including pathogenic bacteria, have been shown to be able to form a pellicle under static conditions. Here, we summarize the growing body of knowledge about pellicle formation in G ram‐negative bacteria, especially about the components of the pellicle matrix. We also propose that the pellicle is a specific biofilm, and that its formation involves particular processes. Since this lifestyle concerns a growing number of bacteria, its properties undoubtedly deserve further investigation.

The chromate efflux pump encoding gene chrA_SO was identified on the chromosome of Shewanella oneidensis MR1. Although chrA_SO is expressed without chromate, its expression level increases when Cr(VI) is added. When deleted, the resulting mutant ΔchrA_SO exhibits a chromate sensitive phenotype compared to that of the wild-type strain. Interestingly, heterologous expression of chrA_SO in E. coli confers resistance to high chromate concentration. Moreover, expression of chrA_SO in S. oneidensis and E. coli significantly improves Cr(VI) reduction. This effect could result either from extracytoplasmic chromate reduction or from a better cell survival leading to enhanced Cr(VI) reduction.

Summary: Partner‐Switching Systems (PSS) are widespread regulatory systems, each comprising a kinase‐anti‐σ, a phosphorylatable anti‐σ antagonist and a phosphatase module. The anti‐σ domain quickly sequesters or delivers the target σ factor according to the phosphorylation state of the anti‐σ antagonist induced by environmental signals. The PSS components are proteins alone or merged to other domains probably to adapt to the input signals. PSS are involved in major cellular processes including stress response, sporulation, biofilm formation and pathogenesis. Surprisingly, the target σ factors are often unknown and the sensing modules acting upstream from the PSS diverge according to the bacterial species. Indeed, they belong to either two‐component systems or complex pathways as the stressosome or Chemosensory Systems (CS). Based on a phylogenetic analysis, we propose that the sensing module in Gram‐negative bacteria is often a CS.

Here, we show that a partner-switching system of the aquatic Proteobacterium Shewanella oneidensis regulates post-translationally σ^S (also called RpoS), the general stress response sigma factor. Genes SO2118 and SO2119 encode CrsA and CrsR, respectively. CrsR is a three-domain protein comprising a receiver, a phosphatase, and a kinase/anti-sigma domains, and CrsA is an anti-sigma antagonist. In vitro, CrsR sequesters σ^S and possesses kinase and phosphatase activities toward CrsA. In turn, dephosphorylated CrsA binds the anti-sigma domain of CrsR to allow the release of σ^S. This study reveals a novel pathway that post-translationally regulates the general stress response sigma factor differently than what was described for other proteobacteria like Escherichia coli. We argue that this pathway allows probably a rapid bacterial adaptation.