Flotillin-mediated membrane fluidity controls peptidoglycan synthesis and MreB movement.

The bacterial plasma membrane is an important cellular compartment. In recent years it has become obvious that protein complexes and lipids are not uniformly distributed within membranes. Current hypotheses suggest that flotillin proteins are required for the formation of complexes of membrane proteins including cell-wall synthetic proteins. We show here that bacterial flotillins are important factors for membrane fluidity homeostasis. Loss of flotillins leads to a decrease in membrane fluidity that in turn leads to alterations in MreB dynamics and, as a consequence, in peptidoglycan synthesis. These alterations are reverted when membrane fluidity is restored by a chemical fluidizer. In vitro, the addition of a flotillin increases membrane fluidity of liposomes. Our data support a model in which flotillins are required for direct control of membrane fluidity rather than for the formation of protein complexes via direct protein-protein interactions.

Every living cell is enclosed by a flexible membrane made of molecules known as phospholipids, which protects the cell from harmful chemicals and other threats. In bacteria and some other organisms, a rigid structure known as the cell wall sits just outside of the membrane and determines the cell's shape. There are several proteins in the membrane of bacteria that allow the cell to grow by assembling new pieces of the cell wall. To ensure these proteins expand the cell wall at the right locations, another protein known as MreB moves and organizes them to the appropriate place in the membrane and controls their activity. Previous studies have found that another class of proteins called flotillins are involved in arranging proteins and phospholipid molecules within membranes. Bacteria lacking these proteins do not grow properly and are unable to maintain their normal shape. However, the precise role of the flotillins remained unclear. Here, Zielinska, Savietto et al. used microscopy approaches to study flotillins in a bacterium known as Bacillus subtilis. The experiments found that, in the presence of flotillins, MreB moved around the membrane more quickly (suggesting it was more active) than when no flotillins were present. Similar results were observed when bacterial cells lacking flotillins were treated with a chemical that made membranes more 'fluid' ­ that is, made it easier for the molecules within the membrane to travel around. Further experiments found that flotillins allowed the phospholipid molecules within an artificial membrane to move around more freely, which increases the fluidity of the membrane. These findings suggest that flotillins make the membranes of bacterial cells more fluid to help cells expand their walls and perform several other processes. Understanding how bacteria control the components of their membranes will further our understanding of how many currently available antibiotics work and may potentially lead to the design of new antibiotics in the future.

Antibacterial activity of monoacetylated alkyl gallates against Xanthomonas citri subsp. citri.

Asiatic citrus canker (ACC) is an incurable disease of citrus plants caused by the Gram-negative bacterium Xanthomonas citri subsp. citri (X. citri). It affects all the commercially important citrus varieties in the major orange producing areas around the world. Control of the pathogen requires recurrent sprays of copper formulations that accumulate in soil and water reservoirs. Here, we describe the improvement of the alkyl gallates, which are potent anti-X. citri compounds, intended to be used as alternatives to copper in the control of ACC. Acetylation of alkyl gallates increased their lipophilicity, which resulted in potentiation of the antibacterial activity. X. citri exposed to the acetylated compounds exhibited increased cell length that is consistent with the disruption of the cell division apparatus. Finally, we show that inhibition of cell division is an indirect effect that seemed to be caused by membrane permeabilization, which is apparently the primary target of the acetylated alkyl gallates.