Two temperature-dependent membrane fluidity regimes in Gram-positive bacteria

It is widely accepted that bacterial cells maintain a constant membrane fluidity in response to temperature changes. This process, known as membrane fluidity homeostasis, occurs through remodeling of plasma membrane composition. We tested this using an assay based on total internal reflection-fluorescence correlation spectroscopy (TIR-FCS) that directly quantifies membrane fluidity as the diffusion speed of a membrane marker in Bacillus subtilis and two other Gram-positive bacteria, Streptococcus pneumoniae and Staphylococcus aureus, across a temperature range of 20{degrees}C to 37{degrees}C. Instead of the expected constant membrane fluidity, we identified a two-component regime: membrane fluidity is maintained at low temperatures (<26{degrees}C), but freely increases at higher temperatures. Significance statementTemperature changes affect the physical properties of the bacterial plasma membrane. Typically, with a reduction in temperature comes a loss of membrane fluidity. It is known that bacteria like Bacillus subtilis adapt their membrane composition when this happens, and it was thought to maintain membrane fluidity constant. We found that this is not true: fluidity is maintained only in a low (<26{degrees}C) temperature regime, while at higher temperatures fluidity is not maintained. We found this to be true for several Gram+ bacteria.