Bacterial cell wall quantification by a modified low-volume Nelson-Somogyi method and its use with different sugars.

The study of peptidoglycan-binding proteins frequently requires in vitro binding assays, in which the isolated peptidoglycan used as a substrate must be carefully quantified. Here, we describe an easy and sensitive assay for peptidoglycan quantification based on a modified Nelson-Somogyi reducing sugar assay. We report the response of this assay to different common sugars and adapt its use to peptidoglycan samples subjected to acid hydrolysis. This method showed better sensitivity than the peptidoglycan quantification method based on the acid detection of diaminopimelic acid. The method described in this work, besides being valuable in the characterization of peptidoglycan-binding proteins, is also useful for the quantification of reducing monosaccharides or polysaccharides after acid or hydrolysis.

Changes in fluidity of the E. coli outer membrane in response to temperature, divalent cations and polymyxin-B show two different mechanisms of membrane fluidity adaptation.

The outer membrane (OM) is an essential component of the Gram-negative bacterial cell envelope. Restricted diffusion of integral OM proteins and lipopolysaccharide (LPS) that constitute the outer leaflet of the OM support a model in which the OM is in a semi-crystalline state. The low fluidity of the OM has been suggested to be an important property of this membrane that even contributes to cell rigidity. The LPS characteristics strongly determine the properties of the OM and the LPS layer fluidity has been measured using different techniques that require specific conditions or are technically challenging. Here, we characterize the Escherichia coli LPS fluidity by evaluating the lateral diffusion of the styryl dye FM4-64FX in fluorescence recovery after photobleaching experiments. This technique allowed us to determine the effect of different conditions and genetic backgrounds on the LPS fluidity. Our results show that a fraction of the LPS can slowly diffuse and that the fluidity of the LPS layer adapts by modifying the diffusion of the LPS and the fraction of mobile LPS molecules.