Use of the fluorescent probe LAURDAN to label and measure inner membrane fluidity of endospores of Clostridium spp.

► A method for measuring membrane fluidity of endospores was developed. ► The fluorescence method can be applied to endospore populations in solution. ► Outer membranes of endospores do not significantly interfere with this method. ► We report that the inner membranes of endospores are highly ordered.