Lateral diffusion of lipopolysaccharide in the outer membrane of Salmonella typhimurium

Gram-negative enteric bacteria are enveloped by two membrane systems. The inner or cytoplasmic membrane is responsible for the major metabolic functions including biosynthetic activities, while the major known functions of the outer membrane are primarily physical: it contains receptors for bacteriophages and bacteriocins; it contributes to the maintenance of cell shape; and it controls access of nutrient solutes and agents such as antibiotics and detergents to the cytoplasmic membrane. Several investigations have indicated that mobility of membrane components, particularly lipopolysaccharide, is essential for biogenesis of the outer membrane, and is a primary event in phage infection. To define more accurately the fluid dynamic properties of the outer membrane as related to function, we have now developed the capability to measure lateral diffusion coefficients in vivo of rhodaminated G30 lipopolysaccharide fused into Salmonella typhimurium G30A filamentous bacteria. The method used extends the FRAP procedure (fluorescence redistribution after photobleaching) to bacteria and the results demonstrate rapid diffusion of lipopolysaccharide (D = 2.0 +/- 0.9 x 10(-10) cm2s-1) over micrometre distances.