Keeping your lawn wet

Abstract

Even before the existence of experimental proof, it was thought that the length of the bacterial flagellum was controlled by capillary action. Flagellin subunits travel through the hollow structure of the flagellum and assemble at the growing tip outside the cell body. The growth rate of a flagellum has been shown to decrease as its length increases, with growth stopping at a flagellum length of approximately 15 μm (Iino, 1969). The same growth rate and terminal length was observed for a flagellum that was sheared off by mechanical forces. This would suggest that length is indeed limited by capillary action. Recent work by Wang and colleagues reports a relatively simple assay system showing that the length of Salmonella flagella is dependent directly on the degree of hydration in their external environment (Wang et al , 2005). The authors take advantage of earlier discoveries by Harshey and colleagues that Salmonella typhimurium , Escherichia coli and Serratia marcescens can switch from a swimming phenotype, in which they use flagellar rotation to propel themselves (swim) through liquid environments, to a swarming phenotype, in which the bacteria use flagellar rotation to crawl (swarm) across surfaces (Alberti & Harshey, 1990; Harshey & Matsuyama, 1994). Furthermore, the bacteria undergo a morphogenetic change when they are taken from a swimming environment and placed on a hydrated surface. A low agar concentration in Petri dishes (0.3%) provides enough hydration to allow the bacteria to swim through the agar. Bacteria isolated from these …