Nanoscale visualization of a fibrillar array in the cell wall of filamentous cyanobacteria and its implications for gliding motility

Abstract

Many filamentous cyanobacteria are motile by gliding, which requires attachment to a surface. There are two main theories to explain the mechanism of gliding. According to the first, the filament is pushed forward by small waves that pass along the cell surface. In the second, gliding is powered by the extrusion of slime through pores surrounding each cell septum. We have previously shown that the cell walls of several motile cyanobacteria possess an array of parallel fibrils between the peptidoglycan and the outer membrane and have speculated that the function of this array may be to generate surface waves to power gliding. Here, we report on a study of the cell surface topography of two morphologically different filamentous cyanobacteria, using field emission gun scanning electron microscopy (FEGSEM) and atomic force microscopy (AFM). FEGSEM and AFM images of Oscillatoria sp. strain A2 confirmed the presence of an array of fibrils, visible as parallel corrugations on the cell surface. These corrugations were also visualized by AFM scanning of fully hydrated filaments under liquid; this has not been achieved before for filamentous bacteria. FEGSEM images of Nostoc punctiforme revealed a highly convoluted, not parallel, fibrillar array. We conclude that an array of parallel fibrils, beneath the outer membrane of Oscillatoria, may function in the generation of thrust in gliding motility. The array of convoluted fibrils in N. punctiforme may have an alternative function, perhaps connected with the increase in outer membrane surface area resulting from the presence of the fibrils.

FIG. 1.

FEGSEM image of Oscillatoria sp. strain A2. The original filament has fragmented to release the single cell visible in the center of the micrograph. The fibrillar array is clearly visible as parallel corrugations on the cell surface, which run at an angle to the long axis of the filament. Part of a small, contaminating bacterium can be seen at the upper right of the micrograph. Scale bar, 500 nm.

FIG. 2.

AFM image of Oscillatoria sp. strain A2. The scan was performed on a sample that had been transferred from an agar plate to a glass slide and was, consequently, dehydrated when scanned. Parts of two cells are visible, with a cell septum running vertically down the center of the micrograph. Scale bar, 500 nm.

FIG. 3.

AFM image of Oscillatoria sp. strain A2. The scan was performed on a sample transferred from an agar plate to a glass slide. Parts of two cells are visible, with a cell septum running from the top left to the bottom middle of the micrograph. Scale bar, 200 nm.

FIG. 4.

FEGSEM image of N. punctiforme. Parts of three cells can be seen, the surfaces of which are highly convoluted as a consequence of the fibrillar array beneath the OM. Note the difference between this convoluted array and the parallel array seen in Oscillatoria strain A2 (Fig. 1). Scale bar, 500 nm.

FIG. 5.

AFM image of N. punctiforme. The scan was performed on a sample transferred from agar to a glass slide and was, consequently, dehydrated when scanned. The convolutions seen in Fig. 4 appear disrupted in places. Scale bar, 100 nm.

FIG. 6.

AFM image of Oscillatoria sp. strain A2. The scan was performed under liquid on a sample immobilized in dental wax. Cell septa can be seen at the bottom left and top right of the micrograph. The fibrillar array shows the same highly regular arrangement seen in FEGSEM images (Fig. 1); the partial disruption of the fibrils apparent in the AFM scans of dry samples (Fig. 2 and 3) is not seen in this fully hydrated sample. Scale bar, 500 nm.