Microscopy of Microbial Gas Vesicles

Gas vesicles are intracellular gas-filled protein-shelled nanocompartments. Their function is to provide buoyancy which allows aerophilic bacteria to float into oxygenated surface waters (Walsby, 1994). They also enable cyanobacteria to float up toward the light, stratifying in layers below the water surface (Walsby, 1994). The gas vesicles collapse and disappear when subjected to abrupt pressure increase (Ramsay et al., 2011). Mature gas vesicles are spindle or cylinder-shaped, and typically 0.1~2 μm in length and 45~250 nm in width (Pfeifer, 2012) (Fig. 1A and B). They begin as a bicone (biconical structure) and later become the mature forms of gas vesicles (Fig. 1C). There are characteristic 4.6-nm striations or ‘ribs’ of the 7~8 kDa gas vesicle protein A that are perpendicular to the long axis of the gas vesicles. The presence of gas vesicles can reduce the volume of the cytoplasm during the later stage of gas vesicle formation, which might help cells to survive under stress conditions (Pfeifer, 2012) (Fig. 1D). Enclosed by a 2 nm-thick hydrophobic protein shell, gas vesicles exclude water by surface tension at the hydrophobic inner surface, but permit gas from outside to freely diffuse in and out of their barrier (Shapiro et al., 2014; Walsby, 1994) (Fig. 1E). Therefore, the gas within the vesicle is in equilibrium with the gas dissolved in the cytoplasm or medium (Pfeifer, 2012). But they are not considered to store gas as pressurized balloons. Although gas vesicles are intensively studied in aquatic microbes such as the cyanobacterium Anabaena flos-aquae and the halophilic archaeon Halobacterium salinarum, they have been also observed even in a soil microbe and an enterobacterium (Daviso et al., 2013). As microbial gas vesicle-like structures, lipidor protein-stabilized gas microbubbles have been used as contrast agents for conventional ultrasound imaging (Shapiro et al., 2014). However, the microbubbles are prone to collapse and bubble fragmentation. There is a growing need in ultrasound-modulated stable contrast agents on the nanoscale or molecular reporters. Gas vesicles derived from natural biological structures could be used as nanoscale molecular reporters for ultrasound imaging, based on their hollow interiors, gas permeability, buoyancy, and optical scattering (Shapiro et al., 2014). Due to their nanoscale size, gas vesicles could potentially exit the intravascular space into solid tumors (Cherin et al., 2017). In addition, they could