Sulfolobus acidocaldarius Microvesicles Are Naturally Occurring Nanoparticles with Unusual Stability against Various Environmental Stressors

Abstract Microvesicles secreted from the thermoacidophilic archaeon S. acidocaldarius (Sa-MVs) contain a membrane made exclusively of tetraether lipids and covered by S-layer proteins. In this study, as the start point to exploit Sa-MVs for biotechnology applications, we used dynamic light scattering, laser Doppler electrophoresis, and cryo-transmission electron microscopy to characterize Sa-MVs and tested their stabilities against various environmental stressors. We found that Sa-MVs have long term (> 4.5 months) stability when stored at room temperature. Under the cell's growth conditions (75°C and pH 2.6), the average size of Sa-MVs is ∼180 nm, which remains unchanged in the temperature range examined (25-80°C). When the pH was increased from 2.6 to 7.2, Sa-MVs decrease their size by ∼40 nm, which probably came from the protein dependent events, rather than conformational changes of tetraether lipid headgroups. The isoelectric point for Sa-MVs in 1 mM KCl is 3.0 while that for liposomes reconstituted from Sa-MV lipids (LUV MV ) is below 2.0. Sa-MVs are stable against autoclaving at neutral pH in terms of size and shape. At pH 2.6, autoclaving changes Sa-MVs from a spherical to an irregular shape, although particle size changes little, while diester liposomes cannot sustain multiple cycles of autoclaving displaying a dramatic increase in size probably due to aggregation. Compared to liposomes made of diesters or the tetraether lipids from the polar lipid fraction E (PLFE) of S. acidocaldarius , Sa-MVs and LUV MV exhibit unusual resistance against the surfactant Triton X-100. These results set the foundation for future functional and biophysical studies of Sa-MVs.