An unusual filamentous, budding bacterium was isolated from several North American hot springs and named Isosphaera pallida. Filaments are composed of spherical cells 2.5–3.0 μm in diameter, with cell growth and division occurring by formation of intercalary buds. These obligately aerobic, heterotrophic isolates closely resemble Isocystis pallida Woronichin, which has been previously described as a cyanobacterium, and later as a yeast, based on collected specimens.
Isolates were salmon-colored due to the presence of carotenoids and contained gas vesicles. Growth occurred at temperatures up to 55° C in defined media using 0.025% glucose or lactate as carbon sources. Glucose concentrations of 0.05% or higher inhibited growth of the culture.
Ultrathin sections observed by TEM revealed an unusual tri-laminar wall structure. Pit-like ultrastructural features were found in the cell wall. Growth of cultures was not inhibited by penicillin G, and the Gram reaction gave variable results.
Cells formed motile, macroscopic aggregates (“comets”) when harvested from liquid cultures and plated on media containing Gelrite (Kelco Co.) as a solidifying agent. Aggregation and motility were observed in both the light and the dark. However, comets were strongly phototactic. Negative stains revealed numerous pili, but not flagella.
We propose that this highly unusual prokaryote be placed in a new genus.
Isosphaera pallidaThermophile Hot spring Phototaxis Gliding motility Gas vesicles Budding Cell wall