Assemblage of Bacterial Saccharic Microfibrils in Sheath Skeleton Formed by Cultured Leptothrix sp. Strain OUMS1

Mitsuaki Furutani, Tomoko Suzu

Abstract

Leptothrix spp., a type of Fe- and Mn-oxidizing bacteria, is characterized by the formation of extracellular Fe- or Mn-encrusted microtubular sheaths in aquatic environments. The sheath matrix is known to be a hybrid of bacterial polymers and aquatic metals and minerals. This study describes the initial phase of assemblage of saccharic microfibrils in the sheath skeleton formed by Leptothrix sp. strain OUMS1 in 1–3 day culture investigated using various light and electron microscopic techniques. The negative staining of OUMS1 cells demonstrated a monotrichous polar flagellum and fibrous materials being secreted from the entire cell surface. Specific staining at light microscopic level revealed that viable bacterial cells were involved in the initial development phase of the immature sheath that contained saccharic polymers comprising various sugars. Transmission electron microscopy proved that secretion occurred from the entire cell surface to form membrane-unbound globules within 1 day of culture. After being released from the cell surface, these globules assembled to form the immature fibrous sheath layer away from the cell, resulting in the intervening space between the cell and the layer. It seems likely that secreted globules continuously moved toward the initial immature sheath layer through the intervening space and contributed to thickening of the layer. Alkaline bismuth staining revealed that the globules and the immature sheath materials contain saccharic polymers. By the second day of culture, electron-dense materials containing metals began depositing on fibrous components of the immature sheath layer. Results led us to conclude that the assembly of bacterial saccharic polymers to form the fibrous layer and the subsequent deposition of aquatic metals on the layer constructed the initial frame of the sheath. This new knowledge of the successive steps of sheath development provides deeper insights into the significance of bacterial saccharic polymers in the initial phase of sheath skeleton assembly.