Initial Assemblage of Bacterial Saccharic Fibrils and Element Deposition to Form an Immature Sheath in Cultured Leptothrix sp. Strain OUMS1

Mitsuaki Furutani,Hideki Hashimoto,Hitoshi Kunoh,Tomoko Suzuki,Hiromichi Ishihara,Jun Takada

doi:10.3390/min1010157

Abstract

In an aquatic environment, the genus Leptothrix produces an extracellular Fe- or Mn-encrusted tubular sheath composed of a complex hybrid of bacterial exopolymers and aqueous-phase inorganic elements. This ultrastructural study investigated initial assemblage of bacterial saccharic fibrils and subsequent deposition of aqueous-phase inorganic elements to form the immature sheath skeleton of cultured Leptothrix sp. strain OUMS1. After one day of culture, a globular and/or thread-like secretion was observed on the surface of the bacterial cell envelope, and secreted bodies were transported across the intervening space away from the cell to form an immature sheath skeleton comprising assembled and intermingled fibrils. Energy dispersive X-ray microanalysis and specific Bi-staining detected a distinguishable level of P, trace Si, and a notable amount of carbohydrates in the skeleton, but not Fe. By the second day, the skeleton was prominently thickened with an inner layer of almost parallel aligned fibrils, along with low level of Fe deposition, whereas an outer intermingled fibrous layer exhibited heavy deposition of Fe along with significant deposition of P and Si. These results indicate that basic sheath-construction proceeds in two steps under culture conditions: an initial assemblage of bacterial saccharic fibrils originated from the cell envelope and the subsequent deposition of aqueous-phase Fe, P, and Si.

Highlights

The genus Leptothrix is a type of Fe/Mn-oxidizing bacteria that belongs to the aquatic β-proteobacteria [1,2,3]
In a previous paper [11], we reported the use of various microscopic techniques to visualize the assemblage of bacterial saccharic fibrils during the formation of a fibrous layer, and the subsequent deposition of aqueous-phase inorganic elements on the layer contributed to the production of the initial sheath skeleton of cultured Leptothrix sp. strain OUMS1 [12]
The scanning transmission electron microscopy (STEM) image of a U/Pb-stained section (Figure 1(a), inset) revealed threads, where one was connected to the cell envelope and another was disconnected at its base, and the apical globules of these threads had a relatively high electron density, suggesting that these structures could be rich in saccharic polymers

Summary

Introduction

The genus Leptothrix is a type of Fe/Mn-oxidizing bacteria that belongs to the aquatic β-proteobacteria [1,2,3]. Demonstrated using electron energy-loss spectroscopy that both sheath- and stalk-fibers of L. ochracea and Gallionella ferruginea, respectively, have a carbon core of bacterial exopolymers and that aqueous-phase iron interacts with oxygen at the surface of the carbon core, resulting in the deposition of iron oxides at the surface These reports support the hypothesis presented by Chan et al [10] that organic polymers play important roles in ecosystems by accumulating biologically important elements and that microbial polymers could scavenge iron oxide particles and induce the crystallization of unexpected phases. An understanding of the initial phase of hybrid construction would provide a better insight into this remarkable bacterium-associated phenomenon This biologically derived organic/inorganic hybrid is chemically and physically active and is considered as a future-oriented promising functional material. We cultured OUMS1 and observed the time course of inorganic deposition in the immature sheath skeleton by transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive X-ray (EDX) spectroscopy

Results and Discussion

Sample and Culturing

Electron Microscopy

Conclusions