Redox‐reactive membrane vesicles produced by Shewanella

Abstract

This manuscript is dedicated to our friend, mentor, and coauthor Dr Terry Beveridge, who devoted his scientific career to advancing fundamental aspects of microbial ultrastructure using innovative electron microscopic approaches. During his graduate studies with Professor Robert Murray, Terry provided some of the first glimpses and structural evaluations of the regular surface arrays (S-layers) of Gram-negative bacteria (Beveridge & Murray, 1974, 1975, 1976a). Beginning with his early electron microscopic assessments of metal binding by cell walls from Gram-positive bacteria (Beveridge & Murray, 1976b, 1980) and continuing with more than 30 years of pioneering research on microbe-mineral interactions (Hoyle & Beveridge, 1983, 1984; Ferris et al., 1986; Gorby et al., 1988; Beveridge, 1989; Mullen et al., 1989; Urrutia Mera et al., 1992; Mera & Beveridge, 1993; Brown et al., 1994; Konhauser et al., 1994; Beveridge et al., 1997; Newman et al., 1997; Lower et al., 2001; Glasauer et al., 2002; Baesman et al., 2007), Terry helped to shape the developing field of biogeochemistry. Terry and his associates are also widely regarded for their research defining the structure and function of outer membrane vesicles from Gram-negative bacteria that facilitate processes ranging from the delivery of pathogenic enzymes to the possible exchange of genetic information. The current report represents the confluence of two of Terry's thematic research streams by demonstrating that membrane vesicles produced by dissimilatory metal-reducing bacteria from the genus Shewanella catalyze the enzymatic transformation and precipitation of heavy metals and radionuclides. Under low-shear conditions, membrane vesicles are commonly tethered to intact cells by electrically conductive filaments known as bacterial nanowires. The functional role of membrane vesicles and associated nanowires is not known, but the potential for mineralized vesicles that morphologically resemble nanofossils to serve as palaeontological indicators of early life on Earth and as biosignatures of life on other planets is recognized.

Shewanella oneidensis MR-1 nanowires are outer membrane and periplasmic extensions of the extracellular electron transport components

“…This dye was previously used to label bacterial nanowires recovered from chemostat cultures (7,22). In all our experiments, the production of filaments coincided with the formation of separate or attached spherical membrane vesicles, another observation consistent with previous electron and atomic force microscopy measurements of Shewanella nanowires (19). In Fig.…”

“…Specifically, the dimensions of these filaments (10,23,24), their association with membrane vesicles (19), and their production during O 2 limitation (7) led us to conclude that these structures are the bacterial nanowires whose conductance was previously measured ex situ under dry conditions (10). Additionally, when we labeled cells grown in O 2 -limited chemostat cultures with the same fluorescent dyes, we observed identical structures with the same composition as the perfusion cultures reported here (see below).…”

“…oneidensis MR-1 Nanowires Are Outer Membrane and Periplasmic Extensions. Membrane vesicles have previously been observed to be associated with Shewanella nanowires (19) (Fig. 1A).…”

“…The extension of outer membrane filaments, and their functionalization with electron transport proteins, may represent a widespread strategy for EET. Virtually all Gram-negative bacteria produce outer membrane vesicles, and can alter the rate of production and composition of those vesicles in response to various stress conditions (19,41). More recent electron microscopy reports describe membrane vesicle chains and related membrane tubes (also referred to as periplasmic tubules) that form cell-cell connections in the social soil bacterium Myxococcus xanthus (42) as well as the Fig.…”

“…Previous reports demonstrated increased production of bacterial nanowires and associated redox-active membrane vesicles in electron acceptor (O 2 )-limited Shewanella cultures (7,10,19). To directly observe this response in vivo, we subjected Shewanella oneidensis MR-1 to O 2 -limited conditions in a microliter-volume laminar perfusion flow imaging platform (Materials and Methods) and monitored the production and growth of extracellular filaments from individual cells with fluorescent microscopy.…”