Biopearling of Interconnected Outer Membrane Vesicle Chains by a Marine Flavobacterium

Abstract

Large surface-to-volume ratios provide optimal nutrient uptake conditions for small microorganisms in oligotrophic habitats. The surface area can be increased with appendages. Here, we describe chains of interconnecting vesicles protruding from cells of strain Hel3_A1_48, affiliating with Formosa spp. within the Flavobacteriia and originating from coastal free-living bacterioplankton. The chains were up to 10 μm long and had vesicles emanating from the outer membrane with a single membrane and a size of 80 to 100 nm by 50 to 80 nm. Cells extruded membrane tubes in the exponential phase, whereas vesicle chains dominated on cells in the stationary growth phase. This formation is known as pearling, a physical morphogenic process in which membrane tubes protrude from liposomes and transform into chains of interconnected vesicles. Proteomes of whole-cell membranes and of detached vesicles were dominated by outer membrane proteins, including the type IX secretion system and surface-attached peptidases, glycoside hydrolases, and endonucleases. Fluorescein-labeled laminarin stained the cells and the vesicle chains. Thus, the appendages provide binding domains and degradative enzymes on their surfaces and probably storage volume in the vesicle lumen. Both may contribute to the high abundance of these Formosa-affiliated bacteria during laminarin utilization shortly after spring algal blooms. IMPORTANCE Microorganisms produce membrane vesicles. One synthesis pathway seems to be pearling that describes the physical formation of vesicle chains from phospholipid vesicles via extended tubes. Bacteria with vesicle chains had been observed as well as bacteria with tubes, but pearling was so far not observed. Here, we report the observation of, initially, tubes and then vesicle chains during the growth of a flavobacterium, suggesting biopearling of vesicle chains. The flavobacterium is abundant during spring bacterioplankton blooms developing after algal blooms and has a special set of enzymes for laminarin, the major storage polysaccharide of microalgae. We demonstrated with fluorescently labeled laminarin that the vesicle chains bind laminarin or contain laminarin-derived compounds. Proteomic analyses revealed surface-attached degradative enzymes on the outer membrane vesicles. We conclude that the large surface area and the lumen of vesicle chains may contribute to the ecological success of this marine bacterium.

Role of Bacterial Cytoskeleton and Other Apparatuses in Cell Communication

“…These chains of interconnected vesicles enlarge the cell surface of bacteria resulting into increased surface enzymes per cell volume ( Bar-Ziv and Moses, 1994 ). These vesicle chains are well reported in various species of Flavobacteria such as F. columnare , F. psychrophilum and also in strain Hel3_A1_48 of a marine flavobacterium ( Fischer et al, 2019 ). Apart from these, Francisella novicida , Myxococcus xanthus , and Shewanella oneidensis also showed chains of vesicles on their cell surfaces ( Subramanian et al, 2018 ; Fischer et al, 2019 ).…”

Eco-evolutionary feedbacks mediated by bacterial membrane vesicles

“…One recently discovered member of Flavobacteriaceae , Formosa strain Hel3_A148, revealed another strategy for the optimization of nutrient uptake that involves BMV release (Fischer et al . 2019 ). As a result of a process called ‘biopearling’—the membranes of these bacteria protrude and form up to 2-μm-long tubular appendages and start to oscillate due to abiogenic physical forces resulting from the competition between membrane curvature and tension that together introduce instability in the tubular structure and produce a chain of linked with short neck vesicles or ‘pearls’; see Box 2 and Fig.…”

“…4B (Fischer et al . 2019 ). Proteomics studies of the BMVs of this strain have shown that the BMVs are enriched in porins (the OmpC-like type), hydrolytic enzymes (including endonucleases, peptidases and glycoside hydrolases), TonB-dependent proteins and gliding motility-associated proteins, while BMVs have also been found to exhibit laminarin-binding properties, which suggests a role in carbohydrate binding and utilization (Fischer et al .…”

A Prokaryotic Membrane Sculpting BAR Domain Protein

“…OMV formation is ubiquitous and has many documented functions 9 . OMEs are less commonly observed, remain attached to the cell, and various morphologies can be seen extending from single cells including Myxococcus xanthus 14,15 , flavobacterium strain Hel3_A1_48 8 , Vibrio vulnificus 10 , Francisella novicida 28 , Shewanella oneidensis 7,29–31 , and as cell-cell connections in Bacillus subtilis 32–34 and Eschericia coli 35 . Several bacterial proteins have demonstrated membrane tubule formation capabilities in vitro 16,36–40 , but despite the growing number of reports, proteins involved in shaping bacterial membranes into OMV/Es have yet to be identified.…”

“…Several bacterial proteins have demonstrated membrane tubule formation capabilities in vitro 16,36–40 , but despite the growing number of reports, proteins involved in shaping bacterial membranes into OMV/Es have yet to be identified. Recently, researchers have begun to suspect that OMV and OME formation has some pathway overlap 8 , and it is proposed that proteins are necessary to stabilize these structures 13 .…”