Bacterial nanotubes as a manifestation of cell death

Abstract

Bacterial nanotubes are membranous structures that have been reported to function as conduits between cells to exchange DNA, proteins, and nutrients. Here, we investigate the morphology and formation of bacterial nanotubes using Bacillus subtilis. We show that nanotube formation is associated with stress conditions, and is highly sensitive to the cells’ genetic background, growth phase, and sample preparation methods. Remarkably, nanotubes appear to be extruded exclusively from dying cells, likely as a result of biophysical forces. Their emergence is extremely fast, occurring within seconds by cannibalizing the cell membrane. Subsequent experiments reveal that cell-to-cell transfer of non-conjugative plasmids depends strictly on the competence system of the cell, and not on nanotube formation. Our study thus supports the notion that bacterial nanotubes are a post mortem phenomenon involved in cell disintegration, and are unlikely to be involved in cytoplasmic content exchange between live cells.

In situ imaging of bacterial membrane projections and associated protein complexes using electron cryo-tomography

“…A recent study showed that the formation of bacterial tubes significantly increases when cells are stressed or dying [28]. Consistent with this, in our cryo-tomograms we saw many MEs and MVs associated with lysed cells (such as in H. pylori, H. hepaticus, and P. luteoviolacea).…”

“…Interestingly, nanotubes involved in cytoplasmic exchange have been reported to be dependent on a conserved set of proteins involved in assembly of the flagellar motor known as the type III secretion system core complex (CORE): FliP/O/Q/R and FlhA/B [18,24]. Recently, it was also shown that the formation of bacterial nanotubes significantly increases under stress conditions or in dying cells, caused by biophysical forces resulting from the action of the cell wall hydrolases LytE and LytF [28].…”

Real-time tracking of bacterial membrane vesicles reveals enhanced membrane traffic upon antibiotic exposure

“…S5A), suggesting that vesicle formation in E. coli is unlikely part of the programmed SOS pathway as proposed in the literature ( 26 ). Another mechanism for vesicle production that is largely debated in the field is cell death: Dying cells release increased amounts of MVs in the intercellular medium ( 11 – 13 ). The contribution of cell death to increased vesicle production was examined in our samples, although we worked with low doses of antibiotics with minor effects on viability during the time length of the experiment (fig.…”

“…In particular, the work of Schooling and Beveridge ( 39 ) established MVs as common constituents of the matrix of Pseudomonas aeruginosa biofilms. In this context, MVs’ signal changes in the extracellular matrix composition, such as the presence of dead cells, are typically associated with massive release of MVs ( 11 , 12 , 13 ). Per our results, MVs could signal changes in the density of cell surface appendages caused by antibiotics or other sources of cellular stress, thereby prompting changes in biofilm architecture as a response to specific environmental conditions.…”

Adaptivity and dynamics in type III secretion systems

“…No functional connection between the export apparatus and nanotubes could be identified. Based on the recent finding that nanotubes are associated with high stress levels and cell death, rather than nutrient extraction (Pospíšil et al, 2020), the export apparatus, which spans the IM, might merely serve as a nucleation point for cell death-induced nanotube formation.…”