Extrusion of extracellular membrane vesicles from hyphal tips of Streptomyces venezuelae coupled to cell-wall stress.

Extracellular vesicle release is a wide-spread and broadly important phenomenon in bacteria. However, not much is known about the mechanism of vesicle release in Gram-positive bacteria. Observations of polarly growing Streptomyces venezuelae by live cell time-lapse imaging reveal release of extracellular membrane vesicles from tips of vegetative hyphae. Vesicle extrusion is associated with spontaneous growth arrests, but often the apical cell survives and can re-initiate growth by forming new hyphal branches. Treatment with vancomycin to block peptidoglycan synthesis leads to a high frequency of lysis and vesicle extrusion, where some hyphae can survive growth arrest and vesicle extrusion and reinitiate growth after antibiotic is washed away. The extruded vesicles do not contain nucleoids and do not appear able to proliferate. Vesicle extrusion is not affected by the Ser/Thr protein kinase AfsK that phosphorylates the DivIVA at hyphal tips, nor is it affected by the intermediate filament-like protein FilP that localizes in gradient-like structures at hyphal tips. Notably, hyphae of a scy mutant, which has an unstable apical polarisome structure, are prone to spontaneous growth arrests and vesicle extrusion even in the absence of antibiotic treatment, supporting the idea that the nature of the growth zone at the hyphal tips is important for this route of extracellular vesicle formation. We speculate that the propensity for vesicle extrusion is a direct consequence of how polar growth is organized at hyphal tips in Streptomyces, with the cell-wall sacculus being weak and susceptible to bursting at the apical zones of growth where peptidoglycan synthesis is primarily taking place.