Alkaline shock protein 23 (Asp23)-controlled cell wall imbalance promotes membrane vesicle biogenesis in Staphylococcus aureus

Membrane vesicles (MVs) are produced by species across all domains of life and have diverse physiological functions as well as promising applications. While the mechanisms for vesiculation in Gram-negative bacteria are well-established, the genetic determinants and regulatory factors responsible for MV biogenesis in Gram-positive bacteria remain largely unknown. Here, we demonstrate that a Q225P substitution in the alternative sigma factor B (SigB) triggers MV production in Staphylococcus aureus strain Newman by hindering the specific binding of SigB to the asp23 promoter, thereby repressing expression of alkaline shock protein 23 (Asp23). Isogenic deletion of asp23 also promotes MV formation in Newman, confirming the critical roles played by sigB and asp23 in modulating S. aureus vesiculation. While bacterial growth and cytoplasmic membrane fluidity are not impaired, mutation of asp23 weakens the cell wall and enhances autolysis, consistent with decreased expression of alpha-type psm and lrgAB that modulate murein hydrolase activity. TEM and proteomic analysis show that Newman and asp23 deletion mutant generate MVs with nearly identical morphology and composition, but virulence-associated factors are significantly enriched in MVs from the asp23 mutant. Overall, this study reveals novel genetic determinants underlying S. aureus vesiculation and advances the understanding of the physiology of MV biogenesis in S. aureus.