Bacterial membrane vesicles are released by all bacteria as part of their normal growth. Both Gram-negative and Gram-positive bacteria produce membrane vesicles, referred to as outer membrane vesicles (OMVs) and membrane vesicles (MVs), respectively. Although initially considered to be artifacts of bacterial growth, it is now well established that bacterial membrane vesicles are a bone fide secretion mechanism used by all bacterial to facilitate intracellular communication, bacterial survival and to promote disease. To date, most of our knowledge regarding the composition and functions of bacterial membrane vesicles is about OMVs produced by Gram-negative bacteria. In contrast, less is understood about MVs produced by Gram-positive bacteria, and in particular we have very limited knowledge regarding their cargo composition, immunogenicity and intracellular fate once within host cells.
Here we discuss factors that regulate the size, composition and immunogenicity of OMVs produced by Gram-negative bacteria. In addition, we discuss our recent findings identifying the cargo composition of MVs, the mechanisms whereby they mediate innate immune responses and their intracellular fate. Collectively, the findings advance our limited knowledge regarding the regulation of bacterial membrane vesicle size, composition and immunogenicity, and identify key factors that need to be considered for the refinement of their use in a range of medical and therapeutic applications.