Bacterial extracellular vesicles: their nature and role in human pathology

Extracellular vesicles have been observed to bud from eukaryotic cells and bacteria. These membranebound, spherical structures range from 20 to 500 nm in diameter and enclose diverse molecular components. In eukaryotic cells, exosomes represent the prototypical extracellular vesicles. Gram-negative bacteria release outer membrane vesicles, whereas Gram-positive bacteria produce membrane vesicles despite their thick peptidoglycan-rich cell wall. More recently, outer-inner membrane vesicles have been described in both saprophytic and pathogenic bacteria. These vesicles carry proteins, lipids, enzymes, virulence factors, various types of RNA, and in some cases, DNA. Bacterial vesicles play critical roles in biofilm formation, antibiotic resistance, DNA transfer, delivery of virulence factors to eukaryotic cells, and immune response regulation. In this review we highlight the importance of these vesicles in human pathology, with particular emphasis on outer-inner membrane vesicles containing chromosomal DNA. We hypothesize that such vesicles may represent a novel adaptative strategy in non-sporulating bacteria to withstand environmental stress. This review is specifically aimed at biomedical professionals who are not specialists in microbiology.