Eukaryotic cells constantly and naturally release extracellular vesicles in numerous biological processes. From physiological processes, such as regulating metabolism (12), to pathological processes, such as infection (7) and cancer (13), EVs all play crucial roles. EVs are highly heterogeneous and generated in different pathways. Currently, the generation of the three types of EVs is relatively clear, and they are apoptotic bodies, exosomes and microvesicles. Apoptotic bodies are released from the cell plasma membrane by increased hydrostatic pressure after apoptosis (14). Exosomes form through several processes: (a) invagination of the endosomal membrane; (b) formation of multivesicular bodies; (c) fusion of multivesicular bodies with the plasma membrane; and (d) release from the cell (15). Microvesicles form by a direct outward budding from the cell membrane. Consistent with their generation procedure, EVs are filled with cytosolic proteins such as HSC70, Alix and TSG101 (16). EVs also contain transmembrane proteins such as CD9, CD63 and CD81 (17, 18). Other molecules in the cell matrix, such as RNA, may be encapsulated into EVs during generation, which enables EVs to transfer regulatory messages between cells (19).

Bacterial EVs (bEVs) were first reported in Gram-negative bacteria in the 1960s (20), while bEVs from Gram-positive bacteria were not reported until 1990 (21). Due to the existence of the thick peptidoglycan cell wall and the lack of periplasmic space and outer membrane in the cell wall, it was thought to be difficult for Gram-positive bacteria to release bEVs; thus, early studies mainly focused on bEVs from Gram-negative bacteria (22, 23). Outer membrane vesicles (OMVs) are well-explored bEVs from Gram-negative bacteria. OMVs bud from the outer layer of the Gram-negative bacterial cell wall and consist of lipopolysaccharide (LPS) and outer-membrane proteins, which are consistent with their cell walls (3). Apart from OMVs, outer-inner membrane vesicles (O-IMVs) are also found in Gram-negative bacteria (24). This kind of bEV contains both outer membranes and cytoplasmic membranes (25) and may be generated by explosive cell lysis (26). Furthermore, bEVs are also detected in Gram-positive bacteria despite their thick cell walls (27). Little is known about the mechanism, and they may be generated by turgor pressure from the cell wall (28) or cell wall­modifying enzymes (29). The last kind of bEV is nanotube, which is tube-like protrusions of the bacterial membrane and functions as bridges between bacteria for substance exchange (30). BEVs play important roles in bacterial biological processes, including transferring DNA (31), killing other bacteria (32), neutralizing phages (33) and transferring virulence to host cells (8).

After infection, host-derived EVs (hEVs) demonstrate both antibacterial and probacterial effects under different conditions (7, 34, 35). On the one hand, the host could resist infection through hEVs in several ways. First, damaged membranes induced by bacterial toxins are repaired by releasing hEVs, which is promoted by the calcium-activated lipid scramblase TMEM16F (36) (Figure 1A). Second, infected cells expel bacteria through hEVs. When infected with uropathogenic Escherichia coli, a lysosomal channel called TRPML3 sensed the change in lysosome pH and led to Ca2+ efflux, and this efflux triggered hEVs release to expel bacteria (37) (Figure 1B). Third, hEVs protect host cells from toxins by binding to toxins as decoys (7). In these hEVs, a special protein, named ADAM10 (38), existed on the surface and was able to bind to alpha-toxin of methicillin-resistant Staphylococcus aureus, which enabled hEVs to neutralize toxin (Figure 1C). Fourth, hEVs participate in innate immunity. Mtb-infected macrophages released hEVs to healthy macrophages, provoking a RIG-I/MAVS/TBK1/IRF3 RNA sensing pathway and the generation of type I interferon (39). HEVs from LPS-activated macrophages induced MyD88/NF-κB signaling and promoted the expression of cytokines and chemokines, such as IL-4, IL-1β, IFN-γ, CCL4 and CCL19 (40) (Figure 1D). Fifth, hEVs participate in adaptive immunity by contributing to the T-cell response. Phagocytic events enhanced DCs to release hEVs, and these hEVs demonstrated MHC molecules on the surface, enabling hEV antigen-presenting capacity (6). Infected macrophages were also able to release MHC molecule-demonstrating hEVs for adaptive responses (41) (Figure 1E).

On the other hand, pathogenic bacteria attack and damage host cells through hEVs. Lethal factor is one of two parts of anthrax lethal toxin. Lethal factors could be translocated to the lumen of host vesicles during assembly and endocytosis, where they were sheltered from degradation for days and could be delivered to the extracellular medium, leading to damage to the host (10) (Figure 1F). Moreover, Listeria monocytogenes took advantage of hEVs to deliver its DNA to bystander cells. These bacterial DNA-containing hEVs inhibited T-cell proliferation and induced apoptosis (42). Thus, Listeria monocytogenes impaired antimicrobial defense via hEVs (Figure 1G). In terms of probiotics, the host can release hEVs to modulate probiotic functions. Colonic epithelial cells released hEVs to Lactobacillus rhamnosus GG, which might mediate the production of p40. Consequently, Lactobacillus rhamnosus GG released p40 into the enteric cavity, protecting epithelial cells against inflammation (35).

Bacterial EVs possess many functions in the interaction between the host and the bacteria. One of the most important functions is to transfer virulence to the host cells. For example, VacA, a Helicobacter pylori (H. pylori) toxin, was constitutively released via bEVs, accounting for 25% of total VacA (43). This bEV-associated VacA was internalized by gastric epithelial cells, which promoted low-grade gastritis to support the persistence of H. pylori (44). Bacteria exploit bEVs as an alternative secretory pathway to cover the shortage of other secretory systems, and the mechanisms by which bEV-associated toxins enter host cells may be completely different (45). Compared with free toxin, bEVs provide protection against digestion by intestinal proteases, thus allowing the bacteria to target host cells distant from the primary colonization sites (46). BEVs are able to travel through barriers. In an in vitro intestinal epithelial cell model, bEVs could be transported across a polarized cell monolayer with an estimated average uptake efficiency of 30%, suggesting the possibility that bEVs might travel across the gastrointestinal epithelium (47). Moreover, studies showed that bEVs were successfully delivered to the brain crossing the blood–brain barrier and were taken up by meningeal macrophages in mice (48, 49).

Apart from virulence transfer, bEVs contribute to the immune evasion of bacteria. Bacteria release bEVs as strategies to resist antimicrobial molecules from the host. BEVs from Salmonella expressed PagC, which was able to bind to complement component C3b and complement inhibitor factor H. These PagC+ bEVs recruited complement inhibitor factor H and degraded active C3 (50). KatA in H. pylori EVs detoxified reactive oxygen species released by immune cells and rescued bacteria from killing during the respiratory burst (51). BEVs are also released to resist polymyxin (52) and antimicrobial fatty acids (53), protecting bacteria from harm.

The respiratory system is one of the most investigated systems in terms of EVs. The main topics involve mycobacterial species such as Bacillus Calmette-Guerin (BCG), Mycobacterium tuberculosis (Mtb) and Pseudomonas aeruginosa (P. aeruginosa).

In terms of the gastrointestinal system, the focus of EV studies relies on H. pylori and Salmonella typhimurium.

HEVs carry specific molecules in both the surface and lumen, and these biomarkers differ according to the sources and diseases, which shows the potential for diagnosis. Recognition of biomarkers in hEVs with a fast examination technique provides better and faster results in diagnosis. For example, the combination of hEV DNA and droplet digital PCR presented higher sensitivity for tuberculosis detection, especially when bacterial loads were low (85). HEVs can also be used in differential diagnosis. The ability to aggregate with bacteria ensured hEVs to distinguish bacterial infection from noninfectious inflammation (86). Differences in akt and CD9 from urinary EVs might be markers for differentiating urinary tract infection from asymptomatic bacteriuria (87).

Specific subtypes of hEVs exert therapeutic effects. MSC-derived EVs protected against infection in acute lung injury (88). Combination with other materials, such as antibiotics and other nanomaterials, also improves the therapeutic effects. Either administrating hEVs with antibiotics or using antibiotic-containing hEVs provided better outcomes in infected mice (89). A dressing with a combination of silver nanoparticles and hEVs promoted angiogenesis and wound healing in P. aeruginosa-infected mice (90).

BEVs contain many pathogen-associated molecular patterns, such as flagella, peptidoglycans and LPS, which not only provoke the innate immune response but also induce the adaptive immune response. Due to their immunogenicity, bEVs are suitable for adjuvants and vaccines. Compared with the standard adjuvants, bEVs from H. pylori efficiently triggered the Th1 immune response, and the response was skewed toward Th2- and Th17-biased immunity against H. pylori, suggesting a higher efficiency in inducing immune responses (91). BEVs from flagellin-deficient Salmonella typhimurium (92) and Pseudomonas pseudomallei (93) also showed competency as adjuvants. In terms of vaccines, bEVs are able to trigger both humoral immunity (94) and cellular immunity (95) and generate memory cells (96). A retrospective case–control study demonstrated that the outer membrane vesicle meningococcal B vaccine protected individuals from gonorrhea owing to cross-protection with 31% effectiveness (97).

Engineered bEVs are suitable vehicles in antitumor treatment. Several ways have been developed to engineer bacteria: (1) expressing tumor-targeting ligands in bEVs to specifically bind to tumor tissues and to increase drug delivery efficiency (98); (2) expressing tumor proteins to induce antitumor responses as vaccines (99); and (3) expressing proteins such as PD1 to enhance the immune response (100). Meanwhile, as natural cargos, bEVs possess the ability to deliver drugs. Chemotherapeutic drugs, such as doxorubicin (101), and gene therapy drugs, such as siRNA (98), can be packaged into bEVs. With these two advantages, bEVs have great potential in antitumor therapy.