Bacterial extracellular vesicles as bioactive nanocarriers for drug delivery: Advances and perspectives

Abstract

Nanosized extracellular vesicles derived from bacteria contain diverse cargo and transfer intercellular bioactive molecules to cells. Due to their favorable intercellular interactions, cell membrane-derived bacterial extracellular vesicles (BEVs) have great potential to become novel drug delivery platforms. In this review, we summarize the biogenesis mechanism and compositions of various BEVs. In addition, an overview of effective isolation and purification techniques of BEVs is provided. In particular, we focus on the application of BEVs as bioactive nanocarriers for drug delivery. Finally, we summarize the advances and challenges of BEVs after providing a comprehensive discussion in each section. We believe that a deeper understanding of BEVs will open new avenues for their exploitation in drug delivery applications.

Keywords: Bacterial extracellular vesicles; Bioactive nanocarriers; Drug delivery; Isolation and purification.

Graphical abstract

Fig. 1

The biogenesis and composition of bacterial extracellular vesicles. The G⁻ BEVs are generated by two main models, blebbing of the outer membrane and explosive cell lysis. The insertion of hydrophobic molecules or the instability of PG biosynthesis into the outer membrane cause blebbing of the outer membrane, which produces classic OMV. The weakness of PG layer by endolysin leads the inner membrane protrudes into the periplasm, which generate explosive EOMV or OIMV. On the other hand, the biogenesis mechanism of G⁺ BEVs release is bubbling cell death. The endolysin degrades the PG layer and triggers bubbling cell death in G⁺ bacteria and produces CMVs. The difference in contents of G⁺ and G⁻ BEVs goes beyond the presence of LPS and includes other molecules, such as nucleic acids, proteins, lipids, and metabolites. LPS: lipopolysaccharide, OM: other membrane, PG: peptidoglycan, IM: Inner membrane.

Fig. 2

The isolation and purification of bacterial extracellular vesicles. (A) After cultured in appropriate time, the bacteria in the fermentation broth could be removed by low-speed centrifugation at 10, 000 g for 1 h. The supernatant is then filtered through a 0.22 μm sterile filter to remove residual bacteria. Subsequently, 100 KDa ultrafiltration membranes is required to concentrate BEVs and remove non-BEVs-associated proteins. The retentate is subjected to ultracentrifugation at 100, 000 g for 3 h to collect the non-purified BEVs, which could be further purified by density gradient centrifugation with iodixanol, and ultracentrifuged at 100, 000 g for 18 h. Finally, BEVs-rich fraction is diluted with PBS and collected by ultracentrifugation at 100, 000 g for 3 h. (B) TEM and NTA have been used to evaluate extracellular vesicles, as exemplified here by extracellular vesicles produced by Lactobacillus rhamnosus GG (scale bar = 50 nm).

Fig. 3

BEVs achieve access to systemic circulation through the disrupted tight junction under microbial disorder and inflammatory environment. Reprinted with permission [92]. Copyright 2020, Springer.

Fig. 4

The application of bacterial extracellular vesicles in drug delivery. As a nano-scale bioactive material, BEVs deliver a variety of active molecules such as nucleic acids, protein, metabolisms etc. for the treatment of gut, brain, bone, and tumors.

Fig. 5

Recognition of BEV-associated molecular patterns by host immune receptors. TLR1/6, TLR2, TLR4, and TLR5 located at the host cell membrane, TLR3, TLR7/8, TLR9 located at the endosomal membranes. The downstream signaling pathways that lead to activation of NF-kB and inflammatory genes. Adaptor molecules: MyD88, TRAP, TRIF, TRAM. TLRs: toll-like receptors. Reprinted with permission [93]. Copyright 2021, Wiley-VCH GmbH.

Fig. 6

The advantages and challenges of bacterial extracellular vesicles. BEVs display many desirable qualities such as easy of industrialization, high efficiency of drug delivery, ease of modification, and ease of bacterial infections diagnose. On the other side, there are still significant challenges for the clinical translation of BEVs as delivery systems or therapeutics, including potential biosafety, complex contents, time-consuming isolation, ambiguous mechanism.