While questions remain about how common bacterial nanotubes are and what they do, researchers have identified key differences in their structure and function that set them apart from pili used for mating, injectisomes that transfer virulence proteins, and flagella that power swimming in many microbes.
Bacterial Nanotubes | Conjugative Pili | Type 3 Secretion Systems, e.g., Injectisomes and Flagella | |
 |  |  | |
| Composition; structure | Lipids; segmented | Proteins; helical | Multiprotein complex; tubular |
| Length | 1–40 μm | 1–2 μm | 0.8–2 μm |
| Width | 30–130 nm; commonly 40–70 nm | 6–11 nm; lumen diameter ~3 nm | 8–10 nm; lumen diameter ~2.5 nm |
| Materials transferred | Antibiotic resistance factors, metabolites, toxins | Plasmids | Injectisomes for the transfer of virulence proteins; flagella for motility |
| Proteins involved in formation | CORE complex (same proteins as the flagellar base) and hydrolases that help make a hole in the cell wall | “Transfer” (Tra) class of proteins such as pilin, TraL, and TraF | The injectisome complex has various proteins such as secretin, stalk protein, and needle filament; the flagellar apparatus has its own set of dedicated proteins that form the base, stalk, and tip. |
Putative Functions of NanotubesNumerous studies have identified various possible roles for bacterial nanotubes, which researchers have observed under different growth conditions. | |||
Function | Species found | Culture conditions | Citations |
Transfer of materials (RNA, proteins, amino acids, toxins) | Bacillus megaterium, B. subtilis, Clostridium acetobutylicum*, Desulfovibrio vulgaris*, E. coli, | Solid or liquid media, depending on the study | Cell, 144:590–600, 2011; Nat Comm, 11:4963, 2020; Nat Comm, 6:6283, 2015; Cell, 177:683–96.e18, 2019 |
Adhesion to mammalian cells | Enteropathogenic E. coli | Liquid media | Cell, 177:683–96.e18, 2019 |
Adhesion to nanopillars and the formation of biofilms | Pseudomonas aeruginosa | Liquid media | Soft Matter, 16:7613–23, 2020 |
Stress response in dying cells placed under pressure | B. subtilis | Liquid media | Nat Comm, 11:4963, 2020 |
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