A 100-Micrometer Bacterium With a 45-Micrometer Mystery Tube: Textbook Definitions Under Pressure

Forum topic · ✨步子哥 · 2026-08-06

Summary

Researchers from Pusan National University, NIPS, Kobe University, and Toyohashi University of Technology have discovered a previously unknown tubular structure inside Profftella armatura, an intracellular symbiotic bacterium of the Asian citrus psyllid (Diaphorina citri), published in npj Imaging (DOI: 10.1038/s44303-025-00107-w). Using SBF-SEM, high-voltage electron tomography, and FISH imaging, the team found 1-43 tubes per cell, each up to 45 micrometers long and about 230 nm in diameter, built from 5-6 fibrils twisted into a right-handed helix resembling a steel cable. The tubes survive detergent lysis without fixation or embedding, indicating extreme structural stability, and are closely associated with ribosomes. Two hypotheses are proposed: mechanical support for the bacterium's extreme elongation (cells up to 136 micrometers, visible to the naked eye) and a role in organizing protein synthesis. The finding challenges the textbook claim that prokaryotes lack organelle-like compartmentalization and suggests symbiotic constraints drove this unexpected internal complexity.

A new study published September 18, 2025 in *npj Imaging* reports the discovery of an entirely unknown tubular ultrastructure inside a bacterium — the kind of "we don't know what this is" finding that has become vanishingly rare in 21st-century biology.

Paper: Song C, Maruyama J, Murata K, Suzaki T, Nakabachi A. *Enigmatic tubular ultrastructure in the bacterial defensive symbiont of the Asian citrus psyllid Diaphorina citri.* npj Imaging, 2025. DOI: 10.1038/s44303-025-00107-w

Key points

The organism: *Profftella armatura* is an obligate intracellular defensive symbiont living in the bacteriome of the Asian citrus psyllid (*Diaphorina citri*), a 3-mm agricultural pest that spreads citrus greening (huanglongbing). Profftella produces toxins that protect the insect; its co-symbiont *Carsonella ruddii* supplies essential amino acids.
The structure: Using serial block-face scanning electron microscopy (SBF-SEM), high-voltage electron tomography (HVET), and FISH, researchers found multiple tubes inside Profftella cells — 1 to 43 per cell, each up to 45 µm long and ~230 nm in diameter with remarkably uniform caliber.
Cable-like architecture: Each tube is made of 5–6 fibrils twisted into a right-handed helix, directly analogous to a steel cable — multiple strands outperforming a single thick rod in strength, flexibility, and bend resistance.
Extreme stability: Tubes released by detergent lysis — with no chemical fixation or resin embedding — retained their shape on EM grids under dehydration and vacuum, with helical striations clearly visible.
A giant bacterium: Profftella cells range from 2.8 to 136 µm in length and 2–5 µm in width — thicker than a human hair (~70 µm) and visible to the naked eye. Tubes occupy ~6.3% of cell volume, stabilizing around 7% once cell volume exceeds 80–90 µm³, suggesting regulated biogenesis rather than random artifacts.

Why it matters: the "prokaryotes are simple" myth erodes further

"No membrane-bound organelles" has long been a defining feature of prokaryotes. Known exceptions have been accumulating: carboxysomes, metabolosomes, magnetosomes, and chromatophores — though most are protein-shell microcompartments rather than true organelles. The timeline of erosion includes:

2000s: magnetosomes confirmed as membrane-bound bacterial organelles
2010s: carboxysome protein-shell architecture resolved
2019: bacterial microtubules discovered (homologs of eukaryotic tubulin)
2020s: liquid-liquid phase separation shown to create membraneless compartments in bacteria
2025: Profftella's tubes — a genuinely novel multi-fibril helical structure with no known counterpart in any organism

Two hypotheses, neither confirmed

1. Mechanical support: A cell 30× longer than its diameter is physically fragile. The tubes may function like a eukaryotic cytoskeleton, stabilizing the extreme elongated morphology — consistent with the cable-like twisted-fibril design. 2. Protein synthesis organization: Ribosomes cluster around the tubes (not randomly distributed), suggesting a possible role analogous to rough endoplasmic reticulum.

The hypotheses are not mutually exclusive — the tubes may be both scaffold and factory. The paper deliberately uses hedged language ("implies," "possible," "may"), reflecting honest uncertainty: chemical composition, biogenesis, exact function, and whether the structure exists in other symbionts all remain unknown.

Constraints breeding innovation

Profftella is an obligate intracellular symbiont with a heavily reduced genome (its partner Carsonella has just ~160 kb, one of the smallest known cellular genomes). The authors argue that symbiotic constraints — genome reduction, maternal transmission bottlenecks, and packing within the bacteriome — may have driven the evolution of novel internal architecture rather than new genes. Complexity emerged not by adding parts, but by organizing existing components in a new way.

Practically, the researchers suggest the structure could offer a target for controlling the citrus psyllid: disrupting the tubes might disable Profftella and strip the pest of its chemical defense.

The broader takeaway: when imaging technology improves — 3D tomography, cryo methods, multimodal approaches — bacterial interiors keep revealing overlooked complexity. Bacteria didn't become complex; we finally became able to see.