News this month from Imperial College London caught my attention with the provocative headline, “New study shows how bacteria evolved powered-up propellers.” An image caption goes on to say that “Dr. Morgan Beeby’s lab has discovered how some flagella — tiny ‘tail’ propellers used by bacteria for swimming — evolved to be more powerful.” As I read the body of the news item and the original paper on which it is based1, it quickly became apparent that the paper does not demonstrate this at all. This is a classic case of the media overstating the evidence for some evolutionary claim.
What Was the Paper’s Key Finding?
The study concerns the flagellum of Campylobacter jejuni, which has a wider ring of stator complexes relative to E. coli or Salmonella, supported by a tripartite scaffold (basal, medial, and proximal disks) that enables higher torque for movement through more viscous environments such as mucous. The medial disk is comprised of PflC and PflD. A key finding of the paper is that PflC has distant homology to a class of enzymes, the HtrA serine proteases, which ordinarily refold or degrade misfolded or damaged proteins in the periplasm. The expression of these proteases is upregulated at high temperatures, enabling cells to survive thermal stress by preventing the accumulation of misfolded proteins. HtrA proteases can also contribute to virulence by cleaving host cell proteins to disrupt epithelial barriers, and also promote evasion of the host immune response by degrading antimicrobial peptides or host defense proteins. The PflC protein, though distantly homologous to this class of enzymes, has apparently lost its catalytic function. Imperial College London asserts that “This finding offers new insights into how molecular machines evolve, and reinforces an emerging understanding of ‘evolution as tinkerer,’ bolting-together pre-existing parts to make new or improve existing (molecular) gadgets.”
Does This Show How the Flagellar Structure Could Have Arisen?
But does this paper provide insight into how the Campylobacter jejuni flagella evolved their higher-powered motors? Not particularly. The researchers document a distant homology between PflC and the HtrA proteases. But this does not demonstrate the feasibility of co-opting an enzyme to form a structural part of the flagellar system by unguided evolutionary processes, which is the key point of contention between advocates of modern evolutionary theory and proponents of intelligent design. Indeed, the paper does not even attempt to address this question.
The Challenges
It is not at all obvious that such a complex transition is evolutionarily feasible, and its plausibility cannot therefore be assumed. In order to form a lattice structure, the PflC proteins would need to develop new binding interfaces such that they can interact tightly with other PflC proteins, in such a way that allows a 17-part ring and a complex layered network. Moreover, the PflC proteins share parts of their structure with neighboring copies, a phenomenon known as domain swapping (this is illustrated in figure 3c). This requires precise alignment between parts of different proteins, as well as flexible linkers so that the domains can move into place.
The PflC proteins also have to be able to associate with the others at precisely the right angles — otherwise, the complex will not correctly assemble. Evolving an entire set of precisely angled interactions while retaining selective utility at every stage seems to be prohibitively improbable.
Interestingly, the authors of the paper purified PflC and analyzed it using size exclusion chromatography and mass photometry. They found that PflC was largely monomeric (i.e., existing as single units) when isolated, and did not form large assemblies or oligomers. Thus, PflC does not spontaneously form a lattice or even stable dimers or multimers — it only does so in the context of the larger flagellar system, when attached to the basal disk (FlgP) of the flagellum.
The authors wondered whether there was something intrinsic to PflC that prevented it from self-assembling outside of the flagellar context. When they experimentally removed the c-terminal domain of PflC, they found that the modified protein formed dimers more readily than before. Thus, they concluded that the n- and c-terminal domains may interact within the same protein, blocking it from binding to other copies and forming oligomers. However, in the context of the flagellar structure, those blocking interactions are released and it is able to form the lattice structure. It is also noteworthy that deletion of PflC results in “destablization of the proximal and medial disks.” This reveals that PflC is crucial for assembly of the scaffold layers. PflD also appears to be important, and its deletion results in “loss of a peripheral cage-like structure between the medial and proximal disks.”
There are thus various coordinated changes that would need to take place before the PflC proteins could be incorporated into the flagellar structure, and it is difficult to envision it evolving gradually in incremental stages. Not to mention the need for coordinating the expression of these proteins with other flagellar genes — i.e., their folding and assembly must be integrated into the existing assembly pathway without disrupting it.
Be Wary of Sensationalistic Headlines
The headline that prompted me to read this paper claimed that the study “shows how bacteria evolved powered-up propellers.” But the paper does not do this at all. All it shows is another example of a flagellar protein that bears homology to a non-flagellar protein. There is not even an attempt to demonstrate the mathematical plausibility of such transitions taking place by undirected processes. This is a good reminder of why it is always important to be cautious upon encountering a sensationalistic headline — always consult the original research for yourself to see whether it delivers the goods that are being promised.
One good statement in the news item is that “How flagellar motors work and how they evolved, however, remains incompletely understood.” That’s true, and it’s still true in light of the findings of this paper.
Notes
- Drobnič T, Cohen EJ, Calcraft T, Alzheimer M, Froschauer K, Svensson S, Hoffman WH, Singh N, Garg SG, Henderson LD, Umrekar TR, Nans A, Ribardo D, Pedaci F, Nord AL, Hochberg GKA, Hendrixson DR, Sharma CM, Rosenthal PB, Beeby M. In situ structure of a bacterial flagellar motor at subnanometre resolution reveals adaptations for increased torque. Nat Microbiol. 2025 Jul;10(7):1723-1740. doi: 10.1038/s41564-025-02012-9. Epub 2025 Jul 1. PMID: 40595286; PMCID: PMC12222027.