In a groundbreaking breakthrough set to shift paradigms within the microbiology and evolutionary biology communities, researchers have unveiled the detailed structure of a functional archaellum in a bacterial member of the Chloroflexota phylum. This discovery challenges long-standing views about the distribution and evolution of motility apparatus across the domains of life, revealing an unexpected convergence of motility structures previously thought to be exclusive to distinct biological kingdoms. The study not only deepens our understanding of microbial locomotion but also offers new avenues for exploring molecular machinery evolution, with potential implications spanning synthetic biology, bioengineering, and microbiome research.
The archaellum, primarily known as a unique motility organelle found in Archaea, is a type of rotary propeller responsible for swimming in many archaeal species. Until now, the presence of archaella in bacteria was unconfirmed and considered unlikely due to the fundamental differences traditionally recognized between the bacterial flagellum and the archaeal archaellum structures. The team of scientists led by Sivabalasarma et al. has overturned this assumption by demonstrating that certain bacterial species within the Chloroflexota phylum not only possess but actively employ a functional archaellum for motility. This is a first-of-its-kind finding that could rewrite the genetic and functional maps associated with prokaryotic motility.
Delving into the structural intricacies of this archaellum, the researchers utilized state-of-the-art cryo-electron microscopy to capture molecular images with near-atomic resolution. The detailed architecture revealed a complex and highly ordered helical filament that mirrors characteristic features classically attributed to archaeal archaella. Notably, the filament showed similarities in the assembly of its filamentous subunits and the interactions with its motor complex, suggesting a conserved evolutionary mechanism governing its construction and function. These observations imply that the archaellum’s design principles might transcend domain boundaries, challenging the dogma separating bacterial flagella and archaella.
Structurally, the archaellum filament is composed of multiple copies of archaellin proteins, arranged in a helical symmetry that facilitates its rotational motion. Unlike bacterial flagella, which assemble from flagellin subunits and operate via a molecular motor distinct in composition, this archaellum evidences a rotary mechanism driven by ATP hydrolysis similar to that known in Archaea. The discovery of homologous ATPase components within the bacterial strain under study underscores an inheritance or lateral gene transfer event bridging two previously segregated evolutionary pools. Importantly, the study clarifies that this apparatus is not a vestigial relic but a fully operational nanomachine enabling bacterial motility in complex environments.
From an evolutionary standpoint, the findings challenge the neat compartmentalization of motility systems once believed to be domain-specific. The presence of a functional archaellum within Chloroflexota bacteria implies either a retained ancestral trait from a common progenitor or sophisticated horizontal gene transfer events that have imparted archaellum-related genes across domain lines. This revelation prompts a reassessment of evolutionary trajectories where motility modules may have been far more fluid and exchangeable than previously assumed. The data spark intriguing hypotheses about the co-evolution of motility systems, microbial ecology, and adaptability allowing organisms to conquer diverse ecological niches.
Moreover, the archaeal-type motility machinery in bacteria raises questions about the selective pressures favoring such a crossover. The Chloroflexota phylum encompasses organisms inhabiting microoxic or thermophilic environments where standard bacterial flagella may be less effective or stable. The archaellum, with its ATP-driven rotary motor, might provide superior motility solutions under these stressors. Functional assays detailed in the study demonstrated the archaellum’s capability to power swimming in viscous media, implying an evolutionary advantage in habitat colonization and resource acquisition.
At the molecular level, the research dissects the gene clusters encoding the archaellum. It was found that Chloroflexota genomes contain operons bearing marked similarity to archaeal archaellum-associated genes, including components encoding archaellins, motor ATPases, and assembly chaperones. Transcriptomic and proteomic analyses confirmed the active expression and incorporation of these elements into functioning archaellar filaments, further validating that these structures are not merely genetic artifacts but operational motility devices. This multi-omics approach affirms the structural observations and points to a tightly regulated genetic program supporting archaellum biosynthesis.
The implications extend to microbial motility research, wherein this discovery raises fresh questions about the distribution and functional diversity of locomotion organelles. Bacteria previously thought limited to flagellum-driven motility may harbor unrecognized variants, broadening the landscape of microbial movement strategies. Furthermore, understanding the assembly, energetics, and regulation of the bacterial archaellum could inspire synthetic biology initiatives aiming to engineer novel nanomachines with tailored mechanical properties, useful for applications ranging from targeted drug delivery to environmental sensing.
This discovery also sheds light on the potential evolutionary ancestral links between archaeal and bacterial motility systems. The archaellum in bacteria might represent a living molecular fossil or an adaptive innovation through gene exchange, redefining the boundary between bacterial and archaeal cellular features. Such insights enrich our comprehension of early life evolution and the intricate genetic interplay that forged the complex microbial communities observable today. The archaellum’s presence in bacteria bridges a long-standing gap and underscores the mosaic nature of prokaryotic genomes shaped by ancient and ongoing genetic flux.
Experimental validations performed by the team included motility assays under varying environmental conditions confirming that motility in Chloroflexota is indeed mediated by the archaellum. Mutational inactivation of key archaellum components abolished swimming ability, demonstrating causality. Additionally, imaging of live cells highlighted the dynamic assembly and rotation of archaellar filaments, paralleling behavior observed in archaeal cells. The coupling of biophysical measurements with molecular biology strengthens the argument for the archaellum’s role as an authentic functional organelle within these bacteria.
The broader ecological consequences of this finding may be profound. The motility endowed by archaella may influence biofilm formation, nutrient cycling, and microbe-host interactions in habitats where Chloroflexota thrive, such as hot springs and subsurface ecosystems. By facilitating active movement, these bacteria can more effectively colonize niches, evade predators, or compete for resources, impacting community structure and ecosystem dynamics. Future studies may unravel how archaellum-driven motility integrates with other cellular functions and microbial cooperative behaviors.
Technological breakthroughs enabling this discovery highlight the power of integrative structural biology combined with genomic editorial insight. Cryo-electron microscopy, coupled with advanced sequence analysis and functional genomics, provided a multi-dimensional understanding of the archaellum. This interdisciplinary strategy sets a new benchmark for uncovering hidden complexities in microbial cellular biology and emphasizes the importance of exploring understudied microbial phyla to unlock novel biology.
Beyond bacterial physiology, the existence of a bacterial archaellum opens the door to rethinking biotechnological exploitation of motility systems. The bacterial versions of the archaellum might be harnessed for designing biomolecular machines capable of controlled movement in engineered systems. Insights into the archaellular assembly could inform the development of nanoscale mechanical devices, with the bacterial chassis potentially offering new functionalities not present in archaeal counterparts.
Ultimately, this study by Sivabalasarma and colleagues catalyzes a paradigm shift in microbiology by revealing that motility machinery once considered exclusive to Archaea also operates effectively in Bacteria of the Chloroflexota phylum. The findings urge renewed investigation into the diversity of microbial organelles, their evolutionary histories, and their multifaceted roles in life’s vast tapestry. As we unravel these molecular secrets, our appreciation for nature’s ingenuity and the evolutionary fluidity of life’s building blocks deepens, opening unexplored frontiers in science and technology.
Subject of Research: Motility structure (archaellum) in bacteria of the Chloroflexota phylum
Article Title: Structure of a functional archaellum in Bacteria of the Chloroflexota phylum
Article References:
Sivabalasarma, S., Taib, N., Mollat, C.L. et al. Structure of a functional archaellum in Bacteria of the Chloroflexota phylum. Nat Microbiol (2025). https://doi.org/10.1038/s41564-025-02110-8
Image Credits: AI Generated
