That question sits at the heart of our recent study, “A two-step actin-mediated strategy enables Campylobacter jejuni to promote mitochondrial aggregation and iron homeostasis for intracellular survival and persistence.” The project began with a simple observation that refused to be simple: host cell mitochondria behaved very differently during infection.
Why look at mitochondria at all?
Campylobacter jejuni is best known as one of the leading causes of bacterial gastroenteritis worldwide. Despite its public-health importance, it has long been considered a relatively “fragile” bacterium — microaerophilic, sensitive to environmental stress, and poorly equipped to survive outside animal hosts.
Yet epidemiological data tell a different story. C. jejuni persists in the environment, survives processing chains, and repeatedly infects humans. This contradiction pushed us to ask: where does C. jejuni hide, and how does it adapt to hostile conditions?
Our lab has been using free-living amoebae as an infection model, an environmental phagocytic cells that share remarkable functional similarities with macrophages. Amoebae engulf bacteria, impose intracellular stress, and rely on mitochondria for energy, redox balance, and innate defence. That made them an ideal system to explore a neglected question: does C. jejuni manipulate host mitochondria to survive intracellularly?
A surprising pattern inside infected cells
Early microscopy experiments gave us our first clue. In infected amoebae, mitochondria were no longer evenly distributed throughout the cytoplasm. Instead, they clustered tightly around internalised C. jejuni.
At first, we were sceptical. Mitochondrial clustering can be an artefact — fixation issues, staining bias, or imaging artefacts can all mislead. So we quantified everything: spatial distribution, radial distance from bacteria, mitochondrial membrane potential, and host cytoskeletal architecture.
The pattern held. Infected cells consistently showed mitochondrial aggregation, increased mitochondrial membrane potential, and altered iron and reactive oxygen species (ROS) dynamics. Importantly, these changes were not signs of host cell death. Instead, they suggested active mitochondrial adaptation.
That raised a key question: who was driving this change — the host, or the bacterium?
Following the actin trail
The next breakthrough came when we looked at the actin cytoskeleton. Actin is often discussed in the context of bacterial entry, but it also plays a central role in positioning organelles, including mitochondria.
We found that C. jejuni uses a two-step, actin-mediated strategy:
- Early actin remodelling promotes bacterial internalisation and intracellular positioning.
- Post-entry actin modulation reshapes mitochondrial organisation, drawing mitochondria into close proximity with the bacteria.
This second step turned out to be critical. By using bacterial mutants and protein-coated bead assays, we were able to separate actin’s role in uptake from its role in intracellular organisation. The bacterium wasn’t just entering the cell, it was rewiring the host’s internal architecture after entry.
Effector proteins with distinct roles
A major challenge was untangling the functions of different C. jejuni effector proteins, particularly the Campylobacter invasion antigens (Cia proteins). These proteins are often discussed collectively, but our data showed they play non-redundant, spatially distinct roles.
One effector primarily promoted actin polymerisation and bacterial uptake. Another acted later, subtly modulating actin dynamics to influence mitochondrial positioning. Disrupting these proteins didn’t just reduce bacterial survival, it abolished mitochondrial aggregation entirely.
This was a crucial insight: mitochondrial clustering was not a by-product of infection, but an actively driven process.
Iron, energy, and survival
Why mitochondria? The answer appears to lie in iron and energy homeostasis.
Mitochondria are central hubs for iron metabolism, ATP production, and ROS signalling. Our proteomics, transcriptomics, and functional assays all pointed in the same direction: infected host cells showed increased mitochondrial metabolic activity, altered iron handling, and tightly regulated ROS levels.
For C. jejuni, this environment is ideal. Close association with metabolically active mitochondria likely provides access to essential nutrients while buffering against oxidative stress. Rather than triggering a destructive immune response, the bacterium appears to co-opt host stress responses for its own benefit.
Challenges behind the scenes
This project was not straightforward. Live imaging in amoebae is technically difficult. Many dyes do not survive fixation, and spectral overlap quickly becomes a problem when imaging bacteria, mitochondria, and actin simultaneously. Quantifying mitochondrial organisation required moving beyond “representative images” to robust, spatial statistics.
Perhaps the biggest challenge was conceptual. Mitochondria are usually discussed in the context of apoptosis or antiviral defence — not bacterial persistence. Convincing ourselves (and later, reviewers) that mitochondrial aggregation could be adaptive rather than pathological required multiple orthogonal approaches and a lot of careful controls.
Why this matters?
This study adds to a growing body of evidence that bacteria do not merely evade host defences — they reshape fundamental host cell processes to create permissive niches. It also highlights mitochondria as underappreciated players in bacterial infections.
More broadly, our findings support the idea that environmental hosts such as amoebae act as training grounds for pathogens. The strategies C. jejuni uses to manipulate amoebae may pre-adapt it for interactions with macrophages and other phagocytic cells in warm-blooded hosts.
What comes next?
We are now exploring how mitochondrial remodelling intersects with other stress-response pathways, including organelle crosstalk and long-term bacterial persistence. Understanding these interactions may reveal new targets for antimicrobial intervention, not by killing bacteria directly, but by disrupting the host environments they depend on.
Behind every paper is a story of unexpected observations, false starts, and gradual clarity. This one began with oddly clustered mitochondria and ended by revealing a strategy that helps a major pathogen survive inside host cells.