When mitochondria go to war: how mitochondria help the innate immunity clear bacteria

For decades, mitochondria have been introduced in classrooms as the “powerhouse of the cell.” They generate energy. This turns out to be not all they do; Mitochondia do a lot more than we once thought.

This week’s blog explores a fascinating paper published last month, April 2026, in Science Immunology, titled

“Mitochondrial fission mediates an evolutionarily conserved antibacterial defence response",

by Mathew J. Sweet’s team at the University of Queensland. The authors show how mitochondria actively participate in antibacterial defence. They do this by reshaping their mitochondria during infection. More specifically, they fight bacteria by fragmenting their mitochondria.

Pathogenic agents must first be recognized by the host, and this recognition takes place through the innate immune system (Janeway, 1989). Innate immune cells detect infectious agents through pattern-recognition Toll-like receptors (TLRs), among other mechanisms. Once activated, they participate in host inflammatory responses, recruiting key immune cells to the site of infection to eliminate the infection (Kawai and Akira, 2011).

This story begins with macrophages, one of the professional hunters of innate immunity. When macrophages detect bacteria such as E. coli, they rapidly activate the inflammatory and antimicrobial pathways. But immune activation is energetically expensive. Cells must quickly reorganize metabolism, signalling pathways, and intracellular architecture to survive the stress of infection. This is where mitochondria enter the story.

Mitochondria are not static bean-shaped organelles floating in the cytoplasm. They constantly undergo two opposing processes with the help of several guanosine triphosphatases (GTPases), some of which have funny names, like OPA1 and OMA1 (Adebayo et al., 2021; Baker et al., 2014). The two opposing forces for mitochondria are:

• Fusion- mitochondria join into interconnected networks

• Fission- mitochondria split into fragments

  
  

Traditionally, fusion has been associated with nutrient abundance and efficient energy production, while fission has often been viewed as a sign of stress or dysfunction.

But this paper challenges the assumption on fission: when macrophages encountered bacteria, their mitochondrial networks fragmented. Initially, one might assume this simply reflects cellular damage. However, the researchers discovered that blocking mitochondrial fission impaired bacterial clearance. In other words, fragmented mitochondria were helping cells fight infection. They enhanced mitochondrial fission by creating a homozygous mutant, fzo- 1−/−, and, conversely, enhanced mitochondrial fusion using the same technique with the mutant drp- 1−/− in C. elegans. They observed that C. elegans worms infected with Pseudomonas aeruginosa (P. aeruginosa) had a significantly reduced number of eggs per worm. It is known that when worms are under stress, such as being infected by P. aeruginosa, they reduce their egg production to conserve energy and protect themselves (Aprison and Ruvinsky, 2014). Moreover, when the mitochondria of these works are skewed towards mitochondrial fission, the number of eggs/worms is unaltered. Worms can retain their fecundity, as shown in figures A-C below from the paper. (Kapetanovic et al.)

Mitochondrial fission, skewed in the mutant fzo- 1-/-, results in lower pathogen relative to when mitochondria are skewed towards fusion in the mutant drp-1 -/-. Image taken directly from the paper by Sweet et al., 2026.

Even more fascinating was how evolutionarily conserved this response is. Similar antibacterial effects of mitochondrial fission were observed not only in mammalian macrophages, but also in Caenorhabditis elegans, suggesting that this defence strategy is ancient.

The paper then explored how mitochondrial fission promotes antibacterial defence.

Fragmented mitochondria activated a mitochondrial stress-response pathway called the mitochondrial unfolded protein response (UPRmt). This pathway involves the transcription factor ATF5 in mammals (and ATFS-1 in worms), which translocates to the nucleus and activates genes that restore mitochondrial homeostasis and promote immune defence.

But the story became even stranger: researchers found that mitochondrial fission also promoted lipid droplet formation.

For years, lipid droplets were mostly dismissed as passive fat-storage structures. Yet recent work increasingly shows that they participate in immunity, inflammation, and stress adaptation. In this study, lipid droplets were a part of the antibacterial response.

Then biology complicated things further: the UPRmt pathway seems to restrain excessive lipid droplet production, suggesting that mitochondria are not simply flipping defence systems “on.” They are carefully balancing multiple stress and immune pathways simultaneously.

When cells sense infection, mitochondria reorganize their networks, induce stress response, restructure metabolism, mobilize lipid droplets, and help orchestrate antibacterial defence. The boundaries between metabolism, immunity, and organelle biology are becoming increasingly blurred. The mitochondria were never just powerhouses after all.

References:

Adebayo, M., S. Singh, A.P. Singh, and S. Dasgupta. 2021. Mitochondrial fusion and fission: The fine-tune balance for cellular homeostasis. FASEB J. 35:e21620.

Aprison, E.Z., and I. Ruvinsky. 2014. Balanced trade-offs between alternative strategies shape the response of C. elegans reproduction to chronic heat stress. PLoS One. 9:e105513.

Baker, M.J., P.A. Lampe, D. Stojanovski, A. Korwitz, R. Anand, T. Tatsuta, and T. Langer. 2014. Stress‐induced OMA1 activation and autocatalytic turnover regulate OPA1‐dependent mitochondrial dynamics. The EMBO Journal. 33:578-593.

Janeway, C.A., Jr. 1989. Pillars Article: Approaching the Asymptote? Evolution and Revolution in Immunology. Cold Spring Harb Symp Quant Biol. 1989. 54: 1–13. The Journal of Immunology. 191:4475-4487.

Kapetanovic, R., S.F. Afroz, J.E.B. Curson, I. Kirmes, D. Ramnath, S. Nothjunge, J. Liu, J.D. Atkinson, A. Ahier, K.D. Raven, M. Bosch, B. Keller, G.M.E.P. Lawrence, K.D. Gupta, M.R. Shakespear, C. Ferguson, C.J. Stocks, N.J. Bokil, G. Matthias, T.T.K. Nguyen, Z.G. Khalil, R.C. Reid, K.A. Hansford, P.M. Hansbro, M.A. Cooper, M.A. Schembri, A. Blumenthal, K. Schroder, D.P. Fairlie, A. Pol, P. Matthias, R.G. Parton, S. Zuryn, and M.J. Sweet. Mitochondrial fission mediates an evolutionarily conserved antibacterial defense response. Science Immunology. 11:eaed2623.

Kawai, T., and S. Akira. 2011. Toll-like receptors and their crosstalk with other innate receptors in infection and immunity. Immunity. 34:637-650.