Endosymbiosis, the phenomenon in which one organism lives inside the cells of another, underlies some of the most important transitions in the history of life. Mitochondria and chloroplasts, the energy-converting organelles of eukaryotic cells, are the descendants of free-living bacteria that took up residence inside ancestral host cells more than a billion years ago. Wolbachia, an alpha-proteobacterium related to the ancestors of mitochondria, is arguably the most successful animal-associated microbe on Earth, colonizing an estimated 50 to 60 percent of all insect and arthropod species as well as filarial nematodes. Depending on the host and the bacterial strain, the relationship ranges from reproductive parasitism to strict mutualism, in which the host can no longer survive without its passenger.

In filarial worms such as Wuchereria bancrofti, the cause of elephantiasis, and Onchocerca volvulus, the agent of river blindness, the mutualism has become so intimate that antibiotic killing of Wolbachia also kills the worm. In insects, the bacterium can supply essential metabolites including heme, purines, pyrimidines, flavin adenine dinucleotide and riboflavin, and it can suppress infections by pathogenic viruses, bacteria and protists. Yet modeling the early stages of such coevolution in insects or nematodes is slow and experimentally awkward, because adaptive changes in these animals take generations to accumulate. Yeast, with its rapid doubling time and fully sequenced, easily manipulated genome, offers an appealing alternative, particularly since recent surveys have revealed that wild yeasts, long considered axenic, actually harbor hidden communities of intracellular bacteria.

To build their model, the Mexican team exploited two deletion mutants of the laboratory strain By4742. The first, ΔRIB1, lacks GTP cyclohydrolase II, the enzyme that catalyzes the very first step of riboflavin biosynthesis. The second, ΔRIB4, lacks lumazine synthase, which manufactures the immediate precursor of the vitamin. When the researchers grew these strains in a synthetic lactate medium lacking riboflavin, both mutants failed to grow, while the wild-type strain reached measurable densities and grew slightly better when the vitamin was added. The auxotrophy was not an artifact of the carbon source: the mutants also failed to grow without riboflavin in dextrose-containing medium, confirming that the defect lay squarely in the vitamin’s synthesis pathway rather than in respiratory metabolism per se.

Biochemical measurements reinforced the picture. Using a Clark-type oxygen electrode, the team found that both mutants consumed oxygen at very low rates, consistent with their dependence on riboflavin-derived cofactors. Riboflavin is the substrate for the synthesis of flavin mononucleotide and flavin adenine dinucleotide, the two redox coenzymes that power oxidative metabolism and a host of other cellular reactions. Fluorescence-based quantification of intracellular flavins showed that the mutants contained roughly half the riboflavin of wild-type cells, and that exogenous supplementation raised their internal levels, with ΔRIB4 responding particularly strongly. The uncoupling agent CCCP increased oxygen consumption in all strains, indicating that the respiratory machinery itself remained responsive and that the bottleneck was cofactor supply.

The pivotal experiment came next. The researchers infected the yeast strains with Wolbachia strain wStr, originally isolated from the planthopper Laodelphax striatellus and maintained in an Aedes albopictus cell line. Infection was accomplished by co-centrifuging bacteria and yeast and was confirmed seven days later by reverse-transcription PCR amplification of the wsp gene, which encodes the bacterium’s major surface protein. A 650-base-pair product appeared in all infected strains but not in uninfected controls. Strikingly, when the infected ΔRIB1 and ΔRIB4 mutants were grown in medium lacking riboflavin, they grew to levels comparable to those achieved with vitamin supplementation, demonstrating that the endosymbiont had functionally complemented the missing biosynthetic steps.

Physiological assays supported the conclusion that the bacterium was genuinely provisioning its host. Oxygen consumption, which had been negligible in the uninfected auxotrophs, rose significantly after infection, with the largest gains seen in maximal, uncoupled respiration. Intracellular riboflavin concentrations increased in both infected mutants, more modestly in wΔRIB1 and more pronouncedly in wΔRIB4. The team also tested whether infection conferred broader physiological benefits by exposing cells to two harsh conditions: one molar sodium chloride, an ionic stress, and ten millimolar hydrogen peroxide, which generates reactive oxygen species. Infected mutants survived these challenges at substantially higher rates than their uninfected counterparts. Under salt stress, survival of ΔRIB1 rose from 34 to 71 percent and that of ΔRIB4 from 11 to 37 percent; under peroxide, ΔRIB1 improved from 41 to 75 percent and ΔRIB4 from 37 to 57 percent. The wild-type strain showed little change, suggesting that the benefit is most visible when the host is metabolically vulnerable.

The mechanism most likely centers on Wolbachia‘s own riboflavin pathway, which retains a conserved set of six genes, ribA, ribD, ribB, ribE, ribC and ribF, across diverse insect-associated strains. Parallel examples abound in nature. The bed bug Cimex lectularius depends on its Wolbachia for B-vitamin synthesis during blood digestion, the brown planthopper Nilaparvata lugens receives both biotin and riboflavin from its symbiont, and in the tomato psyllid, co-resident Liberibacter and Wolbachia jointly enhance host fitness by supplying arginine and riboflavin. Beyond nutrient transfer, the bacterium’s type IV secretion system and its ankyrin repeat-containing proteins may modulate host cell processes, a strategy documented both in mosquitoes and, notably, in S. cerevisiae itself in earlier work by the same group.

That earlier work, published in 2019, showed that Wolbachia pipientis strain wAlbB infecting yeast W303 deregulated mitochondrial oxidative phosphorylation and hastened host death, a clearly parasitic outcome. The new study therefore captures a different face of the same bacterium: in a host with a specific metabolic vulnerability, infection becomes advantageous rather than lethal, at least in the short term. The authors caution that the benefits are likely transient, since prolonged infection has been shown to induce mitochondrial dysregulation and early cell death in yeast, and they note that the iron supplementation used in their media may itself have supported both symbiont growth and mitochondrial function through heme and iron-sulfur cluster assembly. Even so, the system neatly recapitulates the trajectory from parasite to mutualist that Wolbachia follows in nature, and it does so in an organism whose genetics, metabolism and respiratory physiology are among the best characterized in biology.

The researchers argue that the artificial yeast-endosymbiont model could become a standard platform for dissecting the molecular details of metabolic complementation: which compartments participate in riboflavin trafficking, how nutrient exchange is regulated, and how bacterial effectors reshape host physiology. Because the host is a single-celled eukaryote with a short generation time, experiments that would take years in insects can be completed in days. In an era when Wolbachia is being deployed at continental scale to suppress mosquito-borne diseases such as dengue, understanding the spectrum of its interactions with host cells, from exploitation to essential provisioning, has practical as well as evolutionary significance. This study shows that the first steps of that spectrum can now be reproduced, observed and manipulated in a test tube, one vitamin at a time.

Subject of Research: Wolbachia-mediated rescue of riboflavin auxotrophy in Saccharomyces cerevisiae as a model of endosymbiotic mutualism

Article Title: In Saccharomyces cerevisiae ΔRIB1 and ΔRIB4 mutants, riboflavin auxotrophy is reverted by Wolbachia (wStr) infection

Article References: Mendez-Romero, O., Ricardez-García, C., & Uribe-Carvajal, S. (2026). In Saccharomyces cerevisiae ΔRIB1 and ΔRIB4 mutants, riboflavin auxotrophy is reverted by Wolbachia (wStr) infection. International Microbiology. https://doi.org/10.1007/s10123-026-00884-3

Image Credits: AI Generated

DOI: 10.1007/s10123-026-00884-3

Keywords: Wolbachia, Saccharomyces cerevisiae, riboflavin auxotrophy, endosymbiosis, mutualism, vitamin B2, flavin cofactors, mitochondrial respiration, oxidative stress, wStr strain, yeast model, symbiotic complementation