Zeocin is a member of the bleomycin family of antibiotics, compounds prized in molecular biology for their ability to cut DNA but feared for their toxicity. The drug mimics the effects of ionizing radiation, inducing both single- and double-strand breaks in the genetic material as well as base substitutions in the DNA molecule. Earlier work has also linked Zeocin to pro-oxidative, mutagenic and carcinogenic effects in yeast, making it an ideal probe for studying how cells cope with one of the most dangerous classes of chemical insults. Understanding precisely how such compounds injure cells — and which cellular states confer protection — matters far beyond the laboratory, because pharmaceutical residues released into the environment can interact with biological targets at very low concentrations and inflict lasting damage on lipids, proteins and DNA.

The research team, working with the widely used laboratory strain S. cerevisiae BY4741, took the unusual step of comparing three distinct cell populations side by side. The first consisted of logarithmically growing cells, the fast-dividing workhorses of most yeast experiments. The second and third came from stationary-phase cultures that had been starved into dormancy for 168 hours: using a Percoll density gradient, the scientists separated these cultures into true quiescent cells, which retain the capacity to re-enter the cell cycle, and non-quiescent cells, a more damaged fraction that has largely lost the ability to reproduce. Because quiescence is thought to be the most common cellular state found on Earth, the authors argue that testing drugs on resting cells may yield far more realistic predictions of how environmental toxicants affect natural microbial communities than the standard practice of testing only on proliferating cultures.

The experiments focused on a battery of molecular sensors of oxidative stress. Cells were exposed to 50 micrograms per millilitre of Zeocin — a concentration corresponding to the IC50, the dose that halves survival — for one hour, after which the researchers measured intracellular reactive oxygen species, carbonylated proteins, oxidized lipids and total glutathione. Reactive oxygen species, the toxic radicals generated when cellular redox balance collapses, rose in all three populations, but not equally. Proliferating cells and non-quiescent cells each accumulated roughly four times more ROS than their untreated counterparts, while quiescent cells showed only a modest 1.5-fold increase, the smallest response of any group.

Damage to macromolecules told a complementary story. Carbonyl groups, the chemical fingerprints of oxidized proteins, increased about threefold in quiescent cells after Zeocin treatment, whereas logarithmic and non-quiescent cells showed only slight elevations. The authors interpret this paradoxical result as a consequence of the quiescent cells’ very low metabolic rate: with reduced capacity to sequester and degrade damaged molecules, oxidized proteins simply accumulate. Lipid peroxidation, measured as malonaldehyde equivalents, followed a different pattern again, with the highest levels of oxidized lipids appearing in Zeocin-treated proliferating and quiescent cells at 1.88 and 1.4 nanomoles per mole respectively, while non-quiescent cells showed negligible lipid damage whether treated or not. Because lipid peroxidation compromises membrane functionality and permeability, the findings suggest that metabolically active cells, with dynamic transport across their membranes, are simply more vulnerable to xenobiotic penetration.

Glutathione, the tripeptide that serves as the cell’s first line of non-enzymatic antioxidant defence, rose in every treated population, confirming that Zeocin provokes a coordinated stress response. The magnitude of the response tracked metabolic activity: proliferating cells boosted glutathione biosynthesis roughly 2.5-fold, consistent with their high rates of transcription and translation, while quiescent cells managed only about a 30 percent increase. Non-quiescent cells carried the highest absolute concentrations of the tripeptide, likely reflecting their chronically elevated baseline ROS levels rather than a specific response to the drug.

The most striking results concerned DNA. Using constant field gel electrophoresis, which quantifies double-strand breaks as the fraction of DNA released from agarose plugs, the team found that spontaneous break levels depended strongly on growth phase, with non-quiescent cells carrying about 1.5 times more spontaneous breaks than either logarithmic or quiescent cells — an expected consequence of their genomic instability and tendency toward apoptosis. When the researchers then challenged cells with much higher Zeocin concentrations, from 100 to 300 micrograms per millilitre, logarithmic and non-quiescent cells responded with statistically significant induction of double-strand breaks, while quiescent cells showed no significant break induction at any concentration tested. Their DNA remained essentially as intact as that of untreated controls.

Repair capacity added a final layer to the picture. When given 30 and 60 minutes to recover after treatment, logarithmic cells demonstrated the best repair performance, with calculated repair capacity rising to 3.21 at the highest Zeocin dose after an hour of recovery. Non-quiescent cells, by contrast, proved unable to repair the induced breaks at all, with repair capacity values hovering near or below 1 regardless of dose or recovery time. For quiescent cells, repair capacity could not even be calculated, simply because the treatment had failed to produce a statistically significant increase in breaks in the first place. The authors attribute the extraordinary resilience of quiescent cells to their distinctive physiology: thickened cell walls, highly condensed chromosomes and increased thermo- and osmotolerance, features that likely restrict Zeocin’s ability to penetrate the cell and reach its DNA target.

Taken together, the study delivers a clear message for toxicology: the physiological state of a cell is a critical determinant of its resistance to environmental stress. Actively proliferating yeast, with their high metabolic activity, elevated baseline ROS and dynamic membranes, proved the most susceptible to Zeocin’s combined assault on DNA, proteins and lipids, while cells that had entered the G0 quiescent state emerged as the most robust. Because S. cerevisiae shares a high degree of genetic conservation with higher eukaryotes and is currently the only experimental system that permits simultaneous in vivo evaluation of all cellular targets, the findings carry implications well beyond microbiology. They suggest that toxicity screening programmes relying exclusively on dividing cells may systematically overestimate — or in some cases mischaracterize — the hazards that drugs and environmental pollutants pose to the dormant microbial populations that dominate natural ecosystems. Incorporating quiescent cells into standard yeast-based test panels, the authors argue, would make such assays smarter, more sensitive and far more representative of the biological reality that toxic compounds encounter in the wild.

Subject of Research: Assessing Zeocin-induced oxidative stress and DNA damage in proliferating, quiescent and non-quiescent Saccharomyces cerevisiae cells

Article Title: Saccharomyces cerevisiae yeast cells as a test system for assessing Zeocin toxicity

Article References: Marinovska, P., Todorova, T., Tomova, A., Pisareva, E., Boyadzhiev, K., Dimitrov, M., Parvanova, P., Dimitrova, M., Chankova, S., & Petrova, V. (2022). Saccharomyces cerevisiae yeast cells as a test system for assessing Zeocin toxicity. BioRisk, 17, 105-116. https://doi.org/10.3897/biorisk.17.77227

Image Credits: AI Generated

DOI: 10.3897/biorisk.17.77227

Keywords: Saccharomyces cerevisiae, Zeocin, toxicity testing, quiescent cells, oxidative stress, reactive oxygen species, glutathione, DNA double-strand breaks, lipid peroxidation, radiomimetic antibiotic, yeast model, cellular stress response

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