In the eucalypt woodlands of Belair National Park in South Australia, wild koalas are quietly carrying a molecular hallmark of the human antimicrobial resistance crisis, yet new research suggests that this genetic baggage may be far less disruptive to their gut biology than scientists had feared. A study published in the journal Microbial Ecology has found that the presence of the class 1 integron, a genetic element intimately associated with anthropogenic antibiotic pollution, does not meaningfully alter the composition of the gut microbiome in koalas that have never been exposed to antibiotics. The finding offers a rare and valuable window into how antibiotic resistance genes behave in wildlife living outside the shadow of direct human selection pressure.
Antimicrobial resistance, often abbreviated as AMR, has become one of the defining health challenges of the twenty-first century, and its reach now extends well beyond hospitals and farms. Wildlife populations across the globe are increasingly recognised as reservoirs of resistance genes, prompting concern under the One Health framework, which recognises that human, animal, and environmental health are deeply interconnected. Most studies to date have focused on animals in close contact with human activity, such as urban wildlife, livestock, and captive animals, where antibiotic residues and resistant bacteria flow readily between species and environments. Far less is known about what happens when resistance elements appear in animals that have never encountered clinical antibiotics, and whether their mere presence leaves any detectable imprint on the host’s microbial communities.
At the centre of the new study sits a piece of bacterial genetics with a formidable clinical reputation. The class 1 integron, identified by the presence of the intI1 gene, is a DNA platform that acts as a capture-and-dissemination system for antibiotic resistance genes, primarily among Gram-negative bacteria. Integrons function by using an enzyme called integrase to snip gene cassettes out of one location and slot them into another, allowing bacteria to accumulate and shuffle resistance determinants with remarkable efficiency. Because class 1 integrons thrive in environments shaped by human activity, from wastewater treatment plants to agricultural runoff, their presence in wildlife is often interpreted as a biomarker of anthropogenic influence. In the gut, which is widely regarded as one of the most important reservoirs of antibiotic resistance genes in any animal, the question of whether integron carriage reshapes the microbial community is central to understanding how resistance integrates, persists, and evolves.
To address this question, a research team led by Laura Marshall of Macquarie University, working with colleagues including Sabrina Haque, Wayne Boardman of the University of Adelaide, Fiona McDougall, and Michelle Power, analysed faecal samples from 62 wild koalas living in Belair National Park. The samples were collected in October 2022 with the assistance of researchers from Flinders University and veterinary students from the University of Adelaide, under approvals from the Flinders University Animal Ethics Committee and the South Australian Department for Environment and Water. Crucially, each sample could be categorised according to whether the class 1 integron was present or absent, allowing the team to directly compare the gut microbial communities of integron-positive and integron-negative animals within the same population.
The analytical backbone of the study was 16S rRNA gene amplicon sequencing, a technique that targets a slowly evolving component of the bacterial ribosome to profile which microbes are present in a sample and in what relative proportions. By amplifying and sequencing this marker gene from the faecal DNA, the researchers could construct a snapshot of each koala’s gut microbiome without needing to culture the organisms, a critical advantage given that the majority of gut bacteria resist laboratory cultivation. The resulting datasets were then interrogated with a battery of standard community ecology tools designed to detect even subtle differences between microbial assemblages.
The results were strikingly uniform in their message. When the team measured alpha diversity, a family of metrics that captures the richness and evenness of species within a single sample, they found no significant differences between koalas carrying the class 1 integron and those lacking it. Beta diversity analyses, which quantify how communities differ from one another across samples, likewise revealed no meaningful separation between the two groups. Non-metric multidimensional scaling, an ordination technique that arranges samples in a low-dimensional space based on the similarity of their microbial profiles, showed no clustering pattern associated with integron status, indicating that community composition was essentially unrelated to whether the resistance element was present.
Differential abundance analysis, which tests whether particular bacterial taxa are enriched or depleted in one group relative to another, delivered the same verdict: no single taxon was significantly different between integron-positive and integron-negative koalas. Taken together, the findings indicate that the class 1 integron, at least on its own, is not associated with significant shifts in the gut microbiome of this antibiotic-naïve koala population. In other words, the presence of a clinically important resistance element does not appear to require, nor to produce, any substantial reorganisation of the microbial ecosystem in which it resides.
The significance of this null result lies in what it tells us about the ecology of resistance in the absence of antibiotics. A persistent worry in the field is that the acquisition of resistance elements might impose fitness costs on bacteria, potentially reshaping microbial communities, or conversely that certain microbial contexts might favour the spread of resistance once it arrives. If integron carriage were tied to particular microbial backgrounds, it could hint at conditions that promote or constrain the element’s persistence. The koala data suggest that, in a population with no history of antibiotic exposure, the integron can persist without such ecological entanglement, raising important questions about the baseline state of resistance in wildlife before anthropogenic selection pressures take hold.
Koalas make a particularly compelling subject for this kind of work. As an iconic Australian marsupial with a specialised diet of eucalyptus leaves and a gut microbiome adapted to detoxifying the plant’s chemical defences, the koala occupies a relatively contained ecological niche. Populations such as the one in Belair National Park experience some degree of proximity to urban areas, yet the animals themselves are not treated with antibiotics and are unlikely to encounter significant pharmaceutical residues in their food or water. This makes them a useful natural experiment for asking what resistance carriage looks like in a microbiome that has not been sculpted by clinical drug use, and whether the genetic markers of human influence can appear in wildlife without any accompanying microbial disruption.
The study also carries practical implications for conservation and disease management. If integron carriage in wildlife is ecologically neutral in the absence of antibiotic selection, then monitoring programmes may be able to treat the intI1 gene as a standalone indicator of anthropogenic exposure without needing to account for cascading effects on host microbiome health, at least in antibiotic-naïve animals. Conversely, the result underscores that the arrival of antibiotic residues into an environment could change the picture entirely, potentially converting a benign passenger into an active participant in the evolution and spread of resistance. Understanding the pre-antibiotic baseline, as this study does, is an essential reference point against which future environmental perturbations can be measured.
The research was funded by the Morris Animal Foundation under grant D21ZO-507 awarded to Michelle Power, with open access funding organised by the Council of Australian University Librarians and its member institutions. The work was published as an open access article in Microbial Ecology, accepted on 25 August 2026 and published on 4 September 2026, allowing researchers, wildlife managers, and the public to freely examine the full dataset and methods. The authors acknowledged the Wallumattagal clan of the Dharug Nation as the traditional custodians of the lands where laboratory analyses were performed at Macquarie University, and the Kaurna people as custodians of the Belair National Park lands where the samples were collected.
As antimicrobial resistance continues to spread through human, animal, and environmental compartments, studies like this one are helping to map the terrain on which that spread occurs. The message from Belair National Park is a measured one: a resistance element born of human medicine can settle quietly into the gut of a wild animal without leaving a microbial fingerprint, a reminder that the ecology of antibiotic resistance is complex, context-dependent, and far from fully understood. Whether that quiet coexistence survives contact with real-world antibiotic pollution remains a question for future research, but the koalas of South Australia have now provided a rare and carefully documented baseline for answering it.
Subject of Research: The relationship between class 1 integron presence and gut microbiome composition in antibiotic-naïve wild koalas (Phascolarctos cinereus)
Subject of Research: Biology
Article Title: Gut Microbiome Composition is Independent of Class 1 Integron Presence in Antibiotic-naive Wild Koalas (Phascolarctos cinereus)
Article References: Marshall, L., Haque, S., Boardman, W., McDougall, F., & Power, M. (2026). Gut Microbiome Composition is Independent of Class 1 Integron Presence in Antibiotic-naive Wild Koalas (Phascolarctos cinereus). Microbial Ecology. https://doi.org/10.1007/s00248-026-02877-1
Image Credits: AI Generated
DOI: 10.1007/s00248-026-02877-1
Keywords: Antibiotic resistance, Wildlife, Class 1 integron, Gut microbiome, 16S rRNA sequencing, Koala, Antimicrobial resistance, One Health, Antibiotic resistance genes, Microbiome composition, intI1 gene, Alpha and beta diversity
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Morgan Morrow. (September 5, 2026). Wild Koala Gut Microbiomes Unaffected by Integron Presence. Scienmag. https://scienmag.com/wild-koala-gut-microbiomes-unaffected-by-integron-presence/
Morgan Morrow. “Wild Koala Gut Microbiomes Unaffected by Integron Presence.” Scienmag, 5 September 2026, https://scienmag.com/wild-koala-gut-microbiomes-unaffected-by-integron-presence/. Accessed 5 September 2026.
Morgan Morrow. “Wild Koala Gut Microbiomes Unaffected by Integron Presence.” Scienmag. September 5, 2026. https://scienmag.com/wild-koala-gut-microbiomes-unaffected-by-integron-presence/
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