Some of the deadliest pathogens in modern medicine did not arrive suddenly. They crept in, one small genetic change at a time, until decades of quiet adaptation added up to a full-blown crisis. That is the story researchers have now told about Acinetobacter baumannii, a bacterium that thrives in hospital environments and has become one of the most difficult infections to treat, especially for critically ill patients.

An international team led by the University of East Anglia, working with colleagues at the Quadram Institute and collaborators in Canada and Mexico, dug into an unusual resource to trace the pathogen’s history: hospital samples that had been sitting in storage since the 1970s. By reviving and sequencing 226 of these historical isolates using long-read Oxford Nanopore technology, then combining them with more than 1,000 modern genomes collected from six continents, the team assembled the most complete evolutionary record yet compiled for this organism.

The resulting dataset, 1,281 chromosomes in total, let researchers build a detailed evolutionary tree and track, generation by generation, how antimicrobial resistance genes appeared, vanished, and reshuffled inside the bacterium’s genome over half a century.

Lead researcher Dr. Benjamin Evans of UEA’s Norwich Medical School said the project set out to answer a basic but unresolved question about a pathogen that has become a fixture of intensive care units worldwide. Bacteria that cause infections can adapt to the drugs used against them, he explained, and A. baumannii is a especially stubborn example, capable of causing infections that are extremely difficult to treat in vulnerable patients. Until this study, though, the genetic sequence of events behind its rise to dominance had never been fully mapped.

What the team found was not a single dramatic mutation but a pattern of successive waves, each producing bacteria somewhat better equipped to resist antibiotics than the wave before it. Evans described the result as one of the clearest pictures yet of how resistance can build gradually across decades and then tip suddenly in the pathogen’s favor. As he put it, the superbug did not simply appear from nowhere; it took decades to build, and it is still evolving now.

By cross-referencing genetic changes against the dates and locations of the samples, the researchers pinpointed roughly when the decisive shift happened. Evans said the comparisons showed how the bacterium had grown steadily more resistant over time, and that instead of emerging suddenly, it crept into dominance until, by around 2005, it had become the leading lineage of A. baumannii circulating worldwide.

The turning point, the team found, came from the acquisition of two major genetic elements, including a resistance gene known as oxa23, which confers resistance to some of the most powerful antibiotics available. Picking up that gene effectively supercharged the bacterium, making it substantially harder to kill with standard treatments.

The study also complicates the picture of A. baumannii as a single, uniform threat. Genomic analysis split the pathogen into at least four distinct groups, each following its own evolutionary trajectory. Three of the groups show the kind of slow, incremental evolution that looks like a long-running arms race against modern medicine. A fourth, however, stands apart: it appears to have branched off independently and is showing up more frequently in recent samples, a pattern Evans called worrying, since it may signal that a newer and better-adapted variant is already gaining ground.

The research carries direct implications for how hospitals and health authorities approach antibiotic stewardship. Evans argued that understanding how resistant bacteria respond to shifts in antibiotic use over time is essential for shaping policy now and in the future, particularly for a pathogen that already represents a serious threat to healthcare systems worldwide and could render some infections effectively untreatable without new approaches.

Dr. Sadhana Sharma, who leads antimicrobial resistance work at the UK’s Biotechnology and Biological Sciences Research Council, which helped fund the project, said the findings illustrate how a major hospital superbug evolved over decades into distinct, drug-resistant groups that spread globally, underscoring both how resistance accumulates over time and the value of international collaboration and sustained investment in basic bioscience research.

The project brought together the Centre for Microbial Interactions and the Quadram Institute at Norwich Research Park, the Université de Sherbrooke and CISSS Montérégie-Centre in Québec, the Universidad Nacional Autónoma de México, and the Canadian Institute for Advanced Research in Toronto, alongside funding from BBSRC, Canada’s New Frontiers in Research Fund, the Fonds de Recherche du Québec, and CIFAR.

The findings appear in the journal Microbial Genomics under the title “New isolates from the 1970s to early 2000s provide insights into the evolution of Acinetobacter baumannii international clone 2 and its resistome.”

Notes

1. University of East Anglia, news release, “Superbug secrets revealed in new study,” published July 1, 2026, via EurekAlert.

2. Study: “New isolates from the 1970s to early 2000s provide insights into the evolution of Acinetobacter baumannii international clone 2 and its resistome,” Microbial Genomics (2026).

IMAGE CREDIT: Janice Carr.