Researchers have traced more than 50 years of genetic history in Acinetobacter baumannii, a drug-resistant hospital pathogen, and found that it did not become dangerous overnight—it crept toward global dominance through decades of small, accumulating genetic changes.
For the study, published in Microbial Genomics, an international team led by the University of East Anglia assembled a collection of 226 Acinetobacter baumannii samples dating from the 1970s to the early 2000s. Researchers grew the historical isolates in the lab, then extracted and sequenced their DNA. They combined these newly sequenced genomes with more than 1,000 modern genomes collected from six continents, creating a dataset of 1,281 chromosomes in total.
By lining up genetic changes with the dates and locations of each sample, the researchers pinpointed when key resistance traits emerged and how they spread worldwide. They found that the bacterium's dominant lineage—known as international clone 2—did not suddenly become dangerous. Instead, it accumulated resistance in waves, with each wave producing bacteria better equipped to survive antibiotics than the last.
By 2005, this lineage had become the leading form of Acinetobacter baumannii worldwide, a shift the team linked to the acquisition of two major genetic elements, including a gene called oxa23 that confers resistance to powerful antibiotics. This effectively supercharged the bacterium’s ability to survive treatment.
The team also discovered that Acinetobacter baumannii isn't one uniform strain but splits into at least four distinct evolutionary groups. Three evolved gradually while a fourth branched off independently and is showing up more frequently in recent samples— a sign that a newer, better-adapted variant may already be spreading.
“This work is really important because understanding how antibiotic-resistant bacteria respond to changes in antibiotic use over time is essential for guiding policies on how we use antibiotics now and in the future,” said lead researcher Benjamin Evans, from UEA’s Norwich Medical School.
Data from University of East Anglia