Phylogenomic reconstruction indicates human-to-animal movement in pandemic Klebsiella pneumoniae lineages, with evidence of return in ST11
Phylogenomic analysis of four pandemic *Klebsiella pneumoniae* lineages reveals that while human-to-animal transmission is the predominant direction, specific ST11 strains have successfully moved from animals back into human populations, highlighting the need for integrated genomic surveillance to detect such cross-species spillover events.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Bacteria are not static residents of a single environment; they are travelers that move freely between the soil, water, food, and the bodies of living creatures. Among these travelers, Klebsiella pneumoniae is a particularly significant one. It is a bacterium that naturally lives in the gut of healthy people, livestock, and pets, but it can also cause severe, sometimes fatal infections when it escapes into the wrong place, such as a hospital ward or a bloodstream. A major concern for doctors and public health officials is that many strains of this bacterium have become resistant to powerful antibiotics, including a class known as carbapenems, which are often used as a last resort when other drugs fail. Because these bacteria circulate in both humans and animals, scientists have long debated the direction of the traffic. Does the resistance travel from farms and pets into hospitals, or does it move from hospitals into the wider world of animals? Answering this question is crucial for deciding where to focus efforts to stop the spread of drug-resistant infections.
To solve this puzzle, researchers led by Stephen Fordham and his colleagues at Bournemouth University and the University Hospitals Dorset turned to a massive digital archive of bacterial DNA. They did not look at a few isolated cases but instead examined nearly 166,000 genomes of Klebsiella pneumoniae collected from around the world. Using a method that compares the tiny genetic differences between individual bacteria, they searched for groups where human and animal samples were so closely related that they must have shared a recent common ancestor. They focused on four specific, highly successful lineages of the bacteria that are known to cause outbreaks globally. By reconstructing the family trees of these four groups, the team could trace the history of the bacteria to see where they started and which way they moved over time.
The study revealed a complex picture that challenges simple assumptions. For three of the four bacterial lineages they studied, the evidence pointed clearly in one direction: the bacteria originated in humans and moved into animals. In these cases, the animals appeared to act as a dead end, or a "sink," where the bacteria entered but did not necessarily spread back out to infect people again. One of these lineages, known as ST147, did establish a sustained presence in companion animals, particularly cats and dogs in the United States, forming a distinct family group that circulated among pets. However, even in this case, the researchers found no strong evidence that this animal population had sent the bacteria back to infect humans.
The story was different for the fourth lineage, ST11. This group, which is known for carrying genes that make it resistant to carbapenem antibiotics, showed a clear pattern of movement from animals back to humans. The researchers found that a specific version of this bacteria had been circulating in dogs in North America for several years before it appeared in human patients. In fact, the earliest samples of this specific bacterial strain came from dogs in 2019 and 2021, while the first human samples from the same family were not found until 2022. The genetic distance between the animal and human samples was incredibly small, sometimes differing by only a single genetic letter, and the animal samples were always older than the human ones. This sequence of events strongly suggests that the bacteria moved from the pets into the people. Furthermore, once the bacteria entered the human population, it did not just appear as a single case; in two instances, it spread from person to person, creating new clusters of human infections.
The researchers were careful to rule out the possibility that these findings were just a result of how the data was collected or analyzed. They tested their conclusions using different mathematical models and checked to ensure that the results held up even when they adjusted for the fact that more human samples were available than animal samples. They also confirmed that the human and animal samples came from completely different research projects and laboratories, meaning the connection was not an artifact of a single study mixing its own data. While the study could not prove that every single case of infection came directly from a pet, the pattern was consistent enough to conclude that for this specific high-risk lineage, animals can serve as a source of infection for people.
This discovery has important implications for how we monitor and manage antibiotic resistance. For decades, public health efforts have focused heavily on reducing antibiotic use in food-producing animals, such as cows and chickens, to protect human health. While that remains important, this study highlights that companion animals, like dogs and cats, occupy a different and often overlooked space. These animals live in close contact with their owners and often receive medical care in veterinary hospitals that resemble human hospitals in their use of powerful antibiotics. The study suggests that if a dangerous, drug-resistant bacteria establishes itself in a veterinary setting, it can persist there and eventually jump back to humans. Therefore, including veterinary samples in national surveillance systems could provide an early warning system, spotting dangerous lineages in animals before they become a problem in human hospitals. The research does not suggest that pets are the primary source of all antibiotic resistance, but it does show that the flow of bacteria is not a one-way street and that in some cases, the animals we live with can bring these risks back to us.
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