For decades, antibiotics have been humanity’s frontline defense against bacterial infections, yet these essential medications have also led to the rise of drug-resistant “superbugs.” Now, researchers have discovered an ancient strain of bacteria that managed to develop this superpower thousands of years before humans ever invented antibiotics.
A study published Tuesday in the journal Frontiers in Microbiology describes Psychrobacter SC65A.3, a bacterial strain discovered frozen inside 5,000-year-old layers of cave ice in Romania. Testing revealed that SC65A.3 is resistant to 10 modern antibiotics and carries more than 100 genes linked to resistance despite never being exposed to these drugs.
“Studying microbes such as Psychrobacter SC65A.3 retrieved from millennia-old cave ice deposits reveals how antibiotic resistance evolved naturally in the environment, long before modern antibiotics were ever used,” co-author Cristina Purcarea, a senior scientist at the Institute of Biology Bucharest of the Romanian Academy, said in a release.
Ancient superbug unearthed
Antibiotic resistance is an urgent threat to global public health. In the U.S. alone, more than 2.8 million antibiotic-resistant infections occur each year, and more than 35,000 people die as a result, according to the CDC’s 2019 Antibiotic Resistance Threats Report.
This threat has grown in tandem with the rise of antibiotic use. Antibiotic resistance is a classic example of natural selection: when microbes are exposed to a drug, most die, but a few survive thanks to protective genetic traits. Those survivors then pass their resistance genes onto the next generation, which passes them on to the next, giving rise to superbugs.
While exposure to antibiotics amplifies the prevalence of resistance genes, it does not imbue microbes with these protective traits. Those arise naturally through random genetic mutations and the constant pressure to out-perform other microorganisms in the environment, many of which produce their own antimicrobial compounds.
The ancient Psychrobacter SC65A.3 strain is a perfect example of how these natural processes lead to antibiotic resistance. Purcarea and her colleagues found it inside an 82-foot (25-meter) ice core they extracted from Scarisoara Ice Cave in northwestern Romania. The core represents 13,000 years of climatic history, including the 5,000-year-old ice layers that contained SC65A.3.
In the lab, the researchers isolated various bacterial strains from the core and sequenced their genomes to determine which genes allowed the strain to survive such low temperatures and which promote antimicrobial resistance. When they tested SC65A.3 against 28 widely used antibiotics, they found it was resistant to more than a third of them.
“The 10 antibiotics we found resistance to are widely used in oral and injectable therapies used to treat a range of serious bacterial infections in clinical practice,” including tuberculosis, colitis, and urinary tract infections, Purcarea explained.
High risk, high reward
The findings underscore a frequently overlooked public health threat associated with climate change, according to the study’s authors.
“If melting ice releases these microbes, these genes could spread to modern bacteria, adding to the global challenge of antibiotic resistance,” Purcarea said. As the global temperature rises, the risk of releasing ancient superbugs into the environment grows. Studying these bacterial strains, however, can also lead to the discovery of unique enzymes and antimicrobial compounds that inspire new drugs and other biotechnological innovations, Purcarea noted.
The SC65A.3 genome contains 11 genes that may be able to kill or stop the growth of other bacteria, fungi, and viruses, for example. It also contains nearly 600 genes with unknown functions, suggesting that many more novel biological mechanisms could be hiding in this superbug’s DNA.
“These ancient bacteria are essential for science and medicine,” Purcarea said, “but careful handling and safety measures in the lab are essential to mitigate the risk of uncontrolled spread.”
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