They have called her Psychrobacter SC65A.3 and was found under meters of ice in a cave in Romania. It had been buried for about 5,000 years and, despite this, it seems to show resistance to a dozen modern antibiotics. Its discoverers have also proven that it inhibits the growth of other bacteria, including some of the most difficult pathogens to combat. The research, published in the scientific journal, delves into its genetics to explain how a bacteria can be both resistant to drugs and a potential antibiotic against others.
“The species [del género] Psychrobacter are exceptionally cold-resistant bacteria that thrive in extreme environments such as polar sea ice, glaciers, permafrost and the ocean depths,” says Cristina Purcarea, from the Bucharest Institute of Biology and senior author of this research. “Their ability to survive and adapt to various icy habitats indicates the extraordinary resilience of these microbes,” adds the scientist. “However, the particular strain that we recovered from the Scarisoara glacial cave is unique, as demonstrated by the limited identity score of its DNA with homologous species of Psychrobacter,” adds Purcarea. This low percentage of coincidence in the genetic sequence with its closest relatives gives clues to its specificity.
The Scarisoara Cave, in the Romanian Carpathians, is like a giant natural freezer. It houses one of the largest underground glaciers in the world, with an ice block of about 75,000 cubic meters. There the stalactites and stalagmites are a strange mixture of ice and calcite. From one of its rooms, the researchers extracted a 25-meter ice core, dating back about thirteen millennia. At the height of 16.5 meters, which corresponds to 5,335 years ago, according to carbon dating, they found an entire bacterial ecosystem, with microorganisms probably active, not dormant. As its name indicates, the Psychrobacter They are psychrotrophic organisms, adapted to the coldest environments on the planet.
When analyzing the resistance of the SC65A.3 strain to 28 antibiotics from 10 classes used in the treatment of bacterial infections, they saw that it was resistant to a dozen of them, some broad spectrum and others specific, such as clindamycin, lincomycin or vancomycin. “The 10 antibiotics to which we found resistance are widely used in oral and injectable therapies to treat various serious bacterial infections in clinical practice,” says Purcarea. The .
How can a bacteria that has been under 16.5 meters of ice in a remote cave in the Carpathians for 5,335 years be resistant to antibiotics developed a few decades ago at most? “It is not resistant to modern drugs because it has come into contact with them, but because evolution had already provided these microbes with mechanisms to survive chemical threats long before humans invented antibiotics,” says the Romanian scientist. For millions of years, bacteria have had to fight with other bacteria and other microorganisms, especially fungi, to survive, a fight that has left its mark on their genetics “and these same genes can also confer resistance to the antibiotics we use today, even if they are relatively recent,” adds Purcarea.
Iñaki Comas, head of the Tuberculosis Genomics Unit of the Institute of Biomedicine of Valencia/CSIC, believes along the same lines: “The vast majority of genes related to antibiotic resistance had or have other functions and have always existed,” he says. Comas, who has not participated in this research, adds that “bacteria have been fighting with fungi or each other throughout their evolutionary history, and the way they do it is to resist the attacks of others and what are those attacks? All of them are antimicrobial,” he adds. Hence its own potential as an antibiotic.
The same mechanisms that point to resistance are those that make Psychrobacter SC65A.3 a tough organism to crack. Grown in the laboratory and brought into contact with 20 clinical pathogens, they observed how it inhibited the growth of most of them. Some are among the most resistant and feared bacteria, such as Staphylococcus aureusgenerally benign to healthy people, but , or various strains of the Escherichia coli. It also showed outstanding antibacterial activity against Enterobacter spp and three strains of the Klebsiella pneumoniae.
“The cave strain showed an impressive ability to combat 14 harmful pathogens from the well-known ESKAPE group,” says Purcarea, the lead author of the research, in an email. This group includes the six most dangerous species of antibiotic-resistant microbes, according to the definition of the . “This powerful antimicrobial effect is probably due to genes that produce natural antibiotic compounds, which highlights their potential as a source of new treatments,” concludes the researcher.
Sara Hernando-Amado, principal researcher at the National Center for Biotechnology (CNB-CSIC), who has not participated in this research, indicates that a limitation of the work is that “Psychrobacter It is an environmental bacteria, for which there are no clinical cut-off points, which the authors manage using as a reference Acinotabacter [cercana genéticamente] and less phylogenetically related bacteria.” These cut-off points () are reference values to determine whether a microorganism is sensitive (the antibiotic works) or resistant (the drug does not inhibit bacterial growth). The lack of reference values for this microorganism is a weakness recognized by the authors of the study, which means that the multi-resistance discovered has to be considered with caution.
On the other hand, Hernando-Amado adds that “the intrinsic resistance of many bacteria (including environmental ones) to antibiotics and their ability to evolve rapidly and acquire higher levels by mutation is well known, so the key is to understand this process and exploit bacterial Achilles heels associated with resistance.” This could allow the rational use of antibiotics, “combining pairs where the use of one increases sensitivity to the other, for example,” he adds. Despite the limitations, the discoverers of the Psychrobacter SC65A.3 are already searching for the specific biomolecules responsible for its antimicrobial potential.
