Emergence and genomic characterization of a carbapenem- and colistin-resistant Enterobacter chengduensis isolate carrying blaNDM-1 in Northeast China

Imagine the microscopic world inside our bodies as a bustling, crowded city. In this city, there are many different types of bacteria living in neighborhoods like the gut or the lungs. Most of the time, these bacteria are harmless roommates. But sometimes, a few become troublemakers, causing infections that are hard to treat. For decades, doctors have had a "super-weapon" in their arsenal called carbapenems, a powerful type of antibiotic used when other medicines fail. However, just like in a game of chess, the bacteria are learning to counter these moves. They are evolving into "superbugs" that can shrug off these drugs. Even more worrying, some of these superbugs are learning to ignore a second line of defense called colistin, which is often the very last resort for doctors. When a bacterium becomes resistant to both, it's like a criminal who has found a way to break every lock in the city, leaving the police with no keys. This is the dangerous territory scientists are trying to map: finding out exactly how these bacteria are changing, where they are hiding, and what tools they are using to stay alive.

In this study, researchers from a hospital in Northeast China discovered a new kind of bacterial troublemaker that had never been seen in that region before. They found a strain of a bacterium called Enterobacter chengduensis (let's call it "Chengdu") in a patient's respiratory tract. While this species was first identified in 2015, it is still a bit of a mystery compared to its more famous cousins. The team used a high-tech microscope called whole-genome sequencing to read the bacterium's entire instruction manual (its DNA). They found that this specific Chengdu bacterium was a "super-spy" carrying a dangerous weapon: a gene called blaNDM-1. This gene acts like a pair of molecular scissors that can cut apart and destroy carbapenem antibiotics. Even more surprisingly, this bacterium was also resistant to colistin, the last-resort drug, even though the researchers couldn't find the usual "lockpick" genes that usually cause that resistance.

The scientists identified this specific bacterium as belonging to a family group called ST414. When they compared its DNA to other bacteria from around the world, they found it was closely related to strains found in other parts of China, suggesting it traveled or evolved locally. The bacterium was a master of disguise, carrying 15 different genes that helped it resist various antibiotics, including those used for stomach bugs and painkillers. However, it wasn't invincible. The researchers tested it against a new generation of drugs and found that it was still vulnerable to a few specific medicines, including a drug called cefiderocol and a combination of aztreonam and avibactam. It was also still sensitive to amikacin and tigecycline.

One of the most interesting parts of the story is how the bacterium became resistant to colistin. Usually, bacteria carry a specific gene (like mcr) that acts as a shield against colistin. But when the scientists looked at the Chengdu bacterium's DNA, that shield was missing. They also checked the usual suspects for other types of resistance and found nothing. This suggests that the bacterium might be using a secret, hidden trick—perhaps a change in how it controls its own internal machinery—that the scientists haven't fully figured out yet. They suspect the dangerous blaNDM-1 gene is riding on a small, circular piece of DNA called a plasmid, which acts like a USB drive that bacteria can swap with each other to share their "superpowers." However, because the DNA was so repetitive and messy, the scientists couldn't see the whole USB drive clearly yet; they just knew the dangerous file was there.

The study concludes that while this bacterium is a tough opponent, it hasn't won the war yet. It is still susceptible to a few key drugs, which gives doctors a fighting chance. The researchers warn that because this bacterium can be found in both humans and the environment, it might be spreading in ways we don't fully understand. They suggest that we need to keep a very close eye on these bacteria in hospitals and the environment to stop them from becoming a bigger problem. This discovery adds a new chapter to our understanding of how these microscopic cities are evolving, reminding us that the battle against superbugs is an ongoing race between science and nature.