Sphingomonas koreensis, in yellow, on a petri dish
Image: Justesen, et al (Journal of Clinical Microbiology)

In 2016, a mysterious illness spread inside the National Institutes of Health’s Clinical Center, the U.S. government’s most prominent research hospital, in Bethesda, Maryland. Patients were somehow being sickened by an antibiotic-resistant strain of bacteria that practically never causes disease in humans. Two years later, a new study seems to finally have confirmed where this bug likely came from: the hospital’s own plumbing.

During a six-month period in 2016, six patients came down with infections caused by Sphingomonas bacteria. Four of the patients had an antibiotic-resistant strain of a particular species, Sphingomonas koreensis, which was first discovered in some of Korea’s natural mineral water spots in the early 2000s.

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The authors behind the study, published this week in the New England Journal of Medicine, looked through hospital records at the center dating back to the 2000s, and found eight other cases of people with Sphingomonas koreensis in their bodies from 2006 onwards. Then they sequenced the bacterial DNA of samples taken from patients as well as from various environments of the hospital. They also compared these samples to other samples of Sphingomonas koreensis taken from other places and patients outside of the hospital.

The 2016 samples all bore an incredibly close genetic similarity to one another (99.92 percent similar, in fact). They were also closely related to the past samples but not so much to the outside samples, indicating these particular bacteria have been calling the NIH Clinical Center home for some time. The samples taken from faucets and water sources in the patients’ rooms also matched up closely to the 2016 samples, implicating the plumbing system as the likely conduit for infection.

S. koreensis is a gram-negative bacteria. Gram-negative describes bacteria that show up as pinkish or red when exposed to a common chemical stain used to identify them under a microscope. This happens because their cell walls are very different from gram-positive bacteria. That same cell wall also unfortunately makes them naturally resistant to many antibiotics. And the widespread overuse of antibiotics has only made gram-negative bacteria even tougher to deal with, especially in hospitals.

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The bacteria itself isn’t a particularly potent pathogen, though. Prior to the current study, there were only two other documented cases of S. koreensis infection, with the first reported in 2015. And though three of the 12 NIH patients identified with S. koreensis in their body eventually died, it’s possible they were killed by other contributing factors, including other major infections. Nine of the 12 patients, including the three who died, had been given stem-cell transplants beforehand, meaning their immune systems were already weakened. One patient appeared to have only carried the bacteria in their body, not actually becoming sick as a result.

Near as the authors can tell, S. koreensis might have gotten inside the NIH sometime in 2004, when a new building underwent construction. And it survived, at least partly, thanks to a poorly maintained water system. They found both low levels of chlorine in the water, as well as water that was not heated as high as it should have been. Since the hospital took steps to better clean its water, though, there have been no other cases reported.

That said, S. koreensis is only the latest resistant bug to cause an outbreak at the NIH Clinical Center. And given the uphill climb to stop or even slow down the rise of antibiotic resistance, it almost certainly won’t be the last.

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[NEJM]