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One reason for the gene’s rapid spread: Klebsiella is adept at grabbing plasmids—extrachromosomal DNA–containing resistance genes directly from other bacteria.

These chains—the longest extrachromosomal genetic material ever found—most likely enter single-celled archaea and help them break down methane.

“I don’t think anything else that’s been discovered is as big as these guys are,” among previously known extrachromosomal DNA elements, Doolittle says.

Scientists first noticed this so-called extrachromosomal DNA five decades ago.

Those clumps went by many names over the years, such as double minutes and extrachromosomal DNA.