Some common “superbugs” appear to harbor a little-known type of resistance to a last-resort antibiotic, a new study shows, suggesting a worrying new way in which dangerous bacteria can evade one of the few remaining treatment options.

Bacterial populations are normally viewed as either totally impervious to an antibiotic, or totally treatable. But researchers from Emory University identified a different pattern in a certain type of drug-resistant bacteria, in which some cells in a bacterial colony are resistant to a last-resort antibiotic called colistin. This “heteroresistance” isn’t easily detectable in standard lab tests because most of the cells are susceptible to the drug.

The results of their study were published online Tuesday in the journal mBio.

Antibiotic resistance is a growing public health crisis. The Emory study’s focus, Carbapenem-resistant Klebsiella pneumoniae, is among the most common of a group of so-called “superbug” bacteria that are resistant to most antibiotics and kill up to 50% of the people they infect. More than two million people in the U.S. are sickened every year with antibiotic-resistant infections, with at least 23,000 dying, according to the Centers for Disease Control and Prevention.