28 February 2017

The Stealthy Spread of Superbug CRE in U.S. Hospitals

An alarming new study from the Harvard T.H. Chan School of Public Health and the Broad Institute of MIT and Harvard suggests that carbapenem-resistant Enterobacteriaceae (CRE)—a new class of superbug referred to as “nightmare bacteria” by former CDC director Dr. Tom Frieden—may be spreading more widely and more stealthily than was previously thought. The researchers found that CREs are growing in numbers and strength, are far more diverse than expected, and have many more mechanisms for not only resisting antibiotics but also spreading that resistance to other bacteria than have been identified to date. The study’s findings were published in January in the Proceedings of the National Academy of Sciences (PNAS).

The CDC has called CRE a “triple threat”: These superbug bacteria are resistant to all or nearly all available antibiotics, including the carbapenem class, considered the “last line” drugs for such infections. They are associated with high mortality rates. And they have the alarming ability to transmit their antibiotic-resistant genes to other bacteria, threatening to make even common infections, such as E. coli, untreatable. In fact, last year a Pennsylvania woman was infected with a strain of colistin-resistant E. coli bacteria, the first time in the U.S. that scientists found bacteria carrying colistin-resistance gene mrc-1

While the researchers found little evidence of transmission of CRE bacteria from patient to patient in the hospital setting, they did see what William Hanage, associate professor of epidemiology at the Harvard T.H. Chan School of Public Health and the study’s senior author, termed “a riot of diversity” of CRE species as well as a wide variety of genetic traits that enable these bacteria to resist antibiotics. There are genes known to give bacteria resistance to carbapenems, but the researchers also found bacteria that did not carry those signature genes, yet were resistant to carbapenem antibiotics. The resistance mechanisms in those bacteria are not yet known. The researchers also found that these traits are transferring easily among different species of CRE. 

The study found indications of “continued innovation by these organisms to thwart the action of this important class of antibiotics.” According to a recent article in STAT, Dr. Alexander Kallen, an infection control expert at the CDC who refused to comment specifically on the study, agrees that CREs have myriad ways of overcoming drugs. 

Given that there wasn’t much evidence of transmission within hospitals, the question of how the bacteria are spreading remains unknown. Could it be transmitting from person to person asymptomatically?  The researchers recommend an increase in genomic surveillance of CREs. A Harvard press release quotes Hanage as saying, “The best way to stop CRE making people sick is to prevent transmission in the first place. If it is right that we are missing a lot of transmission, then only focusing on cases of disease is like playing Whack-a-Mole; we can be sure the bacteria will pop up again somewhere else.”