By Gary Evans, Medical Writer

Carbapenems are a last-line antibiotic, one of the final weapons in the formulary against multidrug-resistant bacteria. But the ever-evolving bugs are starting to solve this drug class and have found an ingenious way to do it genetic transfer of resistant properties to another Gram-negative bacterium.

Transferred on a plasmid, genetic material enables the recipient bacterium to produce carbapenemases, enzymes that inactivate beta-lactam antibiotics, including carbapenems. The Centers for Disease Control and Prevention (CDC) established its Antibiotic Resistance Laboratory Network (AR Lab Network) in 2016 to support nationwide lab capacity to detect these emerging antibiotic-resistant organisms rapidly.

“Through the AR Lab Network, public health laboratories test carbapenem-resistant Enterobacterales, carbapenem-resistant Pseudomonas aeruginosa (pCRPA), and carbapenem-resistant Acinetobacter baumannii isolates for carbapenemases,” the CDC reports.1 “During October 2012-April 2019, CDC documented 151 isolates harboring multiple carbapenemase genes from 100 patients in the United States.”

The threat of the genetic transfer from these carbapenem-resistant organisms (CRO) to susceptible bacteria has a multiplier effect that increases the risk to patients well beyond this 100, who were a median 63 years of age.

“Among 93 incident cases with available data, 62 (66.7%) occurred in patients who had traveled internationally in the 12 months before their incident culture,” the CDC found. “Among patients with a history of international travel, most (89.5%) had received inpatient healthcare while abroad.”

When the CDC lab began to do its own testing in 2017, a pattern of increasing microbial threats began to emerge.

“Among 89 (84.8%) incident cases reported since AR Lab Network testing began in 2017, a total of 15 were reported in 2017, 51 in 2018, and 23 in the first four months of 2019,” the CDC reported. “Multi-CRO in healthcare facilities are an emerging concern in the U.S.,” the authors concluded. “Although hospitalization outside the U.S. was the most common risk factor, we found a substantial proportion of cases that were probably acquired in healthcare facilities in the U.S.”

The CDC recommendations included screening patients who were hospitalized outside the U.S. recently and investigating possible transmission when a multi-CRO is identified.


  1. Ham C, Mahon G, Bhaurla SK, et al. Gram-negative bacteria harboring multiple carbapenemase genes, United States, 2012-2019. Emerg Infect Dis 2021; Aug 11. doi: 10.3201/eid2709.210456. [Online ahead of print].