Multidrug-Resistant Gene Found in Salmonella

A gene recently linked with drug resistance in harmful bacteria recently popped up in a disturbing and unlikely place. A variant of the multidrug-resistant gene MCR-1 was detected in a common strain of Salmonella, often a food-borne pathogen, in a healthy patient. This is particularly troubling since the gene plays a key role in bacterial ability to resist polymyxin antibiotics, such as colistin, which is a “last resort” for patients with drug-resistant infections.

“This is the first time an MCR-1 gene has been found in Salmonella in a healthy carrier,” noted Biao Kan, PhD, a professor of pathogenic microorganisms and infectious disease control at the Chinese Center for Disease Control and Prevention (CCDCP)’s National Institute for Communicable Disease Control and Prevention in China. He noted that the health of the patient represents a substantial threat to any community because healthy carriers could be passing the resistance gene around without anyone knowing since they are not showing symptoms.

The investigators called the gene MCR-1.3 because it is a variant on MCR-1, a gene discovered in 2015 in China. The MCR-1 gene can be passed from one bacterium to another because it is located on a small, transmissible piece of DNA called a plasmid. The MCR-1.3 variant is also located on a plasmid.

According to the Centers for Disease Control and Prevention (CDC), the MCR-1 gene has been reported in the United States in California, Illinois, South Carolina, Virginia, Pennsylvania, Rhode Island, Delaware, New York, Maryland, New Jersey, and Connecticut. Dr. Kan noted in a public statement on the discovery to the American Society for Microbiology (ASM) that the plasmid carrying the MCR-1.3 gene has a “broad host range,” meaning animals as well as humans could be infected and spread the infection between populations and species, possibly without showing physical signs of infection.

The troublesome gene variant was isolated in 2014 on a plasmid in a colistin-resistant Salmonella typhimurium bacterium as part of a routine analysis of a rectal swab specimen. The rectal swab was part of a standard medical examination received at the Yulin Center for Disease Control and Prevention, the researchers said. After testing, the bacterium proved to be resistant to colistin, among several other antibiotics. The plasmid carrying the MCR-1.3 gene variant does not appear to carry any other antimicrobial resistance genes in S. typhimurium, but it is known to move from S. typhimurium to Escherichia coli and Klebsiella pneumoniae, both of which are common and easily and often transmissible from patient to patient, sometimes without overt symptoms.

Polymyxin antibiotics are relatively “old” by medical standards; they were discovered in 1947. Colistin, also known as polymyxin E, is often considered a “salvage” drug because it can cause kidney and brain damage while eliminating the bacterial infection. Dr. Kan and his team recommended “close monitoring” of the MCR-1 gene and its variants since a patient infected with a bacterium carrying an MCR-1.3 gene variant will likely have no treatment options for fending off the infection.