Salmonella Resistant to Colistin Identified in US

The gene mcr-3.1, which is associated with colistin-resistant Salmonella in Asia, was identified in a patient in the United States by investigators at North Carolina State University.

The research, published in the Journal of Medical Microbiology, highlighted concerns about the spread of microbes with mobile colistin resistance (MCR). Colistin is considered to be the drug of last resort for treating multidrug-resistant Salmonella infections.

Investigators studied 100 Salmonella enterica serotypes isolated between 2014 and 2016 from the southeastern United States. One of the isolates, taken from an 18-year-old female in 2014, was positive for mcr-3.1. The patient had traveled to China 2 weeks prior to the onset of symptoms.

“Drug resistance genes do not respect international boundaries,” corresponding author Siddhartha Thakur, BVSc, MVSc, PhD, professor and director of global health at North Carolina State University, told Contagion^®. “The patient (18-year-old female) had a travel history to China, where plasmid-borne MCR was first reported in 2015. The fact that the US patient isolate was recovered in 2014 tells us that plasmid-borne MCR was already circulating in China. Just the fact that we isolated it in 2015 does not mean that it emerged in that year. International travel is 1 of the key reasons for dissemination of drug resistance genes across the globe.”

Overall, the investigators identified 8 multidrug-resistant isolates and 31 with phenotypic resistance.

“When was the last time we heard good news on drug resistance? It’s like never,” Thakur said. “We are fighting an uphill battle and we are seeing an increase in frequency of resistance to drugs that are the last resort for our physicians. The last new class of antimicrobials were developed almost 4 decades ago. So, with no new class of antimicrobials being developed and increasing resistance to the existing drugs, the overall situation is not encouraging.”

The study is part of an ongoing effort to monitor the spread of antimicrobial-resistant bacteria. Previous research in 2015 found that mcr-3.1 had moved to a plasmid, easing its ability to spread between organisms and countries. The gene previously was reported in countries including Denmark, Australia and Canada, and the new study suggests it has been circulating farther and earlier than previously thought.

Thakur said efforts to maintain surveillance data on antibiotic resistance are important. Such efforts include the World Health Organization’s Global Antimicrobial Surveillance System (GLASS), which has found that antimicrobial resistance is widespread. Thakur also called for more support for new drug development, research, and public education and engagement.

“Research on drug resistance in my lab is centered around the ‘One Health’ concept where we look at the issue at the interface of humans, animals and the environment that we share,” Thakur said. “Even though I am based in one of the top CVM schools, our work was conducted on isolates recovered from a human patient. Finally, we are doing our work at the global scale and not local.”

Earlier this year, an outbreak of Salmonella linked to live poultry was reported across 21 states. That report noted that Salmonella outbreaks are on the rise, with 70 linked to contact with live poultry between 2000 and 2017.