Researchers from North Carolina State University have found a gene that gives Salmonella resistance to antibiotics of last resort in a sample taken from a human patient in the U.S. The find is the first evidence that the gene mcr-3.1 has made its way into the U.S. from Asia.
The news comes as a new Salmonella outbreak has hit the US, leaving 62 people ill across eight states.
There are more than 2,500 known serotypes of Salmonella. In the U.S., Salmonella enterica 4,,12:i:- ST34 is responsible for a significant percentage of human illnesses. The drug resistance gene in question – known as mcr-3.1 – gives Salmonella resistance to colistin, the drug of last resort for treating infections caused by multidrug-resistant Salmonella.
“Public health officials have known about this gene for some time,” says Siddhartha Thakur, professor and director of global health at NC State and corresponding author of the research. “In 2015, they saw that mcr-3.1 had moved from a chromosome to a plasmid in China, which paves the way for the gene to be transmitted between organisms. For example, E. coli and Salmonella are in the same family, so once the gene is on a plasmid, that plasmid could move between the bacteria and they could transmit this gene to each other. Once mcr-3.1 jumped to the plasmid, it spread to 30 different countries, although not – as far as we knew – to the U.S.”
Thakur’s lab is one of several nationally participating in epidemiological surveillance for resistant strains of Salmonella. The lab generates whole genome sequences from Salmonella samples every year as part of routine monitoring for the presence of antimicrobial-resistant bacteria. When veterinary medicine student Valerie Nelson and Ph.D. student Daniel Monte did genome sequencing on 100 clinical human stool samples taken from the southeastern U.S. between 2014 and 2016, they discovered that one sample contained the resistant mcr-3.1 gene. The sample came from a person who had traveled to China two weeks prior to becoming ill with a Salmonella infection.
“This project proved the importance of ongoing sequencing and surveillance,” says Nelson. “The original project did not involve this gene at all.”
“The positive sample was from 2014, so this discovery definitely has implications for the spread of colistin-resistant Salmonella in the U.S.,” Thakur says. “Our lab will continue to try and fill in these knowledge gaps.”
The research, reported by NCSU News Service earlier this month, appears in the Journal of Medical Microbiology and was supported by the National Institutes of Health/Food and Drug Administration (award number 5U 18FD006194-02). Monte and Nelson are first author and co-author, respectively. Prior to his global health role, Thakur was associate director of the emerging infectious diseases program at NC State’s Comparative Medicine Institute.
Sixty-two people in eight US states have fallen ill this year from Salmonella related to fresh papayas imported from Mexico, according to the Centers for Disease Control and Prevention.
The illnesses range from mid-January up to June 8, with the highest number occurring in April. Of those who’ve gotten sick, 23 have been hospitalized.
So far, no deaths are reported.
Salmonella, which rarely affects how food tastes or smells, lives in the intestinal tracts of animals, including birds and people.
If you’re not sure where your papayas have come from, throw them out
The CDC is advising folks in Connecticut, Massachusetts, New Jersey, Pennsylvania, and Rhode Island to avoid eating whole, fresh papayas from Mexico. They also say not to eat fruit salads or mixes including Mexican papayas.
If you encounter papayas and have doubt about their country of origin, the CDC says to be on the safe side and throw them out. The agency recommends washing and sanitizing places where papayas are stored, including counter tops and refrigerator shelves.
Meanwhile, the US Food and Drug Administration wants importers, suppliers, distributors and other food service providers to halt sales across all states of papayas imported from Mexico.
This year’s outbreak is associated with the Salmonella Uganda serotype (species) of the bacteria.
Those who are infected can develop diarrhea, fever, and stomach cramps between 12 and 72 hours following the initial exposure. Patients usually recover on their own in less than a week, but some people do need to be hospitalized.
According to CDC data, 1.2 million Salmonella cases occur each year in the US, with about 450 of the cases leading to death.