For the first time, Spanish researchers have found antimicrobial resistant genes in Salmonella isolated from raw mussels.

Writing in the journal Eurosurveillance, they said the findings are of concern as such analysis is not performed and Galicia, north-west Spain is the third largest producer of mussel aquaculture worldwide and one of the main suppliers of the product to the European market.

The mcr-1 gene in a strain of Salmonella Rissen sequence type (ST) 469 had not been isolated previously in environmental Salmonella from mussels in Spain.

The emergence of plasmid-mediated colistin resistance due to the mcr-1 gene was initially described in Enterobacteriaceae isolates in China and has been documented in Europe in human clinical cases, food products and food-animal production settings.

One driver for the global spread is trade in food animals and meat, although direct global movement by colonized or infected humans is also likely to have a role in distribution, according to researchers.

Emergence of transferable colistin resistance by mcr-1 undermines its revival as the “antibiotic of last resort” for carbapenem-resistant bacterial infections.

Nineteen different Salmonella strains were isolated from 5,907 randomly seleced mussel samples during a monitoring program for the pathogen in shellfish in Galicia from 2012 to 2016. Scientists collected samples from 15 production areas and four processing facilities in Galicia.

They collected 5,560 raw mussel samples from one raft in each production area and 347 cooked mussel samples came from processing facilities.

Researchers placed the raw and cooked mussel samples in sterile bags with frozen gel-packs and transported to the ASMECRUZ Laboratory in Pontevedra, Spain in 4 degrees Celsius refrigerated trucks. All samples were analyzed within 12 hours of arrival at the lab.

Of the 19 Salmonella strains analyzed, four were isolated from cooked mussels. Antimicrobial susceptibility-resistance testing was positive for resistance to at least four of the antibiotics tested. Two strains were resistant to eight and nine antimicrobials, respectively.

Antimicrobial susceptibility testing involved ampicillin, gentamicin, tobramycin, trimethoprim-sulfamethoxazole, cefoxitin, cefuroxime, and cephalothin.

Salmonella strains isolated during a previous study of mollusks from the same marine environment belonged to different serotypes and antimicrobial resistant (AMR) profiles.

Marine environments may represent a source of AMR genes as they are subjected to contamination with terrestrial effluents such as agricultural waste, discharges from human sites, hospitals and industry and sewage treatment plants. Tourism is also a source, especially in summer months when the population increases in coastal areas.

Researchers said results highlight the need for continuing surveillance of mussels.

“There is a need for public health authorities and mussel producers to ensure correct management, an efficient purification process and extensive sanitary control in ready-to-eat mollusks,” the scientists reported. “Implementation of routine pathogens investigations and screening of the presence of resistance genes could contribute to a better understanding of the role of the marine environment and seafood in the transmission of AMR among human pathogens and resident bacteria.”