Spread of the colistin-resistance gene in through plasmid transmission and chromosomal transposition in French goats.

Introduction: Colistin-resistance widely disseminated in food-producing animals due to decades of colistin use to treat diarrhea. The plasmid-borne gene has been extensively reported from bovine, swine and chicken worldwide, but smaller productions such as the goat farming sector were much less surveyed.

Methods: We looked for colistin-resistant isolates presenting plasmid-borne genes of the mcr family in both breeding (=80) and fattening farms (=5). Localization of the gene was performed using Southern blot analysis coupled to short-read and long-read sequencing.

Results: Only the gene was identified in 10% (8/80) of the breeding farms and four over the five fattening farms. In total, 4.2% (65/1561) of the animals tested in breeding farms and 60.0% (84/140) of those tested in fattening farms presented a -positive . The gene was located either on the chromosome (32.2%) or on IncX4 (38.9%) and IncHI2 (26.8%) plasmids. As expected, both clonal expansion and plasmidic transfers were observed in farms where the gene was carried by plasmids. Tn6330 transposition was observed in the chromosome of diverse sequence types within the same farm.

Discussion: Our results show that the gene is circulating in goat production and is located either on plasmids or on the chromosome. Evidence of Tn transposition highlighted the fact that chromosomal insertion does not impair the transmission capability of the gene. Only strict hygiene and biosecurity procedures in breeding farms, as well as a prudent use of antibiotics in fattening farms, can avoid such complex contamination pathways.