Increasing rates of (B) and (N) in human and erythromycin-resistant isolates between 2018 and 2023 in France.

Macrolides are the first-line compounds used for the treatment of campylobacteriosis. Macrolide resistance remains low in France, with mutations in being the main associated resistance mechanism. However, two erythromycin methyltransferases have also been identified(B), which is mainly described in animal reservoirs, and (N), which is strictly described in humans. In France, between 2018 and 2023, erythromycin-resistant species strains were systematically sequenced and analyzed an in-house bioinformatics pipeline, leading to the identification of the resistomes, MLST and cgMLST, as well as the characterization of the source of contamination. In this study, the genomes of 280 erythromycin-resistant strains were sequenced over a 6-year period. The identification of erythromycin-associated resistance markers revealed a predominance of mutations, in 90% of cases, but also -type methyltransferases in 10% of cases: 75% for (N) and 25% for (B). Over this period, an important increase in the rate of -positive isolates was observed: 2% in 2018 compared with 13% in 2023, with 10% for (N) and 3% for (B). (N) has been found exclusively within a CRISPR-Cas9 operon, whereas (B) has been found within diverse types of resistance genomic islands. Each (N)- or (B)-positive isolate had at least two other resistance markers (mostly ciprofloxacin, tetracycline, or ampicillin) and often carried aminoglycoside-associated resistance genes. The majority of the -positive isolates were obtained from chicken. The increasing rates of -positive and multiresistant isolates make the monitoring of erythromycin-resistant strains, specifically within the chicken meat production, a topic of serious importance.