Insights into macrolide resistance in Arcobacter butzleri: potential resistance mechanisms and impact on bacterial fitness and virulence.

BACKGROUND

Macrolides are recommended for treating the emerging enteropathogen Arcobacter butzleri; nonetheless, this bacterium often exhibits highly variable resistance rates, and the mechanisms behind this resistance phenotype remain largely unexplored.

OBJECTIVES

To understand the phenotypic and genotypic consequences associated with the acquisition of erythromycin resistance in A. butzleri, as well as the effects on the fitness of this species.

METHODS

Resistant strains resulting from spontaneous mutations and adaptive laboratory evolution under increasing erythromycin concentrations were examined regarding their cross-resistance and collateral susceptibility profiles. Genetic causes of phenotypic antibiotic resistance were analysed by sequencing and bioinformatics, with functional correlation through ethidium bromide accumulation assays. Growth profiles in the presence and absence of erythromycin, motility and biofilm formation abilities were assessed to detect potential changes in fitness and virulence.

RESULTS

Clones from spontaneous mutation rate evolution demonstrated decreased susceptibility to erythromycin and other classes of antibiotics, associated with mutations in the transcriptional repressor areR, causing overexpression of the AreABC efflux pump. In turn, WGS analysis of the evolved strain showed additional mutations in the ribosomal proteins L4 and L22 and in the areR gene. Furthermore, the acquisition of macrolide resistance altered A. butzleri virulence and entailed a high biological cost.

CONCLUSIONS

The findings of this study have proved that efflux activity contributes synergistically with mutations in the ribosomal proteins L4 and L22 to A. butzleri's high-level macrolide resistance. The results further suggest an impact on the bacterial physiology and virulence, with the increased fitness cost justifying the low worldwide prevalence of high-level resistant circulating strains.