From an emerging pathogen to therapeutic challenges: advances in Achromobacter species, with a focus on A. xylosoxidans and cystic fibrosis.

Achromobacter spp. are aerobic, non-fermentative, Gram-negative opportunistic pathogens belonging to the family Alcaligenaceae, order Burkholderiales, and are widely distributed in natural environments such as soil and water. In patients with cystic fibrosis (CF), Achromobacter spp. have emerged as increasingly prevalent opportunistic pathogens, with detection rates in respiratory samples rising from approximately 2-3% in the 1990s and early 2000s to 7-10% by the early 2020s in European and North American registries, although reported rates vary considerably by center, age group, and identification method. These bacteria also affect immunocompromised individuals outside the CF population. However, precise diagnosis and treatment of Achromobacter infections remain particularly challenging because of progressive difficulties: inaccurate species-level identification, limited clinical breakpoints for most species, and synergistic multidrug resistance mechanisms. This review synthesizes recent epidemiological data, outbreak investigations, and tentative epidemiological cutoff values (ECOFFs) for A. xylosoxidans established by EUCAST. We also present an integrated framework that links motility loss, biofilm formation, and resistance development to describe within-host adaptive evolution in the CF airway, and we highlight the role of biofilm-mediated tolerance and persister cells in chronic infection. By synthesizing multidimensional evidence, this review aims to provide a comprehensive reference framework for basic research and clinical management of Achromobacter, and to identify priority research directions, including the establishment of standardized identification procedures and the advancement of standardized clinical development of phage therapy.