Comparative genome characterization of Achromobacter members reveals potential genetic determinants facilitating the adaptation to a pathogenic lifestyle.

Members of the Achromobacter genus are Gram-negative bacteria including both environmental and clinical isolates, which are increasingly recovered from patients with cystic fibrosis (CF) as emerging pathogens. To better understand the features of the genus and its potential pathogenic mechanisms, six available Achromobacter genomes were compared in this study. The results revealed that: (1) Achromobacter had a pan-genome size of 10,750 genes with 3,398 core genes and a similar global classification of protein functions; (2) the Achromobacter genomes underwent a relatively low recombination that introduced nearly twice nucleotide substitutions less than the point mutation in genome evolution; (3) phylogenomic analysis based on 436 conserved proteins and average nucleotide identity both indicated that the Achromobacter genus had the closest relationship to the human/animal pathogen Bordetella rather than to Alcaligenes. The entire group of Achromobacter clustered with Bordetella in phylogeny, strongly suggesting a common origin, which therefore highlighted the potentially pathogenic nature of Achromobacter from the phylogenetic perspective, and (4) the CF clinical isolate possessed markedly unique genomic features discriminated from the environmental isolate and was equipped with numerous factors that facilitate its adaptation to a pathogenic lifestyle, such as a type III secretion system, a "polysaccharide island" (36.0 kb) of capsular/cellulose synthesis, adhesion-related proteins, alcaligin biogenesis, and several putative toxins. This study provided the first comprehensive genomic comparative analysis for Achromobacter, revealed information to better understand this far less-known genus on the genomic scale, and, importantly, identified potential virulence factors of the Achromobacter pathogen.