Abstract
Acinetobacter baumannii is an emergent and global nosocomial pathogen. In addition to A. baumannii, other Acinetobacter species, especially those in the Acinetobacter calcoaceticus-baumannii (Acb) complex, have also been associated with serious human infection. Although mechanisms of attachment, persistence on abiotic surfaces, and pathogenesis in A. baumannii have been identified, the genetic mechanisms that explain the emergence of A. baumannii as the most widespread and virulent Acinetobacter species are not fully understood. Recent whole genome sequencing has provided insight into the phylogenetic structure of the genus Acinetobacter. However, a global comparison of genomic features between Acinetobacter spp. has not been described in the literature. In this study, 136 Acinetobacter genomes, including 67 sequenced in this study, were compared to identify the acquisition and loss of genes in the expansion of the Acinetobacter genus. A whole genome phylogeny confirmed that A. baumannii is a monophyletic clade and that the larger Acb complex is also a well-supported monophyletic group. The whole genome phylogeny provided the framework for a global genomic comparison based on a blast score ratio (BSR) analysis. The BSR analysis demonstrated that specific genes have been both lost and acquired in the evolution of A. baumannii. In addition, several genes associated with A. baumannii pathogenesis were found to be more conserved in the Acb complex, and especially in A. baumannii, than in other Acinetobacter genomes; until recently, a global analysis of the distribution and conservation of virulence factors across the genus was not possible. The results demonstrate that the acquisition of specific virulence factors has likely contributed to the widespread persistence and virulence of A. baumannii. The identification of novel features associated with transcriptional regulation and acquired by clades in the Acb complex presents targets for better understanding the evolution of pathogenesis and virulence in the expansion of the genus.
Highlights
Acinetobacter baumannii is a nosocomial pathogen implicated with septicemia, pneumonia, and death [1,2,3]
The results confirm that A. baumannii genomes comprise a monophyletic clade that is part of the larger Acinetobacter calcoaceticus-baumannii (Acb) complex
The Acb complex contains environmental isolates not associated with serious human disease [65]; the three isolates sequenced in this study (A. calcoaceticus TG19593, TG19585, TG19588) from the environmental clade, including A. calcoaceticus and A. sp
Summary
Acinetobacter baumannii is a nosocomial pathogen implicated with septicemia, pneumonia, and death [1,2,3]. A. baumannii is truly a global pathogen, as it has been isolated from hospitals throughout the world [4,5,6,7], as well as in wounded soldiers serving in Iraq [8,9,10] and Afghanistan [11]. Acinetobacter spp. are found in most soil and water samples [12], but are a major source of nosocomial infections [13]; the natural environments for the pathogens A. baumannii and A. nosocomialis are not currently known [14]. Most nosocomial Acinetobacter infections are associated with A. baumannii [15], other species have been associated with human disease. A. ursingii has been associated with nosocomial bloodstream infections [16,17]
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