Abstract
Several Acinetobacter strains are important nosocomial pathogens, with Acinetobacter baumannii as the species of greatest concern worldwide due to its multi-drug resistance and recent appearance of hyper-virulent strains in the clinical setting. Acinetobacter colonization of the environment and the host is associated with a multitude of factors which remain poorly characterized. Among them, the secretion systems (SS) encoded by Acinetobacter species confer adaptive advantages depending on the niche occupied. Different SS have been characterized in this group of microorganisms, including T6SS used by several Acinetobacter species to outcompete other bacteria and in some A. baumannii strains for Galleria mellonella colonization. Therefore, to better understand the distribution of the T6SS in this genus we carried out an in-depth comparative genomic analysis of the T6SS in 191 sequenced strains. To this end, we analyzed the gene content, sequence similarity, synteny and operon structure of each T6SS loci. The presence of a single conserved T6SS-main cluster (T6SS-1), with two different genetic organizations, was detected in the genomes of several ecologically diverse species. Furthermore, a second main cluster (T6SS-2) was detected in a subgroup of 3 species of environmental origin. Detailed analysis also showed an impressive genetic versatility in T6SS-associated islands, carrying VgrG, PAAR and putative toxin-encoding genes. This in silico study represents the first detailed intra-species comparative analysis of T6SS-associated genes in the Acinetobacter genus, that should contribute to the future experimental characterization of T6SS proteins and effectors.
Highlights
The Acinetobacter genus comprises a heterogeneous group of strictly aerobic Gram-negative bacterial organisms endowed with great metabolic versatility
The ability of each Acinetobacter strain to eliminate E. coli was used as a proxy for an operative T6SS, as previously reported (Carruthers et al, 2013; Shneider et al, 2013; Weber et al, 2013; Repizo et al, 2015; Fitzsimons et al, 2018)
The T6SS fulfills critical roles in bacterial competition, bacterium–host interaction and other functions associated with bacterial physiology
Summary
The Acinetobacter genus comprises a heterogeneous group of strictly aerobic Gram-negative bacterial organisms endowed with great metabolic versatility. This genus includes numerous nonpathogenic environmental species as well as some with significant pathogenic potential, notably Acinetobacter baumannii, frequently associated with disease in the context of hospital-acquired infections (McConnell et al, 2013). Bacteria use several secretory mechanisms to export effector proteins into the environment or straight into target cells (Costa et al, 2015; Galán and Waksman, 2018). The multicomponent type VI secretion system (T6SS), which is structurally related to the cell-puncturing device of the T4 bacteriophage, was described in Gram-negative bacteria, including Acinetobacter (Weber et al, 2017; Elhosseiny and Attia, 2018). T6SS effectors with diverse enzymatic activities have been identified, including the membrane-, cell wall-, or nucleic acid-targeting antibacterial effectors and the heterogenous group of eukaryote-targeting effectors (Lien and Lai, 2017)
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