Multilocus genetics to reconstruct aeromonad evolution

Abstract

Background: Aeromonas spp. are versatile bacteria that exhibit a wide variety of lifestyles. In an attempt to improve the understanding of human aeromonosis, we investigated whether clinical isolates displayed specific characteristics in terms of genetic diversity, population structure and mode of evolution among Aeromonas spp. A collection of 195 Aeromonas isolates from human, animal and environmental sources was therefore genotyped using multilocus sequence analysis (MLSA) based on the dnaK, gltA, gyrB, radA, rpoB, tsf and zipA genes.

Results: The MLSA showed a high level of genetic diversity among the population, and multilocus-based phylogenetic analysis (MLPA) revealed 3 major clades: the A. veronii, A. hydrophila and A. caviae clades, among the eleven clades detected. Lower genetic diversity was observed within the A. caviae clade as well as among clinical isolates compared to environmental isolates. Clonal complexes, each of which included a limited number of strains, mainly corresponded to host-associated subsclusters of strains, i.e., a fish-associated subset within A. salmonicida and 11 human-associated subsets, 9 of which included only disease-associated strains. The population structure was shown to be clonal, with modes of evolution that involved mutations in general and recombination events locally. Recombination was detected in 5 genes in the MLSA scheme and concerned approximately 50% of the STs. Therefore, these recombination events could explain the observed phylogenetic incongruities and low robustness. However, the MLPA globally confirmed the current systematics of the genus Aeromonas.

Conclusions: Evolution in the genus Aeromonas has resulted in exceptionally high genetic diversity. Emerging from this diversity, subsets of strains appeared to be host adapted and/or "disease specialized" while the A. caviae clade displayed an atypical tempo of evolution among aeromonads. Considering that A. salmonicida has been described as a genetically uniform pathogen that has adapted to fish through evolution from a variable ancestral population, we hypothesize that the population structure of aeromonads described herein suggested an ongoing process of adaptation to specialized niches associated with different degrees of advancement according to clades and clusters.

Figure 1

Unrooted maximum-likelihood tree based on concatenated sequences of the seven housekeeping gene fragments (3993 nt). The tree shows the structure of the studied Aeromonas spp. population, and the relative placement of human (red font), non-human animal (black font) and environmental (blue font) strains was indicated. The horizontal lines represent genetic distance, with the scale bar indicating the number of substitutions per nucleotide position. The numbers at the nodes are support values estimated with 100 bootstrap replicates. Only bootstrap values > 70 are indicated on the tree. The roots of the clades defined in 1 are represented by bold lines. MLPA clusters of strains sharing identical STs or grouped into CCs sharing at least 4 identical alleles at the 7 loci are indicated by frames (red frames for clusters of human strains, grey frames for clusters of non-human animal strains and uncolored frames for clusters of strains of various origins). In these frames, the following characteristics are indicated from left to right: (i) the strain’s clinical involvement when applicable as Inf for infection and Col for colonization; (ii) the SwaI pulsotype of the strains, with strains of identical pulsotypes designated by the same letter, strains with pulsotypes sharing more than 85% of their DNA fragments by A, A1, A2, … and strains with pulsotypes sharing no more than 70% of their DNA fragments by distinct letters, i.e., A, B, C, …; (iii) the names of STs shared by several strains; (iv) the names of CCs sharing at least 5 identical alleles at the 7 loci; and (v) the names of CCs sharing at least 4 identical alleles at the 7 loci. These ST and CC names are indicated to the right of the brackets grouping the strains with identical STs or belonging to the same CC and are followed by the bootstrap value (indicated in parentheses) supporting the corresponding MLPA cluster. (*) indicates that the relative position of the corresponding branch varied according to the method used. ND, not determined.