An Update on the Genus Aeromonas: Taxonomy, Epidemiology, and Pathogenicity

Abstract

The genus Aeromonas belongs to the Aeromonadaceae family and comprises a group of Gram-negative bacteria widely distributed in aquatic environments, with some species able to cause disease in humans, fish, and other aquatic animals. However, bacteria of this genus are isolated from many other habitats, environments, and food products. The taxonomy of this genus is complex when phenotypic identification methods are used because such methods might not correctly identify all the species. On the other hand, molecular methods have proven very reliable, such as using the sequences of concatenated housekeeping genes like gyrB and rpoD or comparing the genomes with the type strains using a genomic index, such as the average nucleotide identity (ANI) or in silico DNA-DNA hybridization (isDDH). So far, 36 species have been described in the genus Aeromonas of which at least 19 are considered emerging pathogens to humans, causing a broad spectrum of infections. Having said that, when classifying 1852 strains that have been reported in various recent clinical cases, 95.4% were identified as only four species: Aeromonas caviae (37.26%), Aeromonas dhakensis (23.49%), Aeromonas veronii (21.54%), and Aeromonas hydrophila (13.07%). Since aeromonads were first associated with human disease, gastroenteritis, bacteremia, and wound infections have dominated. The literature shows that the pathogenic potential of Aeromonas is considered multifactorial and the presence of several virulence factors allows these bacteria to adhere, invade, and destroy the host cells, overcoming the immune host response. Based on current information about the ecology, epidemiology, and pathogenicity of the genus Aeromonas, we should assume that the infections these bacteria produce will remain a great health problem in the future. The ubiquitous distribution of these bacteria and the increasing elderly population, to whom these bacteria are an opportunistic pathogen, will facilitate this problem. In addition, using data from outbreak studies, it has been recognized that in cases of diarrhea, the infective dose of Aeromonas is relatively low. These poorly known bacteria should therefore be considered similarly as enteropathogens like Salmonella and Campylobacter.

Keywords: Aeromonas; epidemiology; immune response; taxonomy.

Figure 1

(A) Phylogenetic tree based on the sequences of the 16S rRNA gene (1498 bp) among 36 species of Aeromonas. (B) Phylogenetic tree based on the concatenated sequences of rpoD and gyrB genes (1098) among the most similar species based on the 16S rRNA gene. The number in the nodes indicates the bootstrap values substitutions estimated by site. (C) Results (%) for the ANI (average nucleotide identity) and isDDH (in silico DNA–DNA hybridization) obtained between the genomes of the most similar species; notice that ANI and isDDH values are ≤96% and ≤70% in all cases, respectively which are the cut-off values established for delimiting Aeromonas spp.

Figure 2

Toll-like receptors (TLRs) pathways that induce the inflammatory response mediated by cytokines (adapted from [278]). Created with Biorender.

Figure 3

Pyroptosis pathway with Toll-like receptors (TLRs) and Nod-like receptors (NLRs) acting in concert (adapted from [275]). Created with Biorender.