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.

Expression of a Shiga-Like Toxin during Plastic Colonization by Two Multidrug-Resistant Bacteria, Aeromonas hydrophila RIT668 and Citrobacter freundii RIT669, Isolated from Endangered Turtles (Clemmys guttata)

“…cause a range of pathologies, they are emerging as an enteric pathogen of public health concern [44]. Results from diarrheal outbreak studies further show that the infective dose of Aeromonas is very low [45]. A. hydrophila has also been found in food samples and has the ability to form mixed biofilms with other pathogens [46].…”

Role of Bacterial Secretion Systems and Effector Proteins – Insight into the Pathogenicity Mechanisms in Aeromonas

“…Bacterial pathogens have evolved a multitude of strategies against prokaryotic competitors and eukaryotic hosts to colonize, invade, and overcome the host immune response (Sha et al, 2005;Fernandez-Bravo & Figueras, 2020). One of important prokaryotic cell functions is protein secretion, which comprises transport of proteins from the cytoplasm to the extracellular medium and/or directly into other bacteria or eukaryotic cells.…”

“…polar (fla) and lateral flagella (laf), pili, capsules, the Slayer, outer membrane proteins (OMP), and lipopolysaccharide (LPS). Moreover, interaction between such pathogenic bacteria as Aeromonas and host cells is produced by their extracellular components and toxins, including haemolytic toxins, such as: 1) aerolysin with haemolytic and cytolytic properties (Aer), 2) cytotoxic enterotoxin (Act) with multiple biological activities, including ability to lyse red blood cells and destruct tissue culture cell lines (Chopra et al, 2000), 3) thermolabile (Alt), and thermostable (Ast) cytotonic enterotoxins (Alt causes elevation of cyclic AMP and prostaglandin levels in intestinal epithelial cells, Ast possess similar properties to Alt) (Albert et al, 2000), 4) serine protease (Ser) with extracellular proteolytic activity, 5) elastase (Ela) with caseinolytic and elastolytic activities (Rasmussen-Ivey et al, 2016), 6) glycerophospholipids, such as cholesterol acyltransferase (GCAT), which attacks membrane phospholipids and leads to lysis of fish tissues (Tomas, 2012), and secretion systems (Ghenghesh et al, 2014;Rasmussen-Ivey et al, 2016;Dlamini et al, 2018;Fernandez-Bravo & Figueras, 2020). To understand the pathogenicity of Aeromonas, the contribution of these factors should be defined.…”

Genetic detection to Aeromonas hydrophila proteolytic activity in milk samples (cows, buffaloes and goats) in Basra governorate

“…Aeromonas species are facultative anaerobic gramnegative microscopic organisms. It related to the family Aeromonadaceae, which is frequently isolated from various food and environmental sources including sea, river, fresh and ground water (1). The amassing information showing that A. hydrophila is the causal operator for 85% of human gastrointestinal issue (2).…”