The Genus Aeromonas a General Approach
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Microbial Pathogenesis 130 (2019) 81–94 Contents lists available at ScienceDirect Microbial Pathogenesis journal homepage: www.elsevier.com/locate/micpath The genus Aeromonas: A general approach T Rafael Bastos Gonçalves Pessoa, Weslley Felix de Oliveira, Diego Santa Clara Marques, Maria Tereza dos Santos Correia, Elba Verônica Matoso Maciel de Carvalho, ∗ Luana Cassandra Breitenbach Barroso Coelho Departamento de Bioquímica, Centro de Biociências, Universidade Federal de Pernambuco (UFPE), Av. Prof. Moraes Rego, s/n, Cidade Universitária, CEP: 50670-420, Recife, Pernambuco, Brazil ARTICLE INFO ABSTRACT Keywords: The genus Aeromonas comprises more than thirty Gram-negative bacterial species which mostly act as oppor- Aeromonas tunistic microorganisms. These bacteria are distributed naturally in diverse aquatic ecosystems, where they are Virulence factors easily isolated from animals such as fish and crustaceans. A capacity for adaptation also makes Aeromonas able to Infection colonize terrestrial environments and their inhabitants, so these microorganisms can be identified from different Diagnosis sources, such as soils, plants, fruits, vegetables, birds, reptiles, amphibians, among others. Infectious processes usually develop in immunocompromised humans; in fish and other marine animals this process occurs under conditions of stress. Such events are most often associated with incorrect practices in aquaculture. Aeromonas has element diverse ranges, denominated virulence factors, which promote adhesion, colonization and invasion into host cells. These virulence factors, such as membrane components, enzymes and toxins, for example, are dif- ferentially expressed among species, making some strains more virulent than others. Due to their diversity, no single virulence factor was considered determinant in the infectious process generated by these microorganisms. Unlike other genera, Aeromonas species are erroneously differentiated by conventional biochemical tests. Therefore, molecular assays are necessary for this purpose. Nevertheless, new means of identification have been considered in order to generate methods that, like molecular tests, can correctly identify these microorganisms. The main objectives of this review are to explain environmental and structural characteristics of the Aeromonas genus and to discuss virulence mechanisms that these bacteria use to infect aquatic organisms and humans, which are important aspects for aquaculture and public health, respectively. In addition, this review aims to clarify new tests for the precise identification of the species of Aeromonas, contributing to the exact and specific diagnosis of infections by these microorganisms and consequently the treatment. 1. Introduction Composing part of the class Gammaproteobacteria, order Aeromonadales and sharing the family Aeromonadaceae with two other It has been a journey of more than one hundred years structuring genera: Tolumonas and Oceanonimonas [3,5], the genus Aeromonas, the genus Aeromonas within the microbiological universe. It is believed currently with 32 recognized species [6], is constituted by facultative that the first isolates were reported in 1890 [1] and, throughout history, anaerobic, Gram-negative, rod-shaped and non-spore-forming bacteria these bacteria have been classified and reclassified among the most of approximately 1–3 μm[2,7] in length. Moreover, they are oxidase- diverse genera, such as Aerobacter, Pseudomonas, Escherichia, and Pro- positive [8], capable of fermenting glucose [9] and characterized by teus, among others [2]. Although there is a plethora of classifications, tolerating increasing concentrations of NaCl varying from 0.3 to 5% these microorganisms share common characteristics, which include gas [7]. Aeromonas are emerging pathogens capable of colonizing and in- production from glucose and their aquatic distribution [1,2]. The term fecting several hosts [10]. They are inhabitants of marine environ- Aeromonas (from the Greek words “Aer” meaning air or gas and “Mona” ments, so fish and other seafood are the most common sources for meaning units) was firstly proposed in 1936 by Kluyver and van Neil, to isolating these microorganisms. Therefore, they are widely known in embrace gas-producing bacteria [3]. However, it was Stanier who in aquaculture as potentially infectious organisms [11] and can cause 1943 officially used “Aeromonas” to denominate the genus where these diseases such as septicemia and furunculosis [12]. In addition, Aero- bacterial species were added [4]. monas can also be isolated from foods, such as vegetables, dairy ∗ Corresponding author. E-mail address: [email protected] (L.C.B.B. Coelho). https://doi.org/10.1016/j.micpath.2019.02.036 Received 10 July 2018; Received in revised form 27 February 2019; Accepted 28 February 2019 Available online 05 March 2019 0882-4010/ © 2019 Elsevier Ltd. All rights reserved. R.B. Gonçalves Pessoa, et al. Microbial Pathogenesis 130 (2019) 81–94 products, beef and pork [13–15]. In humans, these microorganisms are other bacteria. However, there is the possibility that the colors ex- able to cause, most often, gastrointestinal system infections; however, hibited by bacterial colonies in ADA may vary, depending on the such processes if not treated properly can evolve and reach systemic manufacturer of the medium used, since some authors report that levels, generating septicemia [7]. On the other hand, species of Aero- presumptive Aeromonas colonies were yellow [30] while others report a monas are capable of infecting soft tissues, the hepatobiliary system, white color [31] when sown in ADA. causing ocular, respiratory and joint diseases and even bone infections, Use of selective media for Gram-negative bacteria is also common which is generally associated with a previous case of septicemia with regard to the isolation of this bacterial genus. In MacConkey agar, [16–19]. which is widely used in clinical laboratories, the mesophilic group of Aeromonas species produce a diverse and heterogeneous range of Aeromonas generally grows as a non-lactose fermenter, with A. caviae virulence factors. Expression of membrane components, toxins, en- being an exception, as it is most often a fermenting species [5]. Another zymes and several molecules contribute to bacterial pathogenicity [20] selective medium used is Hektoen Enteric Agar, due to its ability to and act in different ways, such as tissue adhesion, immune response inhibit Gram-positive bacteria. Combination of this medium with evasion, and involvement of host cells [10]. In order to disseminate CHROMagar Salmonella Plus and evaluation by MALDI-TOF has been virulence factors, Aeromonas have four types of secretion systems, re- reported as a reliable and practical alternative for detecting Aeromonas sponsible for release of these cell products into the extracellular en- [32]. vironment or even directly into the host cell [21]. The use of media that mainly differentiate the genus Aeromonas Identification of Aeromonas strains at the species level is still a great from similar genera is of extreme importance, since some may exhibit challenge. Due to the genetic heterogeneity existing in this genus, there similarity in phenotypic and even metabolic characteristics, which can is no effective biochemical evidence for this task [10]. Although new lead to an erroneous isolation and consequently an incorrect classifi- methods have been developed using molecular biology techniques, cation. Pseudomonas, for example, share similar physical aspects and amplification of constitutive genes through polymerase chain reaction oxidase tests [7]; apparently, ampicillin-enriched culture media can is still the best option to effectively identify species belonging to this contribute to differentiation [33]. Isolates were seeded in Glutamate genus [22]. Starch Phenol Red (GSP) agar base for selection of Aeromonas colonies, which showed yellow in color, while Pseudomonas colonies were pink 2. Isolation and identification [34]. Another genus to which these bacteria are sometimes erroneously Correct laboratory identification of the genus Aeromonas and its classified is Vibrio. In order to avoid misunderstanding, two media were components is still a great challenge. Several studies have been un- used: Aerosmart AH medium, which produces a yellow color with dertaken with the aim of making detection practical and reproducible, Aeromonas colonies, and Thiosulfate Citrate Bile Salts Sucrose agar thus increasing the reliability of results. (TCBS), where Vibrio species possess a yellowish aspect [35]. A re- The genus Aeromonas is divided, in relation to growth conditions sistance test to vibriostatic agent 2,4-diamino-6,7-diisopropylpteridine and biochemical characteristics, into two main groups: psychrophilic, (O/129) is also recommended to promote differentiation between these composed of non-motile bacteria with good growth between 22 and two genera, since bacteria belonging to the Vibrionaceae family are 25 °C; and mesophilic, which grow well at 35–37 °C and are motile [3,7] characterized as sensitive [7]. However, this methodology does not owing to a single polar flagellum, for the most part [2]. When grown in provide a totally reliable result, since there are reports of Vibrio species a laboratory, some differences related to the appearance of the colonies resistant to the substance O/129 [36]. can be noted. Aeromonas belonging to the psychrophilic group re- Conventional biochemical tests, as well as automated systems, are of presented mainly by A. salmonicida,