Comparison of phenotypical and genetic identification of Aeromonas strains isolated from diseased fish

Aeromonas dhakensis: A Zoonotic Bacterium of Increasing Importance in Aquaculture

Aeromonas dhakensis is increasingly recognised to be an important pathogen responsible for disease losses in warm-water aquaculture and, similar to several other Aeromonas species, it can infect humans. Knowledge of A. dhakensis is accumulating, but this species remains relatively under-investigated compared to its close relative, Aeromonas hydrophila. The significance of A. dhakensis may have been overlooked in disease events of aquatic animals due to issues with reliable identification. Critical to appreciating the importance of this pathogen is the application of dependable molecular tools that enable accurate identification and discrimination from A. hydrophila and other motile aeromonads. This review aims to synthesise the key literature on A. dhakensis, particularly with relevance to aquaculture, including knowledge of the bacterium derived from disease case studies in aquatic hosts. Identification methods and strain phylogeny are discussed, with accurate detection important for prompt diagnosis and for distinguishing strains with heightened virulence. Increasing evidence suggests that A. dhakensis may be more virulent than A. hydrophila and correct identification is required to determine the zoonotic risks posed, which includes concerns for antibiotic-resistant strains. This review provides an impetus to improve species identification in the future and screen strain collections of presumptive Aeromonas spp. retrospectively to reveal the true prevalence and impact of A. dhakensis in aquaculture, the environment, and healthcare settings.

The role of Aeromonas genotyping in virulence for Dicentrarchus labrax

Aeromonas septicemia still represents a serious challenge facing the global aquaculture sector. In the present study, Aeromonas caviae and A. veronii were isolated from four diseased European seabass (Dicentrarchus labrax) farms experiencing a high mortality rate. Diseased fish showed haemorrhages on the external body surface with exophthalmia, cataracts, scale desquamation, skin ulcers and fin erosions. The most common post-mortem findings were congested internal organs, particularly the liver and posterior kidney. Twenty-eight A. Veronii and 11 A. caviae isolates were identified biochemically by the Vitek 2 system and then confirmed by PCR and phylogenetic analysis. Hemolysin (hlyA) and aerolysin (aer) were the most abundant virulence genes in the recovered isolates, followed by cytotoxic enterotoxin (act) and heat-stable enterotoxin (ast). A. caviae was more virulent than A. veronii for D. labrax fingerlings as LD50 ranging between (>1 × 108 -6.2 × 107 ) for A. veronii and (2.9 × 107 -8.3 × 107 ) for A. caviae. The sensitivity test indicated the effectiveness of norfloxacin, doxycycline and oxytetracycline against the tested isolates. Serum cortisol significantly increased in the infected groups, while catalase and glutathione peroxidase activities significantly decreased at 2 days post-infection (DPI) and then increased at 6 DPI. The presence of virulence genes was associated with bacterial pathogenicity expressed in fish mortality rate. Virulence genes also drastically affect cortisol levels more than catalase and glutathione peroxidase levels.

Polyinfection in Fish Aeromoniasis: A Study of Co-Isolated Aeromonas Species in Aeromonas veronii Outbreaks

We studied the phenotypic and genomic characteristics related to the virulence and antibiotic resistance of two Aeromonas strains, which were co-isolated before an outbreak of Aeromonas veronii among diseased seabass on Agathonisi Island, Greece, in April 2015. The first strain, AG2.13.2, is a potentially pathogenic mesophilic variant of Aeromonas salmonicida, and the second, AG2.13.5, corresponds to an Aeromonas rivipollensis related to A. rivipollensis KN-Mc-11N1 with an ANI value of 97.32%. AG2.13.2 lacks the type III secretion system just like other mesophilic strains of A. salmonicida. This characteristic has been associated with lower virulence. However, the genome of AG2.13.2 contains other important virulence factors such as type II and type VI secretion systems, and toxins such as rtxA, aerolysin aer/act, and different types of hemolysins. The strain also carries several genes associated with antibiotic resistance such as the tetE efflux pump, and exhibits resistance to tetracycline, ampicillin, and oxolinic acid. In an in vivo challenge test with gilthead seabream larvae, the A. veronii bv sobria strain AG5.28.6 exhibited the highest virulence among all tested strains. Conversely, both A. salmonicida and A. rivipollensis showed minimal virulence when administered alone. Interestingly, when A. veronii bv sobria AG5.28.6 was co-administered with A. rivipollensis, the larvae survival probability increased compared to those exposed to A. veronii bv sobria AG5.28.6 alone. This finding indicates an antagonistic interaction between A. veronii bv sobria AG5.28.6 and A. rivipollensis AG2.13.5. The co-administration of A. veronii bv sobria AG5.28.6 with Aeromonas salmonicida did not yield distinct survival probabilities. Our results validate that the primary pathogen responsible for European seabass aeromoniasis is Aeromonas veronii bv sobria.

Isolation, genetic characterization, and virulence profiling of different Aeromonas species recovered from moribund hybrid catfish (Clarias spp.)

Background and Aim

The high diversity of Aeromonas spp. results in various pathogenicity levels. This group of bacteria causes a serious disease named motile Aeromonas septicemia (MAS) in catfish (Clarias spp.). This study aimed to characterize the species and virulence gene diversity of Aeromonas spp. isolated from diseased catfish.

Materials and Methods

Nine Aeromonas spp. were isolated from infected catfish cultivated in Java, Indonesia, and they were identified at the phenotypic and molecular levels (16S rDNA). The virulence genes assessed included aer/haem, alt, ast, flaA, lafA, and fstA.

Results

Phylogenetic analysis identified nine isolates of Aeromonas spp.: Aeromonas hydrophila (11.11%), Aeromonas caviae (11.11%), Aeromonas veronii bv. veronii (44.44%), and Aeromonas dhakensis (33.33%). Virulence genes, such as aer/haem, alt, ast, flaA, lafA, and fstA, were detected in all isolates at frequencies of approximately 100%, 66.67%, 88.89%, 100%, 55.56%, and 66.67%, respectively. This study is the first report on A. dhakensis recovered from an Indonesian catfish culture. Furthermore, our study revealed the presence of A. veronii bv veronii, a biovar that has not been reported before in Indonesia.

Conclusion

This finding confirms that MAS was caused by multiple species of Aeromonas, notably A. dhakensis and A. veronii bv veronii, within Indonesian fish culture. The presence of these Aeromonas species with multiple virulence genes poses a significant threat to the freshwater aquaculture industry.

Whole spectrum of Aeromonas hydrophila virulence determinants and the identification of novel SNPs using comparative pathogenomics

Aeromonas hydrophila is a ubiquitous fish pathogen and an opportunistic human pathogen. It is mostly found in aquatic habitats, but it has also been isolated from food and bottled mineral waters. It causes hemorrhagic septicemia, ulcerative disease, and motile Aeromonas septicemia (MAS) in fish and other aquatic animals. Moreover, it might cause gastroenteritis, wound infections, and septicemia in humans. Different variables influence A. hydrophila virulence, including the virulence genes expressed, host susceptibility, and environmental stresses. The identification of virulence factors for a bacterial pathogen will help in the development of preventive and control measures. 95 Aeromonas spp. genomes were examined in the current study, and 53 strains were determined to be valid A. hydrophila. These genomes were examined for pan- and core-genomes using a comparative genomics technique. A. hydrophila has an open pan-genome with 18,306 total genes and 1620 genes in its core-genome. In the pan-genome, 312 virulence genes have been detected. The effector delivery system category had the largest number of virulence genes (87), followed by immunological modulation and motility genes (69 and 46, respectively). This provides new insight into the pathogenicity of A. hydrophila. In the pan-genome, a few distinctive single-nucleotide polymorphisms (SNPs) have been identified in four genes, namely: D-glycero-beta-D-manno-heptose-1,7-bisphosphate 7-phosphatase, chemoreceptor glutamine deamidase, Spermidine N (1)-acetyltransferase, and maleylpyruvate isomerase, which are present in all A. hydrophila genomes, which make them molecular marker candidates for precise identification of A. hydrophila. Therefore, for precise diagnostic and discrimination results, we suggest these genes be considered when designing primers and probes for sequencing, multiplex-PCR, or real-time PCR.

Critical roles of VipB protein on virulence and oxidative stress tolerance in Aeromonas veronii

Aeromonas veronii is a zoonotic pathogen capable of causing sepsis and ulceration in freshwater fish. Recently, reports of numerous cases indicate a marked increase in pathogenicity. Nonetheless, little is known about the pathogenesis of A. veronii infections. In this study, an in-frame mutant of the A. veronii vipB gene was generated to investigate its biological function. Deletion of the vipB gene resulted in a significant 204.71-fold decrease in the LD₅₀ of A. veronii against zebrafish and a 2-fold and 4-fold reduction in the toxicity to EPC cells at 1 h and 2 h of infection, respectively. The virulence-related genes of the mutant ΔvipB all showed significantly reduced expression levels compared to the wild strain. In addition, the motility of the mutant ΔvipB decreased significantly, the adhesion ability to EPC cells was 3.25-fold lower than that of the parental strain, and the oxidative stress tolerance was 2.31-fold lower than that of TH0426 strain. In contrast, the biofilm formation amount of ΔvipB strain increased by 1.65-fold at both 12 h and 24 h. Our findings suggest that the vipB gene is associated with flagella stability, virulence, and oxidative stress tolerance and plays critical roles in the pathogenesis of A. veronii infections.

Occurrence of Aeromonas Species in the Cutaneous Mucus of Barbour’s Seahorses (Hippocampus barbouri) as Revealed by High-Throughput Sequencing

Although several studies have described the bacterial community composition associated with marine fish, there is limited information related to seahorses. Moreover, previous studies have demonstrated that the skin microbiota is useful for determining health status and common disorders in the host. This study, therefore, aimed to explore the skin bacterial community composition in Barbour’s seahorse (Hippocampus barbouri) using high-throughput sequencing of 16S ribosomal RNA genes. Water and sediment samples from the surrounding environment were also analyzed for comparative purposes. The results revealed that sequences affiliated with the Shewanellaceae family were dominant in the skin of female Barbour’s seahorses and sediment samples, whereas sequences affiliated with the Bacillaceae family were dominant in the skin of male Barbour’s seahorses. Interestingly, sequences affiliated with the Aeromonas genus were found in the skin of Barbour’s seahorses, whose abundance was slightly similar between the female and male specimens. Further comparative analysis showed that the presence of Aeromonas species in the skin of Barbour’s seahorses was strongly influenced by the surrounding sediment. Given that some Aeromonas species are known to be important pathogens in humans and fish, these results may be used for further research on the dependency of the skin microbial composition on the environment as well as determine whether the presence of Aeromonas and other detected species has implications on seahorse health.

Requiring Reconsideration of Differences of Aeromonas Infections Between Extra-Intestinal and Intestinal in Hospitalized Patients

Purpose

The purpose of this study is to examine the variations between extra-intestinal and intestinal infections of Aeromonas in terms of strain types, risk factors, drug susceptibility results, and the distribution of drug resistance and virulence genes.

Patients and Methods

A total of 188 Aeromonas strains were identified to the species level using housekeeping genes (rpoD, gyrB, and gyrA). The risk factors for Aeromonas extra-intestinal and intestinal infection, as well as mortality, were retrospectively examined in this study. The broth microdilution method was used to investigate the antimicrobial susceptibility profiles. Touchdown polymerase chain reaction (PCR) assays and DNA sequencing were employed to confirm virulence and the presence of drug resistance genes.

Results

The housekeeping genes identified 188 strains into 7 species. Extra-intestinal isolates generally contained A. caviae and A. hydrophila, while intestinal were A. veronii (p=0.0001). Extra-intestinal infections (158/188) were the main type and accounted for 24/27 of all fatalities. Malignant tumors, hepatobiliary diseases, anemia, and hypoproteinemia were linked to infections. Poor results were associated with septic shock. Using the broth microdilution method, over 80% isolates were susceptible to most antimicrobials, except for ceftazidime (79.8%) and ceftriaxone (69.7%). Except for imipenem, intestinal strains were more susceptible to other medications than extra-intestinal. Using touch-down polymerase chain reaction testing and DNA sequencing, 6 strains, 31 strains, and a strain only had bla _TEM, bla _CphA, and bla _VIM, respectively. Two Aeromonas hydrophila each possessed bla _CphA+ bla _CTXM-M-9, and bla _CphA + bla _CTX-M-1 + bla _{CTX-M-15-like} + bla _TEM; two Aeromonas caviae each possessed bla _NDM + bla _CTX-M-1 +bla _{CTX-M-15-like} + bla _TEM, and bla _NDM + bla _TEM. Thirty-four of the 42 strains mentioned above were isolated from extra-intestinal. Act, aexT, and ascF-G, were in intestinal more frequently, but alt, hlyA, ela, and lip were in extra-intestinal more frequently.

Conclusion

Aeromonas inside and outside intestinal differed in their clinical characteristics, drug susceptibility, drug resistance and virulence genes.

Accurate Identification and Virulence Detection of Aeromonas: a Single-Center Retrospective Study on the Clinical Features and Outcomes Associated with Aeromonas Bacteremia in Southwestern China

In this study, Aeromonas spp. were re-identified, and the clinical aspects associated with Aeromonas bacteremia, as well as drug resistance and virulence genes, were elucidated. A total of 188 isolates were classified into 7 Aeromonas spp. using housekeeping gene sequencing, which was the standard to assess the accuracy of the VITEK MALDI-TOF system and the VITEK2 Compact system. The VITEK MS system and housekeeping gene sequencing had a 39.89% clear coincidence rate, whereas the VITEK2 Compact system and the standard had a 2.13% coincidence rate. Aeromonas bacteremia was associated with septic shock, hematologic malignancy, and post-hepatitic cirrhosis. Hematological malignancy, hypoproteinemia, systemic steroid use, central venous catheterization, and virulence genes act and ast were linked to poor outcomes. Aeromonas bacteremia had a 37.5% mortality rate; however, differences in mortality rates among Aeromonas spp. were observed. According to the broth microdilution method, over 90% of isolates were sensitive to most antimicrobials, except ceftriaxone (83.33%) and imipenem (83.33%). Polymerase chain reaction and DNA sequencing verified the presence of drug resistance genes; bla_CphA was detected in 3 isolates, while bla_NDM-1 was found in one isolate. In summary, common methods for identifying Aeromonas spp. are ineffective. Immunocompromised patients have a higher risk of infection and mortality. Furthermore, carbapenem resistance is a serious problem.

First Record of the Rare Species Aeromonas lusitana from Rainbow Trout (Oncorhynchus mykiss, Walbaum): Comparative Analysis with the Existing Strains

The species Aeromonas lusitana was first described in 2016 with five strains recovered from untreated water and vegetables from Portugal. Since then, no further records exist of this species. During a surveillance study on the presence of Aeromonas in fish farms in Mexico, a new strain (ESV-351) of the mentioned species isolated from a rainbow trout was recovered. It was identified because it clustered phylogenetically with the type strain of A. lusitana based on the analysis of the rpoD gene sequences. In the present study, phenotypic characteristics, antimicrobial resistance profiles, and the presence of putative virulence genes of this novel strain (ESV-351) were determined in parallel to the five isolates from the original species description. Phenotypic differential characteristics exhibited by A. lusitana ESV-351 depicted an evident similarity to the characteristics exhibited by the other evaluated strains. However, the novel strain was positive for the production of indole using conventional methods, while the rest of the strains, including the type strain, were negative for its production. Furthermore, intermediate resistance to ampicillin, amoxicillin-clavulanic acid and cephalothin was detected in both the novel and the type strain. Five different virulence-related genes were detected in the novel strain and in the previously described strains, with the type strain exhibiting the highest number of virulence-related genes. In addition to this, the genome of the novel strain (ESV-351) was sequenced and compared with the genomes from the type strain (A. lusitana CECT 7828T) and other Aeromonas spp. The genomic analysis defined Aeromonas tecta as the closest species to A. lusitana with a highly similar number of predicted proteins. The genomic size, the number of protein-encoding genes and the number of different tRNAs, among other characteristics, make it possible to propose that the ESV-351 strain could potentially have the capacity to adapt to different environments. Genome comparison of the ESV-351 strain with the type strain revealed that both possess a similar sequence of the citrate synthase gene. In addition to this finding, the chromosomal region containing the citrate synthase locus of the novel strain exhibits some similarity to the chromosomal region in the genome of the A. hydrophila type strain and other known human pathogens, such as Vibrio cholerae. This could suggest a possible virulence role for the citrate synthase gene in A. lusitana (ESV-351).

Effects of Jerusalem Artichoke (Helianthus tuberosus) as a Prebiotic Supplement in the Diet of Red Tilapia (Oreochromis spp.)

The aim of the present study was to evaluate the effects of a Jerusalem artichoke-supplemented diet on the blood chemistry, growth performance, intestinal morphology, expression of antioxidant-related genes, and disease resistance against Aeromonas veronii challenge in juvenile red tilapia. A completely randomized design (CRD) was followed to feed red tilapias with three experimental diets: control, 5.0 g/kg JA-supplemented (JA5), or 10.0 g/kg JA-supplemented (JA10) diets in triplicates for 4 weeks. The results revealed that the growth performance, weight gain (WG), specific growth rate (SGR), and average daily gain (ADG) of fish fed diets JA5 and JA10 were significantly higher (p < 0.05) than those of fish fed the control diet. Fish fed the control diet had significantly higher T-bilirubin, D-bilirubin, and ALT in blood serum than fish fed JA5 and JA10, as well as higher BUN than fish fed JA5. The number of goblet cells in the proximal and distal parts of the intestine revealed that the number of acid, neutral, and double-staining mucous cells of fish fed diets JA5 and JA10 was significantly higher (p < 0.05) than in fish fed the control diet. The diets including the prebiotic (JA5 and JA10) were associated with a significant increase in the expression of gpx1 and gst antioxidant-related genes and disease resistance against A. veronii in juvenile red tilapia. Therefore, JA5 and JA10 can be employed as promising prebiotics for sustainable red tilapia farming.

Relative expression and validation of Aeromonas salmonicida subsp. salmonicida reference genes during ex vivo and in vivo fish infection

The genus Aeromonas is found worldwide in freshwater and marine environments and has been implicated in the etiology of human and animal diseases. In fish, among Aeromonas species, A. salmonicida causes massive mortality and great economic losses in marine and continental aquaculture species. Currently, several aspects of the clinical signs and pathogenesis of this Gram-negative bacterium have been described; however, determination of an appropriate reference gene is essential to normalize cellular mRNA data remain unknown. Here we evaluate the stability of seven candidate reference genes to be used for data normalization during ex vivo and in vivo experiments conducted in Atlantic cod, Atlantic salmon, and lumpfish. To assess this, raw Ct values obtained were evaluated by using geNorm, NormFinder, BestKeeper, Delta Ct comparison, and the comprehensive ranking, through the bioinformatic open-access portal RefFinder. We determined that fabD and era were most suitable reference genes in Atlantic cod primary macrophages, hfq and era in Atlantic salmon primary macrophages, rpoB and fabD in lumpfish head kidney samples, and hfq and era in lumpfish spleen. Our study demonstrates that use of multiple reference genes and its validation before measurements helps to minimize variability arising in qPCR studies that evaluate A. salmonicida gene expression in fish tissues. Overall, this study provided with an expanded list of reliable reference genes for A. salmonicida gene expression using qPCR during fish infection studies.

Diversity and antimicrobial susceptibility profiles of Aeromonas spp. isolated from diseased freshwater fishes in Thailand

Motile Aeromonas septicemia (MAS), a disease caused by Aeromonas spp., is recognized as a major disease in freshwater aquaculture. This study aimed to investigate the distribution and diversity of Aeromonas spp. and their antimicrobial susceptibility patterns. A total of 86 isolates of Aeromonas spp. were recovered from diseased freshwater fishes from 13 farms in Thailand. All isolates were identified using biochemical characteristics, matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS), polymerase chain reaction assays, and the gyrB gene sequence analysis. The result of MALDI-TOF MS showed 100% (86 isolates) accuracy at genus-level identification, and 88.4% (76 isolates) accuracy at species-level identification. Six species of Aeromonas were confirmed through nucleotide sequencing and phylogenetic analysis of the gyrB gene Aeromonas veronii (72.1%), Aeromonas jandaei (11.6%), Aeromonas schubertii (9.3%), Aeromonas diversa (3.5%), Aeromonas hydrophila (2.3%), and Aeromonas punctata (1.2%). Antimicrobial susceptibility tests for all isolates revealed resistance against amoxicillin (99%), ampicillin (98%), oxolinic acid (81.4%), oxytetracycline (77%), trimethoprim-sulfamethoxazole (24%), and enrofloxacin (21%). The multiple antibiotic resistance (MAR) index varied between 0.14 and 0.86, with MAR values more than 0.2 in 99% of isolates. Furthermore, four diverse multidrug-resistant (MDR) patterns were found among Aeromonas isolates. Our finding show that A. veronii is the most abundant species in Thai cultured freshwater fish with the highest MDR patterns.

Epidemiological Cutoff Values for Standard Broth Microdilution and Disk Diffusion Susceptibility Testing of Aeromonas hydrophila Isolated from Fish

Aeromonas hydrophila and other closely related Aeromonas species cause motile aeromonad septicemia, a common fish disease. The disease affects many aquaculture sectors potentially requiring antimicrobial treatments. Therefore, researchers and laboratory diagnosticians need criteria called epidemiological cutoff values (ECVs) to determine whether a bacterial isolate has developed decreased susceptibility to an antimicrobial. To generate ECVs for this bacterium, we assembled a diverse collection of 245 isolates previously identified as A. hydrophila from fish. Using rpoD sequencing, we confirmed that 97 of the 245 isolates were A. hydrophila. We allocated the isolates among three laboratories and tested their susceptibility against eight antimicrobials using standard Clinical and Laboratory Standards Institute (CLSI) disk diffusion and broth microdilution methods. The resulting frequency distributions were statistically analyzed to determine wild-type cutoff estimates, which, along with scatterplots, were used to estimate potential ECVs. In collaboration with the CLSI, aquaculture working group, we proposed ECVs for six of the eight antimicrobials tested. Subsequently, the CLSI Subcommittee on Veterinary Antimicrobial Susceptibility Testing reviewed our data and approved the ECVs to be added to the 2020 edition of the VET04 performance standards for antimicrobial susceptibility testing of aquatic bacteria.

Structure of the lipopolysaccharide O-antigen of Aeromonas encheleia strain A4 representing the new PGO1 serogroup of aeromonads prevailing in Polish aquaculture

The structure of the O-specific polysaccharide (OPS) from Aeromonas encheleia strain A4 lipopolysaccharide was investigated. A. encheleia strain A4, classified into the new provisional serogroup PGO1 predominating among aeromonads in Polish aquaculture, was isolated from common carp tissues during an outbreak of MAI/MAS disease on a fish farm. The high-molecular-weight OPS fraction liberated from the lipopolysaccharide after mild acid hydrolysis followed by gel-permeation chromatography was studied with chemical methods, mass spectrometry, and one- and two-dimensional ¹H and ¹³C NMR spectroscopy techniques. Inter-residue correlations were identified in ¹H,¹³C-heteronuclear multiple-bond correlation (HMBC) and ¹H,¹H NOESY experiments. It was found that the O-specific polysaccharide of A. encheleia strain A4 consists of branched pentasaccharide repeating units with the following structure:→2)[α-d-Fucp3NRHb-(1→3)]-α-l-Rhap-(1→3)-β-l-Rhap-(1→4)-α-l-Rhap-(1→3)-β-d-QuipNAc-(1→.

Plasmid-Mediated Fluoroquinolone Resistance Genes in Quinolone-Susceptible Aeromonas spp. Phenotypes Isolated From Recreational Surface Freshwater Reservoir

Aeromonas spp. are recognized as opportunistic pathogens causing diseases. Infections in humans can result mainly in gastrointestinal and wound diseases with or without progression to septicemia. Although Aeromonas spp. are not known uropathogens and they rarely cause urinary tract infection, we hypothesize that the presence of these bacteria in the water and the contact during, e.g., recreational and bathing activity can create the conditions for the colonization of the human body and may result to diseases in various locations, including the urinary tract. Our study presents the occurrence of aeromonad fluoroquinolone-susceptible phenotypes with the presence of plasmid-mediated fluoroquinolone resistance (PMQR) genes in a natural freshwater reservoir occasionally used for recreational activities. Sixty-nine isolates collected during the bathing period were identified by mass spectrometry and screened for the presence of fluoroquinolone-resistant phenotypes and genotypes. Fluoroquinolone susceptibility was determined as minimal inhibitory concentration values. PMQR qnr genes were detected by PCR. Isolates comprising eight species, namely, mainly Aeromonas veronii (50.7% isolates) and Aeromonas media (24.6% isolates) and rarely Aeromonas eucrenophila, Aeromonas caviae, Aeromonas bestiarum, Aeromonas ichthiosmia, and Aeromonas hydrophila, were selected. All isolates were phenotypically susceptible either to ciprofloxacin or levofloxacin. Unexpectedly, at least one to three of the PMQR genes were detected in 42.0% of the fluoroquinolone-susceptible Aeromonas spp. phenotypes. Mainly the qnrS (34.8% isolates) and qnrA (14.5% isolates) determinants were detected. In conclusion, the freshwater reservoir occasionally used for bathing was tainted with aeromonads, with a high occurrence of opportunistic pathogens such as A. veronii and A. media. MALDI‐TOF MS is a powerful technique for aeromonad identification. Our data reveals the mismatch phenomenon between fluoroquinolone-susceptible aeromonad phenotypes and the presence of plasmid-mediated qnr resistance genes. It suggests that phenotypically susceptible bacteria might be a potential source for the storage and transmission of these genes. The exposure during, e.g., a recreational activity may create the potential risk for causing infections, both diagnostically and therapeutically difficult, after expressing the resistance genes and quinolone-resistant strain selection.

Isolation, identification and virulence gene characterization of Aeromona dhakensis isolated from sea lion (Zalophus californianus)

Species of Aeromonas are ubiquitous pathogens of fish and aquatic animals and can infect humans and other animals through the food chain. However, there are few reports of marine mammalian infections. In 2020, a sea lion (Zalophus californianus) died acutely at an aquarium in Harbin, Heilongjiang Province, China. In order to explore the cause of death, we dissected the animal and observed pathological changes. Ogans were aseptically collected and used for bacterial isolation and culture. This revealed that the sea lion had died of sepsis caused by a bacterial infection. Isolated bacteria were investigated by morphology, biochemical phenotype and molecular identification, and this determined the pathogen as A. dhakensis. The isolate contained six virulence genes, hlyA, aerA, act, lafA, ela, fla, and was susceptible to most antibiotics. This is the first report of A. dhakensis associated with septicemia in pinnipeds and a description of its virulence and resistance profiles. Its presence in aquatic environments poses a potential threat to marine mammals.

Assessment of animal diseases caused by bacteria resistant to antimicrobials: kept fish species

In this Opinion, the antimicrobial-resistant bacteria responsible for transmissible diseases that constitute a threat to the health of certain kept fish species have been assessed. Atlantic salmon (Salmo salar), carp (Cyprinus spp.), rainbow trout (Oncorhynchus mykiss), sea bream (Sparus aurata) and tilapia (Oreochromis spp.), selected as representative of the most important fish species and production systems that are commercially reared in fresh and saltwater farms, were the focus of this assessment. The assessment was performed following a methodology based on information collected by an extensive literature review and expert judgement. Details of the methodology used for this assessment are explained in a separate Opinion. The global state of play of antimicrobial resistance in Aeromonas hydrophila, Aeromonas salmonicida, Flavobacterium psychrophilum and Flavobacterium columnare is provided. Among these bacteria, none was identified as being among the most relevant antimicrobial-resistant bacteria in the assessed kept fish species in the EU due to the very limited scientific evidence available.

Surface Glucan Structures in Aeromonas spp

Aeromonas spp. are generally found in aquatic environments, although they have also been isolated from both fresh and processed food. These Gram-negative, rod-shaped bacteria are mostly infective to poikilothermic animals, although they are also considered opportunistic pathogens of both aquatic and terrestrial homeotherms, and some species have been associated with gastrointestinal and extraintestinal septicemic infections in humans. Among the different pathogenic factors associated with virulence, several cell-surface glucans have been shown to contribute to colonization and survival of Aeromonas pathogenic strains, in different hosts. Lipopolysaccharide (LPS), capsule and α-glucan structures, for instance, have been shown to play important roles in bacterial–host interactions related to pathogenesis, such as adherence, biofilm formation, or immune evasion. In addition, glycosylation of both polar and lateral flagella has been shown to be mandatory for flagella production and motility in different Aeromonas strains, and has also been associated with increased bacterial adhesion, biofilm formation, and induction of the host proinflammatory response. The main aspects of these structures are covered in this review.

Genetic diversity and phenotypic characterization of Iodobacter limnosediminis associated with skin lesions in freshwater fish

The relatively unknown genus Iodobacter sp. has been repeatedly isolated from skin ulcers and saprolegniosis on freshwater fish in Finland, especially farmed salmonids. Genetic characterization verified that all 23 bacterial isolates studied here belonged to the species Iodobacter limnosediminis, previously undescribed from the fish microbiota. Whole-genome pulsed-field gel electrophoresis revealed variability between the I. limnosediminis strains, suggesting that they were most likely of environmental origin. Two I. limnosediminis strains caused lesions in 27%-53% of brown trout (Salmo trutta) injected intramuscularly (p ≤ .05). The lesions represented moderate to severe tissue damage, but for most fish, the tissues had been repaired by the end of the experiment through the accumulation of fibrocytes and macrophages at the site of the lesion. I. limnosediminis was reisolated from some lesions and/or internal organs. Phenotypically and biochemically, I. limnosediminis resembles several common bacterial species found in the aquatic environment, as it grows well on several media as whitish medium-sized colonies, is Gram negative and rod-shaped. Here, we characterized I. limnosediminis strains with several methods, including MALDI-TOF. This characterization will help in further investigations into the occurrence and possible involvement of I. limnosediminis in skin lesions of freshwater fish.

Characterization and Antimicrobial Resistance of Environmental and Clinical Aeromonas Species Isolated from Fresh Water Ornamental Fish and Associated Farming Environment in Sri Lanka

The aims of this study were to characterize and investigate antimicrobial susceptibility and presence of integrons in 161 Aeromonas spp. isolated from ornamental freshwater fish farming environment, apparently healthy and diseased fish. Phylogenetic analyses of the gyrB gene sequences identified Aeromonas veronii as the most abundant species (75.8%) followed by Aeromonashydrophila (9.3%), Aeromonas caviae (5%), Aeromonas jandaei (4.3%), Aeromonas dhakensis (3.7%), Aeromonas sobria (0.6%), Aeromonas media (0.6%), and Aeromonas popoffii (0.6%). Susceptibility to thirteen antimicrobials was determined and antimicrobial resistance frequencies were: amoxicillin (92.5%), enrofloxacin (67.1%), nalidixic acid (63.4%), erythromycin (26.1%), tetracycline (23.6%), imipenem (18%), trimethoprim-sulfamethoxazole (16.8%), and gentamicin (16.8%). Multi-drug resistance (MDR) was widespread among the isolates (51.6%, 83/161) with 51.6% (63/122) A. veronii isolates being MDR. In addition, 68.3% of isolates had multiple antibiotic resistance (MAR) indexes higher than 0.2, suggesting that they originated from a high-risk source of contamination where antimicrobials are often used. In all, 21.7% isolates carried class 1 integrons, with 97.1% having gene cassettes, while there were 12 isolates carrying class 2 integron gene cassettes. Our findings highlight that the aquatic environment and ornamental fish act as reservoirs of multidrug resistant Aeromonas spp. and underline the need for a judicious use of antimicrobials and timely surveillance of antimicrobial resistance (AMR) in aquaculture.

Molecular Epidemiology, Virulence Traits and Antimicrobial Resistance Signatures of Aeromonas spp. in the Critically Endangered Iberochondrostoma lusitanicum Follow Geographical and Seasonal Patterns

Despite the fact that freshwater fish populations are experiencing severe declines worldwide, our knowledge on the interaction between endangered populations and pathogenic agents remains scarce. In this study, we investigated the prevalence and structure of Aeromonas communities isolated from the critically endangered Iberochondrostoma lusitanicum, a model species for threatened Iberian leuciscids, as well as health parameters in this species. Additionally, we evaluated the virulence profiles, antimicrobial resistance signatures and genomic relationships of the Aeromonas isolates. Lesion prevalence, extension and body condition were deeply affected by location and seasonality, with poorer performances in the dry season. Aeromonas composition shifted among seasons and was also different across river streams. The pathogenic potential of the isolates significantly increased during the dry season. Additionally, isolates displaying clinically relevant antimicrobial resistance phenotypes (carbapenem and fluroquinolone resistance) were detected. As it inhabits intermittent rivers, often reduced to disconnected pools during the summer, the dry season is a critical period for I. lusitanicum, with lower general health status and a higher potential of infection by Aeromonas spp. Habitat quality seems a determining factor on the sustainable development of this fish species. Also, these individuals act as reservoirs of important antimicrobial resistant bacteria with potential implications for public health.

Aeromonas species from non-chlorinated distribution systems and their competitive planktonic growth in drinking water

Aeromonas is included in the Dutch Drinking Water Decree as an indicator for elevated microbial regrowth in non-chlorinated drinking water distribution systems (DWDS). The temporal and spatial diversity of Aeromonas species in ten DWDS and their planktonic growth characteristics for different carbon sources was investigated. Genotyping of the gyrB gene of isolates showed a non-systematic temporal and spatial variable prevalence of seven different Aeromonas species in these DWDS and no correlation with AOC-P17/NOX and Aeromonas concentrations. Pure cultures of these seven species showed a high affinity to low concentrations (μg/L) of individual amino acids and fatty acids, compounds associated with biomass. Growth occurred at 0.5 μg-C/L of an amino acid mixture. Growth of a mixed community of A. rivuli, A. salmonicida, A. sobria and A. veronii in drinking water occurred in pasteurized samples, however, no growth and decay occurred in competition with the autochthonous bacteria (non-pasteurized samples). This community also failed to grow in non-pasteurized distribution samples from a location with clear increase in planktonic Aeromonas concentrations in the transported drinking water. For competitive planktonic growth of Aeromonas an amino acid concentration of ≥5 μg-C/L is required. AOC-P17/NOX concentrations showed that such concentrations are not expected in Dutch drinking water. Therefore, we suspect that competitive planktonic growth is not the major cause of the observed non-compliance with the Aeromonas standard in non-chlorinated DWSD.Importance The occurrence of the bacterial genus Aeromonas in non-chlorinated drinking water in the Netherlands is regarded as an indication for elevated microbial regrowth in the distribution system. Identification of the prevalent species in ten distribution systems by genotyping yielded seven different species, with A. rivuli, A. veronii and A. sobria as the most dominant ones. Planktonic growth experiments of pure cultures confirmed former published affinity of Aeromonas for certain biomass compounds (amino and fatty acids). In competition with the autochthonous microflora, however, planktonic growth was not observed, only after addition of a threshold amino acid concentration of 5 μg-C/L. Based on our results and further observations we deduced that planktonic growth of Aeromonas in the DWDS is not very likely. Benthic growth in loose deposits and planktonic release is a more plausible explanation for the observed planktonic increase of Aeromonas.

Comparison of the Multiple Platforms to Identify Various Aeromonas Species

We compared several identification methods for Aeromonas genus members, including traditional biochemical testing, multiplex-PCR amplification, mass spectrometry identification, whole-genome sequencing, multilocus phylogenetic analysis (MLPA), and rpoD, gyrA, and rpoD-gyrA gene sequencing. Isolates (n = 62) belonging to the Aeromonas genus, which were came from the bacterial bank in the laboratory, were used to assess the identification accuracy of the different methods. Whole-genome sequencing showed that the Aeromonas spp. isolates comprised A. caviae (n = 21), A. veronii (n = 18), A. dhakensis (n = 8), A. hydrophila (n = 7), A. jandaei (n = 5), A. enteropelogenes (n = 2), and A. media (n = 1). Using the whole-genome sequencing results as the standard, the consistency of the other methods was compared with them. The results were 46.77% (29/62) for biochemical identification, 83.87% (52/62) for mass spectrometric identification, 67.74% (42/62) for multiplex-PCR, 100% (62/62) for MLPA typing, 72.58% for gyrA, and 59.68% for rpoD and gyrA-rpoD. MLPA was the most consistent, followed by mass spectrometry. Therefore, in the public health laboratory, both MLPA and whole-genome sequencing methods can be used to identify various Aeromonas species. However, rapid and relatively accurate mass spectrometry is recommended for clinical lab.

Antimicrobial resistance in Aeromonas species isolated from aquatic environments in Brazil

AIM

The current study was conducted to determine the antimicrobial resistance profile and genetic relatedness of Aeromonas sp. isolated from healthcare and urban effluents, wastewater treatment plant (WWTP) and river water.

METHODS AND RESULTS

We detected the presence of genes conferring resistance to β-lactam, quinolone and aminoglycoside. Multilocus sequence typing was carried out to differentiate the strains, and multilocus phylogenetic analysis was used to identify the species. A total of 28 cefotaxime-resistant Aeromonas sp. strains were identified, harbouring uncommon Guiana-extended-spectrum (GES)-type β-lactamases (GES-1, GES-5, GES-7 and GES-16). Multidrug-resistant Aeromonas sp. were found in hospital wastewater, WWTP and sanitary effluent, and A. caviae was identified as the most prevalent species (85·7%).

CONCLUSION

The release of untreated healthcare effluents, presence of antimicrobials in the environment, in addition to multidrug-resistant Aeromonas sp., are all potential factors for the spread of resistance.

SIGNIFICANCE AND IMPACT OF THE STUDY

We identified a vast repertoire of antimicrobial resistance genes (ARG) in Aeromonas sp. from diverse aquatic ecosystems, including those that encode enzymes degrading broad-spectrum antimicrobials widely used to treat healthcare-associated infections. Hospital and sanitary effluents serve as potential sources of bacteria harbouring ARG and are a threat to public health.

Bacillus subtilis H2 modulates immune response, fat metabolism and bacterial flora in the gut of grass carp (Ctenopharyngodon idellus)

Functional ingredients such as Bacillus subtilis are used in aquaculture to improve fish condition, modulate microbiota and promote a healthy intestinal system. However, the underlying mechanisms of grass carp treated with B. subtilis are not fully characterized. This study investigated the gut microbes of grass carp after treated with B. subtilis H2 (10⁶ CFU/mL) and Aeromonas hydrophila (10⁶ CFU/mL). The intestinal flora was found that the dominant bacterial phyla identified in all samples were Proteobacteria, Actinobacteria, Fusobacteria, Bacteroidetes and Acidobacteria. Compared with the control group, the relative abundance of Proteobacteria and Bacteroidetes in B. subtilis group were significantly increased. In addition, the relative abundances of Aeromonas and Shewanella in A. hydrophila group were more than the control group. For the intestinal transcriptomic profiling of the grass carp treated with B. subtilis H2, 824 different expressed genes (DEGs) between the B. subtilis H2 treated and non-treated groups were detected, including 365 up-regulated and 459 down-regulated genes. Six DEGs were randomly selected for further validation by quantitative real-time RT-PCR (qRT-PCR) and the results were consistent with the RNA-seq data. Additionally, eight immunomodulatory genes (IL-4, IL-11, IFN-α, CSF, FOSB, MAPK12b, IGHV3-11 and IGHV3-21) were significantly up-regulated after treated with B. subtilis H2. Furthermore, almost all the lipid metabolism-associated genes were significantly up-regulated after treated with B. subtilis H2 according to the lipid metabolism pathways. Eleven lipid metabolism-associated genes were selected by qRT-PCR, which showed that the expressions of almost all the selected genes were increased, especially Apob-48, ABCG8 and DGAT. Taken together, our results support that B. subtilis could modulate the immune response, fat metabolism and bacterial assembly in the gut of grass carp.

Distribution and Potential Effects of 17β-Estradiol (E2) on <i>Aeromonas </i>Diversity in Wastewater and Fish Samples

BACKGROUND AND OBJECTIVE

Recently, there has been evidence for the accumulation of steroid hormones in the water environment with negative consequences on fish and humans. However, there is paucity of information on how the steroid hormones influence the microbial community in environmental waters. The objective of this study was to determine the occurrence of 17β-estradiol (E2) and its potential influence on the diversity of Aeromonas spp.

MATERIALS AND METHODS

Wastewater samples were obtained from sewage treatment plants in northern South Africa and fish samples were collected from the Nandoni dam. Aeromonas spp. were isolated using microbiological methods and PCR protocols were used for their identification. A commercial Elisa kit was used for measuring the concentration of 17β-estradiol (E2) from the wastewater samples as well as the fish samples.

RESULTS

17β-estradiol (E2) was found in high concentration in sewage samples varying from 0.32-348.6 pg mL-1 while in fish samples, it ranged from 1.1-73.6 pg mL-1. There was a tendency of samples with high E2 concentrations to have higher diversity of Aeromonas spp., implying that steroid hormones may serve as nutrient for Aeromonas spp. Aeromonas hydrophila was the most prevalent species (71%), followed by A. sobria with (68%).

CONCLUSION

The presence of Aeromonas spp. in environmental waters and fish that is consumed by the local community poses a serious health concern. The high content of E2 in treated wastewater is of serious concern as well. For the first time, the present study showed a positive impact of E2 on Aeromonas growth.

Recent progress and challenges on the bioassay of pathogenic bacteria

The present review (containing 242 references) illustrates the importance and application of optical and electrochemical methods as well as their performance improvement using various methods for the detection of pathogenic bacteria. The application of advanced nanomaterials including hyper branched nanopolymers, carbon-based materials and silver, gold and so on. nanoparticles for biosensing of pathogenic bacteria was also investigated. In addition, a summary of the applications of nanoparticle-based electrochemical biosensors for the identification of pathogenic bacteria has been provided and their advantages, detriments and future development capabilities was argued. Therefore, the main focus in the present review is to investigate the role of nanomaterials in the development of biosensors for the detection of pathogenic bacteria. In addition, type of nanoparticles, analytes, methods of detection and injection, sensitivity, matrix and method of tagging are also argued in detail. As a result, we have collected electrochemical and optical biosensors designed to detect pathogenic bacteria, and argued outstanding features, research opportunities, potential and prospects for their development, according to recently published research articles.

Analysis of virulence and immunogenic factors in Aeromonas hydrophila: Towards the development of live vaccines

Aeromonas hydrophila, a bacterium that is widespread in aquatic environments, is responsible for causing haemorrhagic disease in both aquatic and terrestrial species. With the purpose of developing a live vaccine, herein we have investigated nine strains of A. hydrophila (Ah-01 to Ah-09) isolated from diseased fish. A study of virulence factors that contribute to pathogenicity and immunogenicity in the host Cyprinus carpio suggests that the presence of β-hly, act and fla genes contribute to pathogenesis: strains Ah-01, Ah-02 and Ah-03 (β-hly⁺ /act⁺ /fla⁺ genotype) were highly pathogenic to C. carpio, whereas Ah-05 and Ah-06 (β-hly^- /act^- /fla^- genotype) showed weak pathogenicity. Accordingly, Ah-02 and Ah-03 were selected to prepare inactivated vaccines, whereas Ah-05 and Ah-06 were chosen as live vaccines. Ah-06 live vaccine was found to have the best protective efficacy, with a protective rate of about 85%, whereas rates of other vaccines were significantly lower, in the range 37%-59%. In addition, DNA vaccines based on genes altA, aha and omp showed immune protection rates of 25%, 37.5% and 75%, respectively. Our data demonstrate that the β-hly^- /act^- /fla^- /altA⁺ /aha⁺ /omp⁺ genotype has weak pathogenicity and high immunogenicity, and provide a simple and effective way to screen for live A. hydrophila vaccines.

Crystal Structure of Exotoxin A from Aeromonas Pathogenic Species

Aeromonas exotoxin A (AE) is a bacterial virulence factor recently discovered in a clinical case of necrotising fasciitis caused by the flesh-eating Aeromonas hydrophila. Here, database mining shows that AE is present in the genome of several emerging Aeromonas pathogenic species. The X-ray crystal structure of AE was solved at 2.3 Å and presents all the hallmarks common to diphthamide-specific mono-ADP-ribosylating toxins, suggesting AE is a fourth member of this family alongside the diphtheria toxin, Pseudomonas exotoxin A and cholix. Structural homology indicates AE may use a similar mechanism of cytotoxicity that targets eukaryotic elongation factor 2 and thus inhibition of protein synthesis. The structure of AE also highlights unique features including a metal binding site, and a negatively charged cleft that could play a role in interdomain interactions and may affect toxicity. This study raises new opportunities to engineer alternative toxin-based molecules with pharmaceutical potential.

Characterization of Aeromonas salmonicida and A. sobria isolated from cultured salmonid fish in Korea and development of a vaccine against furunculosis

Previously, Aeromonas sobria and A. salmonicida were identified to be the most prevalent species in salmonid farms in Korea. In this study, we evaluated the biochemical characteristics, antibiotic susceptibility and pathogenicity of A. salmonicida (3 isolates) and A. sobria (8 isolates) isolated from salmonids, and further investigated efficacy of A. salmonicida vaccine. In antibiotic susceptibility test, all of A. sobria isolates were resistant to amoxicillin and ampicillin. Six A. sobria and two A. salmonicida isolates were resistant to oxytetracycline. In challenge test, A. sobria isolates exhibited low pathogenicity in rainbow trout (Oncorhynchus mykiss) while one A. salmonicida isolate showed high pathogenicity with LD₅₀ of 6.4 × 10³  CFU/fish in rainbow trout and coho salmon (Oncorhynchus kisutch). Among virulence factors, secretion apparatus (ascV and ascC) and transcription regulatory protein (exsA) of type 3 secretion system and A-layer protein genes were differentially detected in DNA or cDNA of A. salmonicida isolates, indicating their contribution to the pathogenicity. A formalin-killed vaccine of highly pathogenic A. salmonicida isolate exhibited a protective effect with relative survival rate of 81.8% and 82.9% at 8 weeks and 16 weeks post-vaccination, respectively, in challenge test.

Molecular Typing, Antibiogram and PCR-RFLP Based Detection of Aeromonas hydrophila Complex Isolated from Oreochromis niloticus

Motile Aeromonas septicemia is a common bacterial disease that affects Oreochromis niloticus and causes tremendous economic losses globally. In order to investigate the prevalence, molecular typing, antibiogram and the biodiversity of Aeromonas hydrophila complex, a total of 250 tilapia (Oreochromis niloticus) were collected randomly from 10 private tilapia farms (25 fish/farm) at El-Sharkia Governorate, Egypt. The collected fish were subjected to clinical and bacteriological examinations. The majority of infected fish displayed ulcerative necrosis, exophthalmia, and internal signs of hemorrhagic septicemia. The prevalence of A. hydrophia complex was 13.2%, where the liver was the most predominant affected organ (54.1%). Polymerase chain reaction (PCR) was used to verify the identification of A. hydrophila complex using one set of primers targeting gyrB as well as the detection of virulent genes (aerA, alt, and ahp). All isolates were positive for the gyrB-conserved gene and harbored aerA and alt virulence genes. However, none of those isolates were positive for the ahp gene. The antimicrobial sensitivity was carried out, where the recovered strains were completely sensitive to ciprofloxacin and highly resistant to amoxicillin. All retrieved strains showed the same phenotypic characteristics and were identical based on the restriction fragment length polymorphism (RFLP). Experimentally challenged fish presented a high mortality rate (76.67%) and showed typical signs as in naturally infected ones. In conclusion, the synergism of phenotypic and genotypic characterization is a valuable epidemiological tool for the diagnosis of A. hydrophila complex. RFLP is a fundamental tool for monitoring the biodiversity among all retrieved strains of A. hydrophia.

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

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.

A Mesophilic Aeromonas salmonicida Strain Isolated from an Unsuspected Host, the Migratory Bird Pied Avocet

Aeromonas salmonicida is a Gram-negative bacterium, known as a fish pathogen since its discovery. Although the species was initially considered psychrophilic, a mesophilic subspecies (pectinolytica) and many other mesophilic strains still not attributed to subspecies have been described in the last two decades. These mesophilic strains were sampled from various sources, including humans, and some of them are known to be pathogenic. In this study, we describe a strain, JF2480, which was isolated from the spleen, and also found the kidney and liver of a dead pied avocet (Recurvirostra avosetta), a type of migratory bird inhabiting aquatic environments. A core genome phylogenomic analysis suggests that JF2480 is taxonomically distant from other known A.&nbsp;salmonicida subspecies. The genome sequence confirms that the strain possesses key virulence genes that are present in the typical A. salmonicida psychrophilic subspecies, with the exception of the genes encoding the type three secretion system (T3SS). Bacterial virulence assays conducted on the surrogate host Dictyostelium discoideum amoeba confirmed that the strain is virulent despite the lack of T3SS. Bacterial growth curves showed that strain JF2480 grow well at 40 °C, the body temperature of the pied avocet, and even faster at 41 °C, compared to other mesophilic strains. Discovery of this strain further demonstrates the extent of the phylogenomic tree of this species. This study also suggests that A. salmonicida can infect a wider array of hosts than previously suspected and that we need to rethink the way we perceive A. salmonicida's natural environment.

Molecular Identification and Virulence Potential of the Genus Aeromonas Isolated from Wild Rainbow Trout (Oncorhynchus mykiss) in Mexico

The members of the Aeromonas genus are important foodborne pathogens, with a worldwide distribution. Wild rainbow trout, from the national protected area Santuario del Agua State Park, Corral de Piedra, were analyzed. Species of Aeromonas were isolated from the trout, and their pathogenic potential was analyzed based on different pathogenicity and virulence factors. The isolates were identified as A. allosaccharophila (n = 15), A. sobria (n = 8), A. veronii (n = 3), A. rivipollensis (n = 2), A. piscicola (n = 2), and A. popoffii (n = 1), by RNA polymerase sigma factor (rpoD) gene sequencing. Sequence similarity with the type strain was 92.2 to 99.6% for A. sobria isolates, 97.8 to 98.0% for A. allosaccharophila isolates, 99.2% for the A. popoffii isolate, 99.2 to 100% for A. piscicola isolates, and 98.2 to 99.2% for A. veronii isolates. Notably, isolates A30T2-gills and A30T2-spleen showed sequence similarity of 98.0% with strain A. media CECT 4232T and 99.0% with strain A. rivipollensis P2G1T. Virulence genes were detected by PCR at the following frequencies: fla and serine protease, 96.77%; aerA, 93.54%; aexT, 87.09%; lipases, 74.19%; ascV and ahyB, 67.74%; exu, 61.29%; act, 41.93%; ascF-G, 38.70%; lafA, 32.26%; alt, 6.46%; aopP, 9.67%; and ast, 3.23%. These results indicate that several Aeromonas species had the potential pathogenicity to infect wild rainbow trout in the waterway created by the Corral de Piedra dam, suggesting they could be an emerging zoonotic pathogen.

Phenotypic and molecular detection of Aeromonads infection in farmed Nile tilapia in Southern highland and Northern Tanzania

Aeromonads disease outbreaks are now becoming a common phenomenon in freshwater farmed fish worldwide. In Tanzania, the aquaculture field is increasingly growing save to sustain food protein demand and strengthen household income. To avoid losses that tilapia fish farmers might account, information on magnitude of infection and characteristics of the aetiological agent is vital. This study aimed to establish the prevalence of aeromonads infection in farmed tilapia and assess pond and fish health management practices. A cross sectional study was carried out between February 2017 and October 2018 and a total of 816 whole fish samples were aseptically collected from 32 ponds in Ruvuma, Mbeya, Iringa and Kilimanjaro regions. During sampling, water quality parameters were taken and questionnaires to assess the knowledge of farmers were also provided. Isolation and identification of bacteria was conducted using conventional biotyping and molecular techniques. A total of 201 (80.4%) of 250 isolates that were conventionally identified were confirmed to be aeromonads by amplification of 820 bp rpoD gene, making the overall prevalence of 24.6% (201, n = 816). Sequencing of rpoD gene and phylogenetic analysis revealed two aeromonads species, Aeromonas hydrophila and Aeromonas veronii. To the best of our knowledge this is the first report to establish the prevalence of aeromonads in apparently healthy farmed tilapia in Southern highlands and Northern zone of Tanzania. In addition it was observed that farmers were lacking proper knowledge and awareness on pond management practices and fish health management. In conclusion, the infection rate of aeromonads in apparently health tilapia coupled with lack of proper knowledge and awareness on pond and fish health management by fish farmers in the study area poses risk of diseases outbreaks in their farms in future. Therefore, it is recommended that the farmers should be trained on basic pond and fish health management and control strategies.

Virulence determinants in genetically heterogeneous populations of Aeromonads recovered from an urban lagoon

The diversity and distribution of Aeromonas spp. associated with virulence profiles from the Rodrigo de Freitas Lagoon were investigated using phylogenetic analysis of gyrB/rpoB gene sequences for speciation. The concatenated gyrB/rpoB gene sequences clustered into five species: Aeromonas punctata/caviae (n = 37), A. hydrophila (n = 10), A. dhakensis (n = 16), A. jandaei (n = 1) and A. enteropelogenes/trota (n = 3). The virulence genes (atc/aerA/hlyA/asp/amp) resulted in 19 virulence profiles, distributed heterogeneously among the five Aeromonas species. Out of the 67 isolates, 16% presented five distinct profiles carrying four virulence genes and 7% showed all genes investigated. The hemolytic genes were detected as follows: act 54% (37/67), aerA 36% (24/67), hlyA 26% (18/67) and proteolytic genes such as asp 36% (24/57) and amp in 85% (57/67) were widely distributed in lagoon sampling stations. Meanwhile, 88% (59/67) and 92% (62/67) of the isolates showed hemolytic and proteolytic activity, respectively. Our results demonstrated that concatenated sequences of the gyrB and rpoB genes showed to be an adequate approach for the Aeromonas speciation and prevalence. The high heterogeneity of virulence genes among the species resulted in several virulence profiles, as well as high percentages of hemolytic and proteolytic activity, demonstrating the necessity of further epidemiological surveys of Aeromonas species pathogenicity in an aquatic recreational lagoon.

The genus Aeromonas: A general approach

The genus Aeromonas comprises more than thirty Gram-negative bacterial species which mostly act as opportunistic microorganisms. These bacteria are distributed naturally in diverse aquatic ecosystems, where they are easily isolated from animals such as fish and crustaceans. A capacity for adaptation also makes Aeromonas able to colonize terrestrial environments and their inhabitants, so these microorganisms can be identified from different 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 differentially 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.

The determination of the infectious status and prevalence of motile Aeromonas species isolated from disease cases in rainbow trout (Oncorhynchus mykiss) and aquarium fish

The aims of this study were to determine the prevalence and phylogenetic relationship of motile Aeromonas spp. that might be pathogenic species for rainbow trout in infected/mix infection cases (based upon different outbreaks on fish farms). A total of 99 motile Aeromonas isolates (and three reference strains) were analysed that were isolated from four different fish species in different sizes of fish (0.1-3,000 g), different months and water temperatures (6.1-21.2°C). The biochemical characteristics of the isolates were determined using conventional tests and a rapid test kit. Additionally, molecular identification was performed using the gyrB housekeeping gene region and with glycerophospholipid-cholesterol acyltransferase polymerase chain reaction (GCAT-PCR). The sequencing results obtained from the gyrB gene region were deposited in the GenBank database, and phylogenetic relationships were determined with the BioNumerics 7.6 database. Nearly half of the Aeromonas isolates that were isolated from rainbow trout showing signs of disease were determined to be possible infectious agents. Aeromonas species exhibit biochemical variability for many characters, so some Aeromonas species tested negative for GCAT-PCR despite that this test was created especially for Aeromonas identification. The phylogenetic tree based upon gyrB contained 10 different phylogroups that were based on 96% cut-off value in gyrB gene region.

Limited performance of MALDI-TOF for identification of fish Aeromonas isolates at species level

The aim of this study was to evaluate the usefulness of the MALDI-TOF MS to identify 151 isolates of Aeromonas obtained mostly from diseased fish. MALDI-TOF MS correctly identified all isolates to the genus level but important differences in the percentage of isolates correctly identified depending on the species were observed. Considering exclusively the first identification option, Aeromonas bestiarum, Aeromonas hydrophila, Aeromonas salmonicida, Aeromonas veronii and Aeromonas sobria were the best identified with results >95%. However, considering the first and second identification options, the only species that showed values >90% was A. hydrophila. Overall, when the database was supplemented with 14 new spectra, the number of accurate identifications increased (41% vs. 55%) and the number of inconclusive identifications decreased (45% vs. 29%), but great differences in the success of species-level identifications were found. Species-distinctive mass peaks were identified only for A. hydrophila and A. bestiarum (5003 and 7360 m/z in 95.5% and 94.1% of their isolates, respectively). This work demonstrates the utility of MALDI-TOF MS for Aeromonas identification to the genus level, but there is no consistency for the accurate identification of some of the most prevalent species implicated in fish disease.

Aeromonas shuberti as a cause of multi-organ necrosis in internal organs of Nile tilapia, Oreochromis niloticus

A disease with white spots in internal organs of Nile tilapia occurred in Zhanjiang, southern China. Multiple, white nodules, 0.8-2.2 mm in diameter, were scattered throughout the liver, spleen and kidney of diseased fish. Signs of nodules reproduced after artificial infection with the isolated strain. Isolated bacteria were Gram-negative, facultative anaerobic, motile, short rod-shaped, with a length of 1.2-2.2 μm. Morphological and biochemical tests, as well as phylogenetic analysis, all strongly indicated that the isolate from tilapia is identical to Aeromonas schubertii (A. schubertii) which temporary named LF1708 strain. Antibiotic sensitivity assays showed the LF1708 is sensitive to 24 of 27 tested antibiotics. Pathogenicity test revealed that the isolate at the dose of 3.75 × 10⁶ CFU/g killed 100% of experimental tilapia within 2 days and the dose of 1 × 10⁷ CFU/g killed 100% of experimental zebrafish within 1 day. Histopathology of diseased tilapia infected with A. schubertii showed numerous necrotic lesions widely distributed in spleen, liver and kidney, and infiltration with a large number of bacteria. To our knowledge, this was the first report that associated A. schubertii with mortality in tilapia.

Development of a species-specific PCR-RFLP targeting rpoD gene fragment for discrimination of Aeromonas species

PURPOSE

The taxonomy of Aeromonas keeps expanding and their identification remains problematic due to their phenotypic and genotypic heterogeneity. In this study, we aimed to develop a rapid and reliable polymerase chain reaction-restriction fragment length polymorphism assay targeting the rpoD gene to enable the differentiation of aeromonads into 27 distinct species using microfluidic capillary electrophoresis.

METHODOLOGY

A pair of degenerate primers (Aero F: 5'-YGARATCGAYATCGCCAARCGB-3' and Aero R: 5'-GRCCDATGCTCATRCGRCGGTT-3') was designed that amplified the rpoD gene of 27 Aeromonas species. Subsequently, in silico analysis enabled the differentiation of 25 species using the single restriction endonuclease AluI, while 2 species, A. sanarelli and A. taiwanensis, required an additional restriction endonuclease, HpyCH4IV. Twelve type strains (A. hydrophila ATCC7966^T, A. caviae ATCC15468^T, A. veronii ATCC9071^T, A. media DSM4881^T, A. allosaccharophila DSM11576^T, A. dhakensis DSM17689^T, A. enteropelogens DSM7312^T, A. jandaei DSM7311^T, A. rivuli DSM22539^T, A. salmonicida ATCC33658^T, A. taiwanensis DSM24096^T and A. sanarelli DSM24094^T) were randomly selected from the 27 Aeromonas species for experimental validation.Results/key findings. The twelve type strains demonstrated distinctive RFLP patterns and supported the in silico digestion. Subsequently, 60 clinical and environmental strains from our collection, comprising nine Aeromonas species, were used for screening examinations, and the results were in agreement.

CONCLUSION

This method provides an alternative method for laboratory identification, surveillance and epidemiological investigations of clinical and environmental specimens.

Delineation of Taxonomic Species within Complex of Species: Aeromonas media and Related Species as a Test Case

Aeromonas media is an opportunistic pathogen for human and animals mainly found in aquatic habitats and which has been noted for significant genomic and phenotypic heterogeneities. We aimed to better understand the population structure and diversity of strains currently affiliated to A. media and the related species A. rivipollensis. Forty-one strains were included in a population study integrating, multilocus genetics, phylogenetics, comparative genomics, as well as phenotypics, lifestyle, and evolutionary features. Sixteen gene-based multilocus phylogeny delineated three clades. Clades corresponded to different genomic groups or genomospecies defined by phylogenomic metrics ANI (average nucleotide identity) and isDDH (in silico DNA-DNA hybridization) on 14 whole genome sequences. DL-lactate utilization, cefoxitin susceptibility, nucleotide signatures, ribosomal multi-operon diversity, and differences in relative effect of recombination and mutation (i.e., in evolution mode) distinguished the two species Aeromonas media and Aeromonas rivipollensis. The description of these two species was emended accordingly. The genome metrics and comparative genomics suggested that a third clade is a distinct genomospecies. Beside the species delineation, genetic and genomic data analysis provided a more comprehensive knowledge of the cladogenesis determinants at the root and inside A. media species complex among aeromonads. Particular lifestyles and phenotypes as well as major differences in evolution modes may represent putative factors associated with lineage emergence and speciation within the A. media complex. Finally, the integrative and populational approach presented in this study is considered broadly in order to conciliate the delineation of taxonomic species and the population structure in bacterial genera organized in species complexes.

Chlorinated and ultraviolet radiation -treated reclaimed irrigation water is the source of Aeromonas found in vegetables used for human consumption

Wastewater is increasingly being recognized as a key water resource, and reclaimed water (or treated wastewater) is used for irrigating vegetables destined for human consumption. The aim of the present study was to determine the diversity and prevalence of Aeromonas spp. both in reclaimed water used for irrigation and in the three types of vegetables irrigated with that water. Seven of the 11 (63.6%) samples of reclaimed water and all samples of vegetables were positive for the presence of Aeromonas. A total of 216 Aeromonas isolates were genotyped and corresponded to 132 different strains that after identification by sequencing the rpoD gene belonged to 10 different species. The prevalence of the species varied depending on the type of sample. In the secondary treated reclaimed water A. caviae and A. media dominated (91.4%) while A. salmonicida, A. media, A. allosaccharophila and A. popoffii represented 74.0% of the strains in the irrigation water. In vegetables, A. caviae (75.0%) was the most common species, among which a strain isolated from lettuce had the same genotype (ERIC pattern) as a strain recovered from the irrigation water. Furthermore, the same genotype of the species A. sanarellii was recovered from parsley and tomatoes demonstrating that irrigation water was the source of contamination and confirming the risk for public health.

Characterization of a new strain of Aeromonas dhakensis isolated from diseased pacu fish (Piaractus mesopotamicus) in Brazil

This study provides a detailed description and characterization of a strain of Aeromonas dhakensis isolated from a diseased juvenile Piaractus mesopotamicus obtained from the fish farm of the National Center for Continental Fish Research and Conservation (CEPTA/ICMBio), in the state of São Paul, Brazil. Biochemical tests using the VITEK 2 automated bacterial identification system identified the isolate to genus level; however, further molecular analysis of the 16S rRNA, gyrB and rpoD genes showed that the strain belonged to the species A. dhakensis. As expected, the isolated A. dhakensis strain was resistant to ampicillin and ampicillin/sulbactam, as resistance to ampicillin is a typical characteristic of the genus Aeromonas. Resistance to cefoxitin and meropenem was also observed, but the strain was susceptible to most of the tested antibiotics. The isolated strain of A. dhakensis caused acute haemorrhagic septicaemia in experimentally infected P. mesopotamicus, with a fifty per cent lethal dose of 1.14 × 10⁵  CFU/fish. This is the first report of the occurrence of an A. dhakensis strain causing an infection in a fish species of South America, providing important epidemiologic data relating to this important pathogenic species.