Virulence diversity among bacteremic Aeromonas isolates: ex vivo, animal, and clinical evidences

Whole-genome sequencing-based species classification, multilocus sequence typing, and antibiotic resistance mechanisms of the clinical Aeromonas complex

Background

Multidrug-resistant strains of the genus Aeromonas can produce various β-lactamases that confer resistance to a broad spectrum of β-lactams, which poses a significant public health threat due to their emergence and spread in clinical settings and natural environments. Therefore, a comprehensive investigation into the antibiotic resistance mechanisms of Aeromonas is scientifically significant.

Methods

Between 2018 and 2021, 78 clinical Aeromonas isolates were collected from human clinical specimens. The MicroScan WalkAway system and average nucleotide identity (ANI) analyses were used to classify the bacterial species. Antibiotic susceptibility was determined through the minimum inhibitory concentration (MIC) test via the agar dilution method. To determine the resistance mechanism and the structure of the resistance gene-related sequences, molecular cloning, whole-genome sequencing and bioinformatic analysis were performed.

Results

Among the 78 Aeromonas isolates studied in this work, obtained from various specimens from different clinical departments, 77 were classified into seven known species by ANI analysis. Most of the isolates were A. caviae (34.6%, 27/78), followed by A. hydrophila (25.6%, 20/78). Multilocus sequence typing (MLST) revealed that they belonged to 72 sequence types (STs), including 52 new STs. A total of 334 resistance genes of 30 antibiotic resistance genotypes were identified from the genomes, more than half (55.99%, 187/334) of which were β-lactamase genes. The isolates showed much higher rates of resistance to penicillins (penicillin G, 98.7%) and first-generation cephalosporins (cefazolin, 96.2%), but lower resistance rates to fourth-generation cephalosporins (cefepime, 6.4%), monobactams (aztreonam, 5.1%), and carbapenems (imipenem, 1.3% and meropenem, 5.1%). Structural analyses of some β-lactamase genes (such as bla NDM-1 and bla PER-3) related sequences revealed that they were generally associated with mobile genetic elements.

Conclusion

The investigation of the correlation between the distribution of β-lactamase genes and Aeromonas resistance phenotypes in this study suggested an urgent need for rigorous monitoring and control to counteract the escalating public health threat posed by the increase in Aeromonas strains harboring extended-spectrum β-lactamase and metallo-β-lactamase genes.

Age-Dependent Variations in the Distribution of Aeromonas Species in Human Enteric Infections

Aeromonas species are enteropathogens that cause gastroenteritis with a unique three-peak infection pattern related to patient age. The contributions of individual Aeromonas species to age-related infections remain unknown. Multi-locus sequence typing (MLST) was performed to determine the species of Aeromonas strains from Australian patients with gastroenteritis. Public database searches were conducted to collect strains of enteric Aeromonas species, identified by either MLST or whole genome sequencing with known patient age. Violin plot analysis was performed to assess Aeromonas infection distribution across patients of different ages. Generalized additive model (GAM) analysis was employed to investigate the relationship between Aeromonas species and patient age. A total of 266 strains of seven Aeromonas species met the selection criteria, which were used for analyses. The violin plots revealed distinct patterns among individual Aeromonas species in relation to patient age. The GAM analyses identified a significant association between Aeromonas species and patient age (p = 0.009). Aeromonas veronii (153 strains) showed the highest probability of infection in most ages, particularly among young adults. Aeromonas caviae (59 strains) is more common in young children and adults over 60 years of age. The probability of infection for Aeromonas hydrophila (34 strains) and Aeromonas dhakensis (9 strains) was generally low, there was a slight increase in individuals aged 50-60 for A. hydrophila and over 60 years for A. dhakensis. These findings provide novel evidence of the varied contributions of different Aeromonas species to human enteric infections related to patient age, offering valuable insights for epidemiology and clinical management.

Aeromonas veronii-associated bacteremia in the course of treatment of acute myeloid leukemia: a case report and review of the literature

PURPOSE

This study aimed to report a case of acute myeloid leukemia (AML) complicated by Aeromonas veronii infection-induced bacteremia and to review relevant literature on the etiology, prevention, treatment, and prognosis of bacteremia in immunocompromised populations, aiming to reduce mortality in individuals with hematologic and other end-stage diseases and improve patient outcomes.

METHODS AND RESULTS

We reported the case of a 23-year-old male patient with relapsed AML characterized by AML1:ETO and ASXL positivity, classified as a high-risk group. The patient presented with fever, abdominal pain, diarrhea, nausea, and vomiting after consuming partially cooked fish. The patient was admitted with high leucocytes, C-reactive protein, procalcitonin, and interleukin-6 levels. Peripheral blood high-throughput sequencing (Next-Generation Sequencing NGS) confirmed infection with Aeromonas veronii, while an abdominal CT scan indicated a liver abscess with gas formation. Culture of the drainage fluid from the ultrasound-guided liver abscess puncture demonstrated growth of Aeromonas veronii. Based on the sensitivity results, the patient was treated with intravenous ciprofloxacin and cefoperazone-sulbactam. After treatment with antibiotics, blood transfusion, liver protection, and azacitidine 100 mg ih, combined with dry white sand(interferon alpha-1B, interleukin-2, and thalidomide), the critical condition of the patient improved, and he was discharged. This study was approved by the Ethics Committee of Medical Research in the Second Affiliated Hospital of Henan University of Science and Technology. Informed consent was obtained from this patient and we have obscured the patient's identifying information. All methods were carried out in accordance with relevant guidelines and regulations.

CONCLUSION

When patients with a recurrence of AML have a history of consuming or contacting aquatic products, clinicians should be vigilant about Aeromonas veronii infection. The presence of Aeromonas veronii in peripheral blood must alert clinicians to the possibility of severe sepsis and septic shock. Early diagnosis and prompt treatment are crucial to reducing patient mortality.

Description of a pathogenic strain of Aeromonas dhakensis isolated from Ancherythroculter nigrocauda in an inland region of China

Aeromonas dhakensis is reported as an emerging pathogenic species within the genus Aeromonas and is widely distributed in tropical coastal areas. This study provided a detailed description and characterization of a strain of A. dhakensis (202108B1) isolated from diseased Ancherythroculter nigrocauda in an inland region of China. Biochemical tests identified the isolate at the genus level, and the further molecular analysis of concatenated housekeeping gene sequences revealed that the strain belonged to the species A. dhakensis. The isolated A. dhakensis strain was resistant to five antibiotics, namely, penicillin, ampicillin, clindamycin, cephalexin, and imipenem, while it was susceptible to or showed intermediate resistance to most of the other 15 tested antibiotics. The isolated strain of A. dhakensis caused acute hemorrhagic septicemia and tissue damage in artificially infected A. nigrocauda, with a median lethal dose of 7.76 × 104 CFU/fish. The genome size of strain 202108B1 was 5 043 286 bp, including 1 chromosome and 4 plasmids. This is the first detailed report of the occurrence of infection caused by an A. dhakensis strain causing infection in an aquaculture system in inland China, providing important epidemiological data on this potential pathogenic species.

Case Report: A rare infection of multidrug-resistant Aeromonas caviae in a pediatric case with acute lymphoblastic leukemia and review of the literature

Aeromonas caviae infection of the bloodstream and intestine is a rare and severe opportunistic infection in immunocompromised people. In Southwest China, we first reported a case of bloodstream and intestinal infection with multidrug-resistant (MDR) Aeromonas caviae in a 4-year-old child with T-cell acute lymphoblastic leukemia. Blood and stool cultures were used to identify the infection. The selection of antibiotics was based on clinical expertise and medication sensitivity tests. We used linezolid, levofloxacin, and polymyxin B to treat the patient aggressively. Aeromonas caviae infection is uncommon in juvenile acute lymphoblastic leukemia. Doctors should be aware of the likelihood of opportunistic infection during the post-chemotherapy bone marrow suppression period. We further conducted a review of the literature and performed a detailed analysis of Aeromonas infection in pediatric leukemia. It is becoming increasingly apparent that antibiotic is abused domestically and abroad, resulting in the sharp increase of MDR bacteria. In general, most of the Aeromonas isolates are susceptible to third- or fourth-generation cephalosporins, aminoglycosides, quinolones, and carbapenem, but drug-resistant strains are being reported increasingly. We summarized the drug resistance rate of Aeromonas caviae and Aeromonas hydrophila in China in the last 10 years. Early recognition and effective treatment will improve prognosis and reduce mortality.

A 26-year-old man with multiple organ failure caused by Aeromonas dhakensis infection: a case report and literature review

Background

Infections in humans are mainly caused by Aeromonas hydrophila, Aeromonas caviae, and Aeromonas veronii. In recent years, Aeromonas dhakensis has been recognized as widely distributed in the environment, with strong virulence. However, this bacterial infection usually does not appear in patients with pneumonia as the first symptom.

Case report

We report a 26-year-old man who was admitted to the hospital with community-acquired pneumonia as the first symptom and developed serious conditions such as hemolytic uremic syndrome, multiple organ dysfunction, and hemorrhagic shock within a short period. He died after 13 h of admission, and the subsequent metagenomic-next generation sequencing test confirmed the finally identified pathogen of infection as A. dhakensis.

Conclusion

Aeromonas is a rare pathogen identified in the diagnosis of community-acquired pneumonia. Hence, doctors need to develop their experience in identifying the difference between infections caused by pathogenic microorganisms. Medical attention is essential during the occurrence of respiratory symptoms that could be controlled by empirical drugs, such as cephalosporins or quinolones. When patients with community-acquired pneumonia present hemoptysis and multiple organ dysfunction in clinical treatment, an unusual pathogen infection should be considered, and the underlying etiology should be clarified at the earliest for timely treatment.

Haemolysin Ahh1 secreted from Aeromonas dhakensis activates the NLRP3 inflammasome in macrophages and mediates severe soft tissue infection

Severe soft tissue infections caused by Aeromonas dhakensis, such as necrotizing fasciitis or cellulitis, are prevalent in southern Taiwan and around the world. However, the mechanism by which A. dhakensis causes tissue damage remains unclear. Here, we found that the haemolysin Ahh1, which is the major virulence factor of A. dhakensis, causes cellular damage and activates the NLR family pyrin domain containing 3 (NLRP3) inflammasome signalling pathway. Deletion of ahh1 significantly downregulated caspase-1, the proinflammatory cytokine interleukin 1β (IL-1β) and gasdermin D (GSDMD) and further decreased the damage caused by A. dhakensis in THP-1 cells. In addition, we found that knockdown of the NLRP3 inflammasome confers resistance to A. dhakensis infection in both THP-1 NLRP3-/- cells and C57BL/6 NLRP3-/- mice. In addition, we demonstrated that severe soft-tissue infections treated with antibiotics combined with a neutralizing antibody targeting IL-1β significantly increased the survival rate and alleviated the degree of tissue damage in model mice compared control mice. These findings show that antibiotics combined with therapies targeting IL-1β are potential strategies to treat severe tissue infections caused by toxin-producing bacteria.

Prevalence and virulence potential of Aeromonas spp. isolated from human diarrheal samples in North East Italy

IMPORTANCE

In this work, we demonstrate the epidemiologic relevance of the Aeromonas genus as the cause of infective diarrhea in North East Italy, both in children and adult subjects, with the significative presence of highly pathogenic strains. Aeromonas strains possess a heterogeneous armamentarium of pathogenicity factors that allows the microbe to affect a wide range of human intestinal epithelial cell processes that justify the ability to induce diarrhea through different mechanisms and cause diseases of variable severity, as observed for other gastrointestinal pathogens. However, it remains to be determined whether specific genotype(s) are associated with clinical pictures of different severity to implement the diagnostic and therapeutic approaches for this relevant enteric pathogen.

Identification and characterization of a novel 6'-N-aminoglycoside acetyltransferase AAC(6')-Va from a clinical isolate of Aeromonas hydrophila

Background

Aeromonas species have been identified as agents responsible for various diseases in both humans and animals. Multidrug-resistant Aeromonas strains pose a significant public health threat due to their emergence and spread in clinical settings and the environment. The aim of this study was to determine a novel resistance mechanism against aminoglycoside antimicrobials in a clinical isolate.

Methods

The function of aac(6')-Va was verified by gene cloning and antibiotic susceptibility tests. To explore the in vivo activity of the enzyme, recombinant proteins were expressed, and enzyme kinetics were tested. To determine the molecular background and mechanism of aac(6')-Va, whole-genome sequencing and bioinformatic analysis were performed.

Results

The novel aminoglycoside N-acetyltransferase gene aac(6')-Va confers resistance to several aminoglycosides. Among the antimicrobials tested, ribostamycin showed the highest increase (128-fold) in the minimum inhibitory concentration (MIC) compared with the control strains. According to the MIC results of the cloned aac(6')-Va, AAC(6')-Va also showed the highest catalytic efficiency for ribostamycin [kcat/Km ratio = (3.35 ± 0.17) × 104 M-1 s-1]. Sharing the highest amino acid identity of 54.68% with AAC(6')-VaIc, the novel aminoglycoside N-acetyltransferase constituted a new branch of the AAC(6') family due to its different resistance profiles. The gene context of aac(6')-Va and its close relatives was conserved in the genomes of species of the genus Aeromonas.

Conclusion

The novel resistance gene aac(6')-Va confers resistance to several aminoglycosides, especially ribostamycin. Our finding of a novel resistance gene in clinical A. hydrophila will help us develop more effective treatments for this pathogen's infections.

A Systematic Review of Culture-Based Methods for Monitoring Antibiotic-Resistant Acinetobacter, Aeromonas, and Pseudomonas as Environmentally Relevant Pathogens in Wastewater and Surface Water

PURPOSE OF REVIEW

Mounting evidence indicates that habitats such as wastewater and environmental waters are pathways for the spread of antibiotic-resistant bacteria (ARB) and mobile antibiotic resistance genes (ARGs). We identified antibiotic-resistant members of the genera Acinetobacter, Aeromonas, and Pseudomonas as key opportunistic pathogens that grow or persist in built (e.g., wastewater) or natural aquatic environments. Effective methods for monitoring these ARB in the environment are needed to understand their influence on dissemination of ARB and ARGs, but standard methods have not been developed. This systematic review considers peer-reviewed papers where the ARB above were cultured from wastewater or surface water, focusing on the accuracy of current methodologies.

RECENT FINDINGS

Recent studies suggest that many clinically important ARGs were originally acquired from environmental microorganisms. Acinetobacter, Aeromonas, and Pseudomonas species are of interest because their ability to persist and grow in the environment provides opportunities to engage in horizontal gene transfer with other environmental bacteria. Pathogenic strains of these organisms resistant to multiple, clinically relevant drug classes have been identified as an urgent threat. However, culture methods for these bacteria were generally developed for clinical samples and are not well-vetted for environmental samples. The search criteria yielded 60 peer-reviewed articles over the past 20 years, which reported a wide variety of methods for isolation, confirmation, and antibiotic resistance assays. Based on a systematic comparison of the reported methods, we suggest a path forward for standardizing methodologies for monitoring antibiotic resistant strains of these bacteria in water environments.

Metformin-associated severe lactic acidosis combined with multi-organ insufficiency induced by infection with Aeromonas veronii: A case report

RATIONALE

Lactic acidosis is a disease in which lactic acid accumulates in the blood and causes acidosis in the patient. The criteria for diagnosis are a lactate level of >2 mmol/L in the blood and a blood pH of <7.2.

PATIENT CONCERNS

A 72-year-old Asian female with a history of diabetes for 20+ years was admitted to the hospital with the chief complaint of "dry mouth, polydipsia for 20+ years, loss of appetite for 5+ days, vomiting for 1-day." She was admitted with a blood gas pH of 6.795, and a lactate level of >30 mmol/L.

DIAGNOSES

Type 2 diabetes mellitus with lactic acidosis, ketoacidosis, chronic renal insufficiency, hypertensive disease, and coronary arteriosclerotic heart disease.

INTERVENTIONS

She was treated with symptomatic rehydration and ketone reduction immediately, but then became unconscious and was admitted to the intensive care unit, where she was administered symptomatic support and continuous renal replacement therapy. As the blood culture showed Aeromonas veronii, she was administered a sensitive antibiotic in conjunction.

OUTCOMES

However, after achieving a stable internal environment and good infection control, the patient's family decided to discontinue treatment because of persistent heart failure with acute exacerbation of chronic renal insufficiency complicated by gastrointestinal bleeding.

LESSONS

Lactic acidosis has low incidence, poor prognosis, and high morbidity and mortality rates. Special attention should be paid to infection-induced acidosis, especially in patients with combined multi-organ insufficiency. Early diagnosis and active management can improve the patient prognosis.

Differences in flaA gene sequences, swimming motility, and biofilm forming ability between clinical and environmental isolates of Aeromonas species

The flagellin A gene (flaA) sequences, swimming motility, and biofilm forming ability were investigated in order to reveal the genetic and functional differences of flagella between clinical and environmental isolates of Aeromonas species. Twenty-eight clinical and 48 environmental strains of Aeromonas species isolated in Okinawa Prefecture of Japan were used in this study. The full-length flaA genes of these strains were sequenced and aligned, and a phylogenetic tree was constructed. In addition, swimming motility and biofilm forming ability were evaluated by conventional methods. Aeromonas veronii biovar sobria and A. hydrophila clearly divided into clinical and environmental strain clusters in the flaA phylogenetic classification, and the six and 13 specific amino acids respectively, of FlaA of both species were different in clinical and environmental strains. Furthermore, the flaA size of the clinical strain of A. veronii bv. sobria was mainly 909, 924, and 939 bp, and the size of A. hydrophila was 909 bp. The swimming motility of clinical isolates of both species was lower than the environmental isolates; however, the biofilm forming ability of the clinical isolates was high. Thus, the clinical isolates of A. veronii bv. sobria and A. hydrophila had different genetic and functional characteristics of flagellin than the environmental isolates. The characteristics of flagellin could serve as indicators to distinguish between clinical and environmental isolates of the both species. It may contribute to diagnosis of these diseases and the monitoring of clinical strain invasion into the natural environment.

Epidemiology of Aeromonas Species Bloodstream Infection in Queensland, Australia: Association with Regional and Climate Zones

Aeromonas species can cause severe bloodstream infection (BSI) however, few studies have examined their epidemiology in non-selected populations. The objective of this study was to describe the incidence and determinants of Aeromonas species BSI in Queensland, Australia. A retrospective population-based cohort study was conducted during 2000-2019. Aeromonas species BSI were identified by laboratory surveillance and clinical and outcome information through data linkages to statewide databases. A total of 407 incident Aeromonas species BSI were identified with an age- and sex-standardized incidence of 5.2 per million residents annually. No trend in annual incidence rate during two decades of surveillance was demonstrated. Significant variable monthly occurrences were observed with highest rates during warmer, wetter months, and lowest rates during winter and dry periods. There was significant variability in incidence accordingly to region and climate zones, with higher rates observed in tropical north regions and lowest in southeastern corner. The highest incidence was observed in very remote and hot areas in Queensland. Cases were infrequent in children and risk was highest in elderly and males. Seventy-eight patients died within 30 days with a case-fatality rate of 19%. Older age, non-focal infection, higher Charlson score, and monomicrobial bacteremia were independent risk factors for death. Demographic and climatic changes may increase the burden of these infections in future years.

Complexation of a Polypeptide-Polyelectrolytes Bioparticle as a Biomaterial of Antibacterial Activity

The development of biomaterials to enable application of antimicrobial peptides represents a strategy of high and current interest. In this study, a bioparticle was produced by the complexation between an antimicrobial polypeptide and the biocompatible and biodegradable polysaccharides chitosan-N-arginine and alginate, giving rise to a colloidal polyelectrolytic complex of pH-responsive properties. The inclusion of the polypeptide in the bioparticle structure largely increases the binding sites of complexation during the bioparticles production, leading to its effective incorporation. After lyophilization, detailed evaluation of colloidal structure of redispersed bioparticles evidenced nano or microparticles with size, polydispersity and zeta potential dependent on pH and ionic strength, and the dependence was not withdrawn with the polypeptide inclusion. Significant increase of pore edge tension in giant vesicles evidenced effective interaction of the polypeptide-bioparticle with lipid model membrane. Antibacterial activity against Aeromonas dhakensis was effective at 0.1% and equal for the isolated polypeptide and the same complexed in bioparticle, which opens perspectives to the composite material as an applicable antibacterial system.

Comparison of species, virulence genes and clones of Aeromonas isolates from clinical specimens and well water in Okinawa Prefecture, Japan

AIMS

To reveal the sources of Aeromonas infection in Okinawa Prefecture of Japan, the species, virulence genes and clones of strains isolated from clinical specimens and well water were compared.

METHODS AND RESULTS

The properties of both isolates were investigated by sequencing of rpoD, detection of 10 virulence genes using PCR and genotyping with pulsed-field gel electrophoresis. In all, 68 clinical and 146 well water strains of Aeromonas were isolated and the main species were A. caviae, A. dhakensis, A. hydrophila and A. veronii biovar sobria. Aeromonas dhakensis possessed various virulence genes; however, A. caviae possessed only fla. The same or similar clones were distributed in certain areas of Okinawa and one clone had survived several months in the biliary system of two patients, respectively.

CONCLUSION

Although the same Aeromonas clone was not isolated from clinical and well water samples, our study revealed the detected patterns of virulence genes in both isolates, the distribution of identical/similar clones in the Okinawan environment and long-time survival in patient's organs.

SIGNIFICANCE AND IMPACT OF THE STUDY

We investigated the association between Aeromonas patients and well water exposure. This study provides the properties of species, virulence genes and clones of Aeromonas isolated from samples of these origins.

Taxonomy, virulence determinants and antimicrobial susceptibility of Aeromonas spp. isolated from bacteremia in southeastern China

Background

The study aimed to elucidate the species taxonomy, clinical manifestations, virulence gene profiles and antimicrobial susceptibilities of Aeromonas strains isolated from life-threatening bacteremia in southeastern China.

Methods

Clinical samples of Aeromonas causing bacteremia were isolated from a teaching hospital in Wenzhou from 2013 to 2018 and a retrospective cohort study was performed. Aeromonas strains were identified at species level by housekeeping gene gyrB. Virulence and drug resistance-associated genes were screened by polymerase chain reaction (PCR) and antimicrobial susceptibility testing (AST) was performed by the VITEK 2 Compact system.

Results

A total of 58 Aeromonas isolated from patients with bacteremia were collected during 6 years (2013–2018). 58 isolates were identified to five different species, where Aeromonas dhakensis appeared to be the predominant species (26/58), followed by Aeromonas veronii (13/58), Aeromonas caviae (10/58), Aeromonas hydrophila (7/58) and Aeromonas jandaei (2/58). 16 of 58 patients had poor prognosis. Poor prognosis was significantly associated with liver cirrhosis and inappropriate empirical antimicrobials therapy. The progression of bacteremia caused by Aeromonas was extremely fast, especially in A. dhakensis infections. Virulence genes aer, lip, hlyA, alt, ast, and act, were detected at ratios of 24.1% (14/58), 62.1% (36/58), 65.5% (38/58), 58.6% (34/58), 15.5% (9/58) and 65.5% (38/58), respectively. Antimicrobial susceptibility testing exhibited that 9 out of 58 isolates were identified as multi-drug resistant (MDR) organism. The bla_TEM gene was identified in all 9 MDR isolates. bla_SHV, bla_AQU-1, bla_MOX, bla_CepH, bla_CphA and aac(6′)-Ib-cr were detected in 4 isolates, 2 isolates, 1 isolate, 3 isolates, 8 isolates, and 3 isolates, respectively. The majority of Aeromonas strains maintained susceptible to 3rd generation cephalosporins, aminoglycosides, fluoroquinolones and furantoin.

Conclusions

The prevalence and dangerousness of Aeromonas infections, especially A. dhakensis, are underestimated in clinic. Continuous monitoring is essential to keep track of MDR Aeromonas due to the increasing prevalence recently and a more effective measure is required to control the spread of resistance determinants.

UvrY is required for the full virulence of Aeromonas dhakensis

Aeromonas dhakensis is an emerging human pathogen which causes fast and severe infections worldwide. Under the gradual pressure of lacking useful antibiotics, finding a new strategy against A. dhakensis infection is urgent. To understand its pathogenesis, we created an A. dhakensis AAK1 mini-Tn10 transposon library to study the mechanism of A. dhakensis infection. By using a Caenorhabditis elegans model, we established a screening platform for the purpose of identifying attenuated mutants. The uvrY mutant, which conferred the most attenuated toxicity toward C. elegans, was identified. The uvrY mutant was also less virulent in C2C12 fibroblast and mice models, in line with in vitro results. To further elucidate the mechanism of UvrY in controlling the toxicity in A. dhakensis, we conducted a transcriptomic analysis. The RNAseq results showed that the expression of a unique hemolysin ahh1 and other virulence factors were regulated by UvrY. Complementation of Ahh1, one of the most important virulence factors, rescued the pore-formation phenotype of uvrY mutant in C. elegans; however, complementation of ahh1 endogenous promoter-driven ahh1 could not produce Ahh1 and rescue the virulence in the uvrY mutant. These findings suggest that UvrY is required for the expression of Ahh1 in A. dhakensis. Taken together, our results suggested that UvrY controls several different virulence factors and is required for the full virulence of A. dhakensis. The two-component regulator UvrY therefore a potential therapeutic target which is worthy of further study.

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.

Comprehensive Analysis Reveals the Evolution and Pathogenicity of Aeromonas, Viewed from Both Single Isolated Species and Microbial Communities

The genus Aeromonas is a common gastrointestinal pathogen associated with human and animal infections. Due to the high level of cross-species similarity, their evolutionary dynamics and genetic diversity are still fragmented. Hereby, we investigated the pan-genomes of 29 Aeromonas species, as well as Aeromonas species in microbial communities, to clarify their evolutionary dynamics and genetic diversity, with special focus on virulence factors and horizontal gene transfer events. Our study revealed an open pan-genome of Aeromonas containing 10,144 gene families. These Aeromonas species exhibited different functional constraints, with the single-copy core genes and most accessory genes experiencing purifying selection. The significant congruence between core genome and pan-genome trees revealed that core genes mainly affected evolutionary divergences of Aeromonas species. Gene gains and losses revealed a high level of genome plasticity, exhibited by hundreds of gene expansions and contractions, horizontally transferred genes, and mobile genetic elements. The selective constraints shaped virulence gene pools of these Aeromonas strains, where genes encoding hemolysin were ubiquitous. Of these strains, Aeromonas aquatica MX16A seemed to be more resistant, as it harbored most resistance genes. Finally, the virulence factors of Aeromonas in microbial communities were quite dynamic in response to environment changes. For example, the virulence diversity of Aeromonas in microbial communities could reach levels that match some of the most virulent Aeromonas species (such as A. hydrophila) in penetrated-air and modified-air packaging. Our work shed some light onto genetic diversity, evolutionary history, and functional features of Aeromonas, which could facilitate the detection and prevention of infections.IMPORTANCE Aeromonas has long been known as a gastrointestinal pathogen, yet it has many species whose evolutionary dynamics and genetic diversity had been unclear until now. We have conducted pan-genome analysis for 29 Aeromonas species and revealed a high level of genome plasticity exhibited by hundreds of gene expansions and contractions, horizontally transferred genes, and mobile genetic elements. These species also contained many virulence factors both identified from single isolated species and microbial community. This pan-genome study could elevate the level for detection and prevention of Aeromonas infections.

Aeromonas Isolates from Fish and Patients in Tainan City, Taiwan: Genotypic and Phenotypic Characteristics

The present study aimed to isolate Aeromonas from fish sold in the markets as well as in sushi and seafood shops and compare their virulence factors and antimicrobial characteristics with those of clinical isolates. Among the 128 fish isolates and 47 clinical isolates, Aeromonas caviae, A. dhakensis, and A. veronii were the principal species. A. dhakensis isolates carried at least 5 virulence genes, more than other Aeromonas species. The predominant genotype of virulence genes was hlyA lip alt col ela in both A. dhakensis and A. hydrophila isolates, alt col ela in A. caviae isolates, and act in A. veronii isolates. A. dhakensis, A. hydrophila, and A. veronii isolates more often exhibited hemolytic and proteolytic activity and showed greater virulence than A. caviae isolates in Caenorhabditis elegans and the C2C12 cell line. However, the link between the genotypes and phenotypes of the studied virulence genes in Aeromonas species was not evident. Among the four major clinical Aeromonas species, nearly all (99.0%) A. dhakensis, A. hydrophila, and A. veronii isolates harbored bla _CphA, which encodes a carbapenemase, but only a minority (6.7%, 7/104) were nonsusceptible to carbapenem. Regarding AmpC β-lactamase genes, bla _AQU-1 was exclusively found in A. dhakensis isolates, and bla _MOX3 was found only in A. caviae isolates, but only 7.6% (n = 6) of the 79 Aeromonas isolates carrying bla _AQU-1 or bla _MOX3 exhibited a cefotaxime resistance phenotype. In conclusion, fish Aeromonas isolates carry a variety of combinations of virulence and β-lactamase resistance genes and exhibit virulence phenotypes and antimicrobial resistance profiles similar to those of clinical isolates.IMPORTANCE Aeromonas species can cause severe infections in immunocompromised individuals upon exposure to virulent pathogens in the environment, but the characteristics of environmental Aeromonas species remain unclear. Our study showed that several pathogenic Aeromonas species possessing virulence traits and antimicrobial resistance similar to those of Aeromonas isolates causing clinical diseases were present in fish intended for human consumption in Tainan City, Taiwan.

Isolation, identification and characterisation of an emerging fish pathogen, Acinetobacter pittii, from diseased loach (Misgurnus anguillicaudatus) in China

Although members of the genus Acinetobacter have emerged as important nosocomial pathogens causing severe human infections, there are few reports about their occurrence as fish pathogens. In this study, five bacterial strains were isolated from diseased loach (Misgurnus anguillicaudatus) cultured in a farm in China. The diseased loach displayed shedding of skin mucus and many petechial haemorrhages all over the body. Based on sequence analyses of 16S rRNA and rpoB genes, the isolates were identified as Acinetobacter pittii. An experimental infection assay confirmed their pathogenicity to loach. The results of artificial infection in zebrafish (Barchydanio rerio) and nematode (Caenorhabditis elegans) suggested that, as well as loach, these A. pittii isolates are pathogenic and highly virulent to these organisms. Multilocus sequence typing analysis revealed that all the isolates belong to sequence type (ST) 839, which may be the dominant clone causing fish disease and exhibits a close phylogenetic relationship with ST396 from human clinical samples in Korea or Taiwan China. This is the first report demonstrating that A. pittii is an emerging causal agent of mass mortality in loach and poses significant risks to fish culturing besides causing human clinical infection worldwide.

Biocide resistant and antibiotic cross-resistant potential pathogens from sewage and river water from a wastewater treatment facility in the North-West, Potchefstroom, South Africa

Exposure to antibiotics, biocides, chemical preservatives, and heavy metals in different settings such as wastewater treatment plants (WWTPs) may apply selective pressure resulting in the enrichment of multiple resistant, co- and cross-resistant strains of bacteria. The purpose of this study was to identify and characterize potentially pathogenic triclosan (TCS) - and/or, chloroxylenol (PCMX) tolerant bacteria from sewage and river water in the North-West, Potchefstroom, South Africa. Several potential pathogens were identified, with Aeromonas isolates being most abundant. Clonal relationships between Aeromonas isolates found at various sampling points were elucidated using ERIC-PCR. Selected isolates were characterized for their minimum inhibitory concentrations against the biocides, as well as antibiotic resistance profiles, followed by an evaluation of synergistic and antagonistic interactions between various antimicrobials. Isolates were also screened for the presence of extracellular enzymes associated with virulence. High-performance liquid chromatography revealed the presence of both biocides in the wastewater, but fingerprinting methods did not reveal whether the WWTP is the source from which these organisms enter the environment. Isolates exhibited various levels of resistance to antimicrobials as well as several occurrences of synergy and antagonisms between the biocides and select antibiotics. Several isolates had a very high potential for virulence but further study is required to identify the specific virulence and resistance genes associated with the isolates in question.

Severe acute cellulitis and sepsis caused by Aeromonas spp. in a dog on immunosuppressive therapy

OBJECTIVE

To describe the clinical presentation, diagnostic investigation, and medical management of a dog on immunosuppressive therapy that developed a severe soft tissue infection attributed to Aeromonas hydrophila/caviae.

CASE SUMMARY

A 5-year-old female neutered Border Collie dog was presented for investigation of a rapidly growing skin lesion. The dog had been diagnosed with immune-mediated thrombocytopenia and was receiving immunosuppressive therapy for 5 weeks. Physical examination at initial presentation revealed no abnormalities except a 6 cm raised, erythematous, firm, and painful swelling on the ventral abdomen. Within 12 hours of admission, the lesion had expanded to cover much of the ventrum and some areas had begun to slough. The patient had also become obtunded and exhibited pyrexia, tachypnea, tachycardia as well as extreme pain around the lesion. The dog's clinical signs and hematology results were consistent with sepsis. Histopathology showed severe acute suppurative cellulitis and panniculitis and a heavy growth of A. hydrophila/caviae was obtained on tissue culture. The infection was treated with trimethoprim sulphadiazine, based on culture and susceptibility results.

UNIQUE INFORMATION PROVIDED

This is the first reported case of severe panniculitis and cellulitis caused by Aeromonas spp. in a dog. Aeromonas spp. should be considered a differential diagnosis for cases of severe soft tissue infection, especially in immune-compromised animals or those with a history of aquatic exposure.

Three species of Aeromonas (A. dhakensis, A. hydrophila and A. jandaei) isolated from freshwater crocodiles (Crocodylus siamensis) with pneumonia and septicemia

Hundreds of farmed Siamese crocodiles (Crocodylus siamensis) died during July 2016 at a farm in Wenchang, Hainan, China. In two necropsied crocodiles, we observed symptoms of dermatorrhagia, hepatomegaly and hepatic congestion. Pulmonitis was diagnosed by pulmonary congestion and pulmonary fibrinous exudate. Septicaemia was diagnosed by isolation of three Aeromonas species from blood and visceral tissues; A. dhakensis, A. hydrophila and A. jandaei were identified by biochemical and molecular tests. We used a zebrafish model to determine the half-maximal lethal dose (LD₅₀ ), and A. dhakensis was found to be the most virulent species, with an LD₅₀ of 8·91 × 10⁵ CFU per ml. The results of a drug sensitivity test indicated that these species were sensitive to 11 antibiotics. This is the first report of A. dhakensis, A. hydrophila and A. jandaei being isolated from a mixed infection in Siamese crocodiles. SIGNIFICANCE AND IMPACT OF THE STUDY: In this study, we isolated three species of Aeromonas (A. dhakensis, A. hydrophila and A. jandae) from farmed Siamese crocodiles with fatal fibrinous pneumonia and septicaemia. This is the first description of a mixed infection with three Aeromonas species among captive crocodilians.

Effects of hexavalent chromium on intestinal histology and microbiota in Bufo gargarizans tadpoles

Chromium is detrimental heavy metal pollutants and can enter and affect aquatic organisms. In our experiment, B. gargarizans embryos were chronically exposed in different concentrations of Cr (VI) (0, 13, 104, and 416 μg Cr⁶⁺ L^-1) until reached Gosner stage 38. We measured morphological parameters of the body and intestine of B. gargarizans tadpoles, and examined alteration of intestinal tissue. Furthermore, we analyzed the intestinal microbial community of B. gargarizans tadpoles using 16S rRNA gene amplicon sequencing. Our research demonstrated that Cr (VI) exposure caused alteration of intestinal tissue structure in 416 μg Cr⁶⁺ L^-1 treatment groups. Total body length, body wet weight, intestinal length and wet weight of B. gargarizans tadpoles were significantly declined at 416 μg Cr⁶⁺ L^-1. In addition, 16S rRNA gene sequencing revealed that Cr (VI) exposure significantly altered the intestinal microbiota diversity and composition, and perturbed the community structure of the microbiota. As for the intestinal microbiota, at the phylum level, Fusobacteria significantly changed in all Cr (VI) treated groups. Saccharibacteria and TM6_Dependentiae were detected only in the high dose exposure groups. At the genus level, Aeromonas was significantly decreased in Cr (VI) treated groups. According to the results of functional prediction, Cr (VI) exposure affected metabolism and increased risk of disease by inducing the alterations of intestinal microbiota structure. Taken together, the present study provide a new framework elucidating the toxic effects Cr (VI) exposure on B. gargarizans tadpoles associated with intestinal histology and microbiota.

Diagnostic methods for identifying different Aeromonas species and examining their pathogenicity factors, their correlation to cytotoxicity and adherence to fish mucus

Aeromonas spp. are ubiquitous in the aquatic environment, acting as facultative or obligate pathogens for fish. Identifying Aeromonas spp. is important for pathogenesis and prognosis in diagnostic cases but can be difficult because of their close relationship. Forty-four already characterized isolates of Aeromonas spp. were analysed by 16S rRNA gene sequencing, by gyrase B sequencing, by analysing their fatty acid profiles, by biochemical reactions and by MALDI-TOF MS. To determine their pathogenicity, cytotoxicity, adhesion to mucus and the expression of 12 virulence factors were tested. The susceptibility of the isolates towards 13 different antibiotics was determined. MALDI-TOF MS was found to be an acceptable identification method for Aeromonas spp. Although the method does not detect all species correctly, it is time-effective and entails relatively low costs and no other methods achieved better results. A high prevalence of virulence-related gene fragments was detected in almost all examined Aeromonas spp., especially in A. hydrophila and A. salmonicida, and most isolates exhibited a cytotoxic effect. Single isolates of A. hydrophila and A. salmonicida showed multiple resistance to antibiotics. These results might indicate the potentially pathogenic capacity of Aeromonas spp., suggesting a risk for aquatic animals and even humans, given their ubiquitous nature.

Characterization of multidrug-resistant Gram-negative bacilli isolated from hospitals effluents: first report of a blaOXA-48-like in Klebsiella oxytoca, Algeria

The antibiotic susceptibility profile and antimicrobial resistance determinants were characterized on Gram-negative bacilli (GNB) isolated from Algerian hospital effluents. Among the 94 isolates, Enterobacteriaceae was the predominant family, with Escherichia coli and Klebsiella pneumoniae being the most isolated species. In non-Enterobacteriaceae, Acinetobacter and Aeromonas were the predominant species followed by Pseudomonas, Comamonas, Pasteurella, and Shewanella spp. The majority of the isolates were multidrug-resistant (MDR) and carried different antimicrobial resistance genes including bla_CTX-M, bla_TEM, bla_SHV, bla_OXA-48-like, bla_OXA-23, bla_OXA-51, qnrB, qnrS, tet(A), tet(B), tet(C), dfrA1, aac(3)-IIc (aacC2), aac(6')-1b, sul1, and sul2. The qacEΔ1-sul1 and intI2 signatures of class 1 and class 2 integrons, respectively, were also detected. Microarray hybridization on MDR E. coli revealed additional resistance genes (aadA1 and aph3strA, tet30, mphA, dfrA12, bla_cmy2, bla_ROB1, and cmlA1) and classified the tested strains as commensals, thus highlighting the potential role of humans in antibiotic resistance dissemination. This study is the first report of bla_OXA-48-like in Klebsiella oxytoca in Algeria and bla_OXA-23 in A. baumannii in Algerian hospital effluents. The presence of these bacteria and resistance genes in hospital effluents represents a serious public health concern since they can be disseminated in the environment and can colonize other hosts.

Isolation and identification of pathogenic Aeromonas veronii from a dead Yangtze finless porpoise

Diseases caused by Aeromonas veronii in freshwater fish have been widely reported, but other species such as aquatic mammals have probably been overlooked. Here, we identified one isolate of A. veronii from a Yangtze finless porpoise Neophocaena asiaeorientalis asiaeorientalis exhibiting disease and mortality, and subsequently confirmed its virulence in artificial infection of BALB/c mice. The bacterial isolate was identified as A. veronii based on physiological, biochemical, and phenotypic features, and homology of the 16S rRNA, cpn60, rpoB, dnaJ and gyrB genes. Our results expand the known host spectrum of A. veronii, which is of great importance for the etiology of porpoise, dolphin, and other cetacean diseases.

RIOK-1 Is a Suppressor of the p38 MAPK Innate Immune Pathway in Caenorhabditis elegans

Innate immunity is the primary defense mechanism against infection in metazoans. However, aberrant upregulation of innate immune-signaling pathways can also be detrimental to the host. The p38 MAPK/PMK-1 innate immune-signaling pathway has been demonstrated to play essential roles in cellular defenses against numerous infections in metazoans, including Caenorhabditis elegans. However, the negative regulators that maintain the homeostasis of this important innate immune pathway remain largely understudied. By screening a focused RNAi library against the kinome of C. elegans, we identified RIOK-1, a human RIO kinase homolog, as a novel suppressor of the p38 MAPK/PMK-1 signal pathway. We demonstrated that the suppression of riok-1 confers resistance to Aeromonas dhakensis infection in C. elegans. Using quantitative real time-PCR and riok-1 reporter worms, we found the expression levels of riok-1 to be significantly upregulated in worms infected with A. dhakensis. Our genetic epistasis analysis suggested that riok-1 acts on the upstream of the p38 MAPK/pmk-1 genetic pathway. Moreover, the suppression of riok-1 enhanced the p38 MAPK signal, suggesting that riok-1 is a negative regulator of this innate pathway in C. elegans. Our epistatic results put riok-1 downstream of skn-1, which encodes a p38 MAPK downstream transcription factor and serves as a feedback loop to the p38 MAPK pathway during an A. dhakensis infection. In conclusion, riok-1 is proposed as a novel innate immune suppressor and as a negative feedback loop model involving p38 MAPK, SKN-1, and RIOK-1 in C. elegans.

Draft Genome Sequence of Aeromonas lusitana sp. nov. Strain DSM 24905T, Isolated from a Hot Spring in Vila-Real, Portugal

Aeromonas lusitana sp. nov. is an isolate derived from a study aimed at characterizing Aeromonas spp. from water sources used for recreation and agricultural purposes and assessing the implications these organisms have for human and animal health. We present here the 4.52-Mbp draft genome sequence of this novel species.

Aeromonas caviae mimicking Vibrio cholerae infectious enteropathy in a cholera-endemic region with possible public health consequences: two case reports

BACKGROUND

Aeromonas species have been documented to yield false positive results in microbiological tests for Vibrio cholerae. They share many biochemical properties with Vibrio species, with which they were jointly classified in the family Vibrionaceae until genotypic information provided new insights. Aeromonas species are increasingly associated with gastrointestinal infections, albeit with great apparent variation in pathogenicity and virulence both between and within species of the genus. We report two cases with clinically mild cholera-like symptoms, at a time when a cholera outbreak was unfolding in other regions of the country (Tanzania). These are the first cases to be reported with Aeromonas mimicking cholera in our area.

CASE PRESENTATION

Two patients were admitted at the isolation unit designated by the Kilimanjaro Christian Medical Centre for emerging infectious diseases and provided informed consent about regular stool analysis and culture under the provisional diagnosis of gastroenteritis. The first patient was a 23-year-old black African woman with a 2-day history of watery diarrhea and vomiting associated with a temperature of 39.7 °C. The second patient was a 47-year-old black African woman with a 2-day history of diarrhea and vomiting with a temperature of 37.7 °C, and she was hemodynamically stable. Both patients were isolated in a specific area for infection control and treated with fluids and orally administered rehydration solution, ciprofloxacin, metronidazole, and paracetamol. Stool culture was done. The isolated colonies were reported as V. cholerae and transferred to the research laboratory of Kilimanjaro Clinical Research Institute for confirmation using whole genome sequencing. Microbiological testing determined colonies isolated from stool to be V. cholerae, and warranted the conclusion "presumptive cholera." Whole genome sequencing, however, established the presence of Aeromonas caviae rather than V. cholerae.

CONCLUSIONS

The co-existence of Aeromonas species with V. cholerae in cholera-endemic regions suggests the possibility that a proportion of suspected cholera cases may be Aeromonas infections. However, with close to no epidemiological data available on Aeromonas infection in cases of diarrhea and dysentery in Sub-Saharan Africa, it is not currently possible to establish the extent of misdiagnosis to any degree of certainty. Whole genome sequencing was shown to readily exclude V. cholerae as the etiological agent and establish the presence of Aeromonas species.

A Disease Model of Muscle Necrosis Caused by Aeromonas dhakensis Infection in Caenorhabditis elegans

A variety of bacterial infections cause muscle necrosis in humans. Caenorhabditis elegans has epidermis and bands of muscle that resemble soft-tissue structures in mammals and humans. Here, we developed a muscle necrosis model caused by Aeromonas dhakensis infection in C. elegans. Our data showed that A. dhakensis infected and killed C. elegans rapidly. Characteristic muscle damage in C. elegans induced by A. dhakensis was demonstrated in vivo. Relative expression levels of host necrosis-associated genes, asp-3, asp-4, and crt-1 increased significantly after A. dhakensis infection. The RNAi sensitive NL2099 rrf-3 (pk1426) worms with knockdown of necrosis genes of crt-1 and asp-4 by RNAi showed prolonged survival after A. dhakensis infection. Specifically knockdown of crt-1 and asp-4 by RNAi in WM118 worms, which restricted RNAi only to the muscle cells, conferred significant resistance to A. dhakensis infection. In contrast, the severity of muscle damage and toxicity produced by the A. dhakensis hemolysin-deletion mutant is attenuated. In another example, shiga-like toxin-producing enterohemorrhagic E. coli (EHEC) known to elicit toxicity to C. elegans with concomitant enteropathogenicty, did not cause muscle necrosis as A. dhakensis did. Taken together, these results show that Aeromonas infection induces muscle necrosis and rapid death of infected C. elegans, which are similar to muscle necrosis in humans, and then validate the value of the C. elegans model with A. dhakensis infection in studying Aeromonas pathogenicity.

The Animal Model Determines the Results of Aeromonas Virulence Factors

The selection of an experimental animal model is of great importance in the study of bacterial virulence factors. Here, a bath infection of zebrafish larvae is proposed as an alternative model to study the virulence factors of Aeromonas hydrophila. Intraperitoneal infections in mice and trout were compared with bath infections in zebrafish larvae using specific mutants. The great advantage of this model is that bath immersion mimics the natural route of infection, and injury to the tail also provides a natural portal of entry for the bacteria. The implication of T3SS in the virulence of A. hydrophila was analyzed using the AH-1::aopB mutant. This mutant was less virulent than the wild-type strain when inoculated into zebrafish larvae, as described in other vertebrates. However, the zebrafish model exhibited slight differences in mortality kinetics only observed using invertebrate models. Infections using the mutant AH-1ΔvapA lacking the gene coding for the surface S-layer suggested that this protein was not totally necessary to the bacteria once it was inside the host, but it contributed to the inflammatory response. Only when healthy zebrafish larvae were infected did the mutant produce less mortality than the wild-type. Variations between models were evidenced using the AH-1ΔrmlB, which lacks the O-antigen lipopolysaccharide (LPS), and the AH-1ΔwahD, which lacks the O-antigen LPS and part of the LPS outer-core. Both mutants showed decreased mortality in all of the animal models, but the differences between them were only observed in injured zebrafish larvae, suggesting that residues from the LPS outer core must be important for virulence. The greatest differences were observed using the AH-1ΔFlaB-J (lacking polar flagella and unable to swim) and the AH-1::motX (non-motile but producing flagella). They were as pathogenic as the wild-type strain when injected into mice and trout, but no mortalities were registered in zebrafish larvae. This study demonstrates that zebrafish larvae can be used as a host model to assess the virulence factors of A. hydrophila. This model revealed more differences in pathogenicity than the in vitro models and enabled the detection of slight variations in pathogenesis not observed using intraperitoneal injections of mice or fish.

Establishment of Infection Models in Zebrafish Larvae (Danio rerio) to Study the Pathogenesis of Aeromonas hydrophila

Aeromonas hydrophila is a Gram-negative opportunistic pathogen of fish and terrestrial animals. In humans, A. hydrophila mainly causes gastroenteritis, septicaemia, and tissue infections. The mechanisms of infection, the main virulence factors and the host immune response triggered by A. hydrophila have been studied in detail using murine models and adult fish. However, the great limitation of studying adult animals is that the animal must be sacrificed and its tissues/organs extracted, which prevents the study of the infectious processes in the whole living animal. Zebrafish larvae are being used for the analysis of several infectious diseases, but their use for studying the pathogenesis of A. hydrophila has never been explored. The great advantage of zebrafish larvae is their transparency during the first week after fertilization, which allows detailed descriptions of the infectious processes using in vivo imaging techniques such as differential interferential contrast (DIC) and fluorescence microscopy. Moreover, the availability of fluorescent pathogens and transgenic reporter zebrafish lines expressing fluorescent immune cells, immune marker genes or cytokines/chemokines allows the host-pathogen interactions to be characterized. The present study explores the suitability of zebrafish larvae to study the pathogenesis of A. hydrophila and the interaction mechanisms between the bacterium and the innate immune responses through an infection model using different routes for infection. We used an early-embryo infection model at 3 days post-fertilization (dpf) through the microinjection of A. hydrophila into the duct of Cuvier, caudal vein, notochord, or muscle and two bath infection models using 4 dpf healthy and injured larvae. The latter resembled the natural conditions under which A. hydrophila produces infectious diseases in animals. We compared the cellular processes after infection in each anatomical site by confocal fluorescence imaging and determined the implication of inflammatory immune genes by measuring gene expression by qPCR.

Aeromonas dhakensis, an Increasingly Recognized Human Pathogen

Aeromonas dhakensis was first isolated from children with diarrhea in Dhaka, Bangladesh and described in 2002. In the past decade, increasing evidence indicate this species is widely distributed in the environment and can cause a variety of infections both in human and animals, especially in coastal areas. A. dhakensis is often misidentified as A. hydrophila, A. veronii, or A. caviae by commercial phenotypic tests in the clinical laboratory. Correct identification relies on molecular methods. Increasingly used matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) may be able to identify Aeromonas specie rapidly and accurately. A. dhakensis has shown its potent virulence in different animal models and clinical infections. Although several virulence factors had been reported, no single mechanism is conclusive. Characteristically A. dhakensis is the principal species causing soft tissue infection and bacteremia, especially among patients with liver cirrhosis or malignancy. Of note, A. dhakensis bacteremia is more lethal than bacteremia due to other Aeromonas species. The role of this species in gastroenteritis remains controversial. Third generation cephalosporins and carbapenems should be used cautiously in the treatment of severe A. dhakensis infection due to the presence of AmpC ββ-lactamase and metallo-β-lactamase genes, and optimal regimens may be cefepime or fluoroquinolones. Studies of bacterial virulence factors and associated host responses may provide the chance to understand the heterogeneous virulence between species. The hypothesis A. dhakensis with varied geographic prevalence and enhanced virulence that compared to other Aeromonas species warrants more investigations.

Pan-genome analysis of Aeromonas hydrophila, Aeromonas veronii and Aeromonas caviae indicates phylogenomic diversity and greater pathogenic potential for Aeromonas hydrophila

Aeromonas species are important pathogens of fishes and aquatic animals capable of infecting humans and other animals via food. Due to the paucity of pan-genomic studies on aeromonads, the present study was undertaken to analyse the pan-genome of three clinically important Aeromonas species (A. hydrophila, A. veronii, A. caviae). Results of pan-genome analysis revealed an open pan-genome for all three species with pan-genome sizes of 9181, 7214 and 6884 genes for A. hydrophila, A. veronii and A. caviae, respectively. Core-genome: pan-genome ratio (RCP) indicated greater genomic diversity for A. hydrophila and interestingly RCP emerged as an effective indicator to gauge genomic diversity which could possibly be extended to other organisms too. Phylogenomic network analysis highlighted the influence of homologous recombination and lateral gene transfer in the evolution of Aeromonas spp. Prediction of virulence factors indicated no significant difference among the three species though analysis of pathogenic potential and acquired antimicrobial resistance genes revealed greater hazards from A. hydrophila. In conclusion, the present study highlighted the usefulness of whole genome analyses to infer evolutionary cues for Aeromonas species which indicated considerable phylogenomic diversity for A. hydrophila and hitherto unknown genomic evidence for pathogenic potential of A. hydrophila compared to A. veronii and A. caviae.

Clinical implications of species identification in monomicrobial Aeromonas bacteremia

BACKGROUND

Advances in Aeromonas taxonomy have led to the reclassification of aeromonads. Hereon, we aimed to re-evaluate the characteristics of Aeromonas bacteremia, including those of a novel species, Aeromonas dhakensis.

METHODOLOGY/PRINCIPAL FINDINGS

A retrospective study of monomicrobial Aeromonas bacteremia at a medical center in southern Taiwan from 2004-2011 was conducted. Species identification was based on rpoB sequencing. Of bacteremia of 153 eligible patients, A. veronii (50 isolates, 32.7%), A. dhakensis (48, 31.4%), A. caviae (43, 28.1%), and A. hydrophila (10, 6.5%) were the principal causative species. A. dhakensis and A. veronii bacteremia were mainly community-acquired and presented as primary bacteremia, spontaneous bacterial peritonitis, or skin and soft-tissue infection, whereas A. caviae was associated with hospital-onset bacteremia. The distribution of the AmpC β-lactamase and metallo-β-lactamase genes was species-specific: bla(AQU-1), bla(MOX), or bla(CepH) was present in A. dhakensis, A. caviae, or A. hydrophila, respectively, and bla(CphA) was present in A. veronii, A. dhakensis, and A. hydrophila. The cefotaxime resistance rates of the A. caviae, A. dhakensis, and A. hydrophila isolates were higher than that of A. veronii (39.5%%, 25.0%, and 30% vs. 2%, respectively). A. dhakensis bacteremia was linked to the highest 14-day sepsis-related mortality rate, followed by A. hydrophila, A. veronii, and A. caviae bacteremia (25.5%, 22.2%, 14.0%, and 4.7%, respectively; P = 0.048). Multivariate analysis revealed that A. dhakensis bacteremia, active malignancies, and a Pitt bacteremia score ≥ 4 was an independent mortality risk factor.

CONCLUSIONS/SIGNIFICANCE

Characteristics of Aeromonas bacteremia vary between species. A. dhakensis prevalence and its associated poor outcomes suggest it an important human pathogen.