Insight Into Whole Genome of Aeromonas veronii Isolated From Freshwater Fish by Resistome Analysis Reveal Extensively Antibiotic Resistant Traits

Phylogenomics of novel clones of Aeromonas veronii recovered from a freshwater lake reveals unique biosynthetic gene clusters

Aquatic ecosystems serve as crucial reservoirs for pathogens and antimicrobial resistance genes, thus presenting a significant global health risk. Here, we investigated the phylogenomics of Aeromonas veronii from Lake Wilcox in Ontario. Among the 11 bacterial isolates, nine were identified as A. veronii. Notably, 67% of A. veronii isolates were potential human pathogens. Considerable genetic diversity was noted among the A. veronii isolates, suggesting the lake as a reservoir for multiple human pathogenic strains. Comparison of the A. veronii sequenced with global A. veronii genomes highlighted significant genetic diversity and suggests widespread dissemination of strains. All the isolates carried chromosomal genes encoding resistance to β-lactams. Although virulence gene content differed between human and non-human pathogenic strains, type III secretion systems was associated with human pathogenic isolates. The assessment of AMR genes in global isolates showed that β-lactam and tetracycline resistance genes were predominant. Although the machine learning-based pangenome-wide association approach performed did not yield any source-based genes, some genes were enriched in a few isolates from different sources. The mrkABCDF operon that mediates biofilm formation and genes encoding resistance to colistin, chloramphenicol, trimethoprim, and tetracycline were enriched in animal products, whereas macrolide resistance genes and Inc plasmid-types were linked to the aquatic environment. Novel biosynthetic gene clusters were identified, suggesting that A. veronii with varying pathogenic potential could produce unique secondary metabolites. There is a need for continuous tracking of pathogens in aquatic ecosystems to contribute to our understanding of their evolutionary dynamics and the ecological roles of their genetic elements.

IMPORTANCE

Lakes and other aquatic ecosystems can harbor harmful bacteria that can make people sick and resist antibiotics, posing a significant global health risk. In this study, we investigated Aeromonas veronii, a Gram-negative bacteria found in Lake Wilcox in Ontario. We used various techniques, including whole-genome sequencing (WGS), to analyze the bacteria and found that many of the isolates had the potential to cause human disease. We also discovered significant genetic diversity among the isolates, indicating that the lake may be a reservoir for multiple human pathogenic strains. All isolates carried genes that confer resistance to antibiotics, and some virulence genes were associated with human pathogenic isolates. This study highlights the importance of monitoring aquatic ecosystems for harmful bacteria to better understand their evolution, potential for human pathogenicity, and the ecological roles of their genetic elements. This knowledge can inform strategies for preventing the spread of antibiotic-resistant bacteria and protecting public health.

The Antibacterial Efficacy and Mechanism of Tea Polyphenol Against Drug-Resistant Aeromonas veronii TH0426 In Vitro

The emergence of Motile Aeromonas Septicemia (MAS) caused by Aeromonas veronii in sturgeon farming has become a significant concern due to its high mortality impact on the aquaculture industry. The threat posed by MAS highlights the urgent need for effective control measures to combat bacterial infections in sturgeon populations. Tea polyphenol (TP) has demonstrated promising antibacterial properties against livestock and poultry bacterial infections. However, its antibacterial efficacy and mechanism in bacterial diseases of aquatic animals remain largely unexplored. This study aimed to investigate the in vitro antibacterial effect and mechanism of TP on fish-borne drug-resistant A. veronii TH0426 by assessing the impact of TP on TH0426 cell growth, antibiofilm activity, morphology, as well as measuring electrical conductivity, DNA extravasation, lactate dehydrogenase (LDH) activity, protein, and DNA contents. Results demonstrated that the minimum inhibitory concentration and the minimum bactericidal concentration of TP on TH0426 were 1024 and 2048 μg/mL, respectively. After a 4 h treatment, the growth of TH0426 was completely inhibited at the concentration of 1024 and 2048 μg/mL of TP. Meanwhile, TP exhibited a significant antibiofilm activity. Both scanning electron microscope and transmission electron microscope analyses revealed disrupted cell membrane structure, irregular cell morphology, and loss of intracellular contents following TP treatment. Moreover, increased cell membrane permeability induced by TP led to intracellular ion and DNA leakage, resulting in elevated electrical conductivity and DNA extravasation. Furthermore, TP decreased LDH activity, protein concentration and content, DNA fluorescence intensity, and density in a time-dependent manner, indicating inhibition of protein metabolism and DNA synthesis. In conclusion, TP exhibits potent antibacterial properties by inhibiting biofilm formation, disrupting cell membrane integrity, and interfering with protein metabolism and DNA synthesis in drug-resistant A. veronii TH0426 in vitro.

Changing epidemiological trend of Aeromonas species and ciprofloxacin sensitivity in South India

Aeromonas species can cause a wide range of clinical infections. Several reports of drug resistance among the Aeromonas species have been reported, but our observations have differed. Here we present the changing susceptibility pattern of antibiotics for Aeromonas species over 14 years (January 2010-February 2024) at a tertiary care hospital in South India.

Identification and characterization of a novel type II toxin-antitoxin system in Aeromonas veronii

The bacterial type II toxin-antitoxin (TA) system is a rich genetic element that participates in various physiological processes. Aeromonas veronii is the main bacterial pathogen threatening the freshwater aquaculture industry. However, the distribution of type II TA system in A. veronii was seldom documented and its roles in the life activities of A. veronii were still unexplored. In this study, a novel type II TA system AvtA-AvtT was predicted in a fish pathogen Aeromonas veronii biovar sobria with multi-drug resistance using TADB 2.0. Through an Escherichia coli host killing and rescue assay, we demonstrated that AvtA and AvtT worked as a genuine TA system, and the predicted toxin AvtT actually functioned as an antitoxin while the predicted antitoxin AvtA actually functioned as a toxin. The binding ability of AvtA with AvtT proteins were confirmed by dot blotting analysis and co-immunoprecipitation assay. Furthermore, we found that the toxin and antitoxin labelled with fluorescent proteins were co-localized. In addition, it was found that the transcription of AvtAT bicistronic operon was repressed by the AvtAT protein complex. Deletion of avtA gene and avtT gene had no obvious effect on the drug susceptibility. This study provides first characterization of type II TA system AvtA-AvtT in aquatic pathogen A. veronii.

Two colistin resistance-producing Aeromonas strains, isolated from coastal waters in Zhejiang, China: characteristics, multi-drug resistance and pathogenicity

Introduction

Aeromonas spp. are ubiquitous inhabitants of ecosystems, and many species are opportunistically pathogenic to humans and animals. Multidrug-resistant (MDR) Aeromonas species have been widely detected in hospitals, urban rivers, livestock, and aquatic animals.

Results

In this study, we identified two Aeromonas isolates, namely Aeromonas veronii 0728Q8Av and Aeromonas caviae 1029Y16Ac, from coastal waters in Zhejiang, China. Both isolates exhibited typical biochemical characteristics and conferred MDR to 11 kinds of antibiotics, remaining susceptible to ceftazidime. Whole-genome sequencing revealed that both isolates harbored multiple antibiotic resistance genes (ARGs) and several mobile genetic elements (MGEs) on the chromosomes, each containing a resistance genomic island (GI), a typical class 1 integron, a transposon, and various insertion sequences (ISs). Most ARGs were situated within the multiple resistance GI, which contained a class 1 integron and a transposon in both Aeromonas isolates. Furthermore, a chromosomal mcr-3.16 gene was identified in A. veronii 0728Q8Av, while a chromosomal mcr-3.3 was found in A. caviae 1029Y16Ac. Both mcr-3 variants were not located within but were distanced from the multidrug resistance GI on the chromosome, flanking by multiple ISs. In addition, a mcr-3-like was found adjacent to mcr-3.16 to form a tandem mcr-3.16-mcr-3-like-dgkA structure; yet, Escherichia coli carrying the recombinants of mcr-3-like did not exhibit resistance to colistin. And an incomplete mcr-3-like was found adjacent to mcr-3.3 in A. caviae 1029Y16Ac, suggesting the possibility that mcr-3 variants originated from Aeromonas species. In vivo bacterial pathogenicity test indicated that A. veronii 0728Q8Av exhibited moderate pathogenicity towards infected ayu, while A. caviae 1029Y16Ac was non-virulent.

Discussion

Thus, both Aeromonas species deserve further attention regarding their antimicrobial resistance and pathogenicity.

Systemic and mucosal immune responses in red tilapia (Oreochromis sp.) following immersion vaccination with a chitosan polymer-based nanovaccine against Aeromonas veronii

A mucoadhesive chitosan polymer-based nanoplatform has been increasingly recognized as an effective mucosal vaccine delivery system for fish. The present study aimed to investigate the effectiveness of immersion vaccination with a chitosan polymer-based nanovaccine to elicit an immune response in serum and mucus of red tilapia and evaluate its protective efficacy after immersion challenge with a heterogenous strain of Aeromonas veronii UDRT09. Six hundred red tilapia (22 ± 1.8 g) were randomly allocated into four experimental groups: control, empty-polymeric nanoparticle (PC), formalin-killed vaccine (FKV), and chitosan polymer-based nanovaccine (CS-NV) in triplicate. The specific IgM antibody levels and their bactericidal activity were assessed in serum and mucus for 28 days after immersion vaccination and followed by immersion challenge with A. veronii. The immersion vaccine was found to be safe for red tilapia, with no mortalities occurring during the vaccination procedure. The specific IgM antibody levels and bactericidal activity against A. veronii in both serum and mucus were significantly higher in red tilapia vaccinated with CS-NV compared to the FKV and control groups at all time points. Furthermore, the serum lysozyme activity, ACH50, and total Ig levels demonstrated a significant elevation in the groups vaccinated with CS-NV compared to the FKV and control groups. Importantly, the Relative Percentage Survival (RPS) value of the CS-NV group (71 %) was significantly higher than that of the FKV (15.12 %) and PC (2.33 %) groups, respectively. This indicates that the chitosan polymer-based nanovaccine platform is an effective delivery system for the immersion vaccination of tilapia.

Addressing Nanovaccine Strategies for Tilapia

Tilapia is the world's most extensively farmed species after carp. It is an attractive species for aquaculture as it grows quickly, reaching harvest size within six to seven months of production, and provides an important source of food and revenue for many low-income families, especially in low- to middle-income countries. The expansion of tilapia aquaculture has resulted in an intensification of farming systems, and this has been associated with increased disease outbreaks caused by various pathogens, mostly bacterial and viral agents. Vaccination is routinely used to control disease in higher-value finfish species, such as Atlantic salmon. At the same time, many tilapia farmers are often unwilling to vaccinate their fish by injection once the fish have been moved to their grow-out site. Alternative vaccination strategies are needed to help tilapia farmers accept and use vaccines. There is increasing interest in nanoparticle-based vaccines as alternative methods for delivering vaccines to fish, especially for oral and immersion administration. They can potentially improve vaccine efficacy through the controlled release of antigens, protecting antigens from premature proteolytic degradation in the gastric tract, and facilitating antigen uptake and processing by antigen-presenting cells. They can also allow targeted delivery of the vaccine at mucosal sites. This review provides a brief overview of the bacterial and viral diseases affecting tilapia aquaculture and vaccine strategies for farmed tilapia. It focuses on the use of nanovaccines to improve the acceptance and uptake of vaccines by tilapia farmers.

First Report of Aeromonas veronii as an Emerging Bacterial Pathogen of Farmed Nile Tilapia (Oreochromis niloticus) in Brazil

Brazil is one of the world's leading producers of Nile tilapia, Oreochromis niloticus. However, the industry faces a major challenge in terms of infectious diseases, as at least five new pathogens have been formally described in the last five years. Aeromonas species are Gram-negative anaerobic bacteria that are often described as fish pathogens causing Motile Aeromonas Septicemia (MAS). In late December 2022, an epidemic outbreak was reported in farmed Nile tilapia in the state of São Paulo, Brazil, characterized by clinical signs and gross pathology suggestive of MAS. The objective of this study was to isolate, identify, and characterize in vitro and in vivo the causative agent of this epidemic outbreak. The bacterial isolates were identified as Aeromonas veronii based on the homology of 16S rRNA (99.9%), gyrB (98.9%), and the rpoB gene (99.1%). A. veronii showed susceptibility only to florfenicol, while it was resistant to the other three antimicrobials tested, oxytetracycline, enrofloxacin, and amoxicillin. The lowest florfenicol concentration capable of inhibiting bacterial growth was ≤0.5 µg/mL. The phenotypic resistance of the A. veronii isolate observed for quinolones and tetracycline was genetically confirmed by the presence of the qnrS2 (colE plasmid) and tetA antibiotic-resistant genes, respectively. A. veronii isolate was highly pathogenic in juvenile Nile tilapia tested in vivo, showing a mortality rate ranging from 3 to 100% in the lowest (1.2 × 104) and highest (1.2 × 108) bacterial dose groups, respectively. To our knowledge, this study would constitute the first report of highly pathogenic and multidrug-resistant A. veronii associated with outbreaks and high mortality rates in tilapia farmed in commercial net cages in Brazil.

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.

Aeromonas hydrophila ST251 and Aeromonas dhakensis are major emerging pathogens of striped catfish in Vietnam

Introduction

Aeromonads are ubiquitous in aquatic environments and several species are opportunistic pathogens of fish. Disease losses caused by motile Aeromonas species, particularly Aeromonas hydrophila, can be challenging in intensive aquaculture, such as at striped catfish (Pangasianodon hypophthalmus) farms in Vietnam. Outbreaks require antibiotic treatments, but their application is undesirable due to risks posed by resistance. Vaccines are an attractive prophylactic and they must protect against the prevalent strains responsible for ongoing outbreaks.

Methods

This present study aimed to characterize A. hydrophila strains associated with mortalities in striped catfish culture in the Mekong Delta by a polyphasic genotyping approach, with a view to developing more effective vaccines.

Results

During 2013-2019, 345 presumptive Aeromonas spp. isolates were collected at farms in eight provinces. Repetitive element sequence-based PCR, multi-locus sequence typing and whole-genome sequencing revealed most of the suspected 202 A. hydrophila isolates to belong to ST656 (n = 151), which corresponds to the closely-related species Aeromonas dhakensis, with a lesser proportion belonging to ST251 (n = 51), a hypervirulent lineage (vAh) of A. hydrophila already causing concern in global aquaculture. The A. dhakensis ST656 and vAh ST251 isolates from outbreaks possessed unique gene sets compared to published A. dhakensis and vAh ST251 genomes, including antibiotic-resistance genes. The sharing of resistance determinants to sulphonamides (sul1) and trimethoprim (dfrA1) suggests similar selection pressures acting on A. dhakensis ST656 and vAh ST251 lineages. The earliest isolate (a vAh ST251 from 2013) lacked most resistance genes, suggesting relatively recent acquisition and selection, and this underscores the need to reduce antibiotics use where possible to prolong their effectiveness. A novel PCR assay was designed and validated to distinguish A. dhakensis and vAh ST251 strains.

Discussion

This present study highlights for the first time A. dhakensis, a zoonotic species that can cause fatal human infection, to be an emerging pathogen in aquaculture in Vietnam, with widespread distribution in recent outbreaks of motile Aeromonas septicaemia in striped catfish. It also confirms vAh ST251 to have been present in the Mekong Delta since at least 2013. Appropriate isolates of A. dhakensis and vAh should be included in vaccines to prevent outbreaks and reduce the threat posed by antibiotic resistance.

Aeromonas species isolated from aquatic organisms, insects, chicken, and humans in India show similar antimicrobial resistance profiles

Aeromonas species are Gram-negative bacteria that infect various living organisms and are ubiquitously found in different aquatic environments. In this study, we used whole genome sequencing (WGS) to identify and compare the antimicrobial resistance (AMR) genes, integrons, transposases and plasmids found in Aeromonas hydrophila, Aeromonas caviae and Aeromonas veronii isolated from Indian major carp (Catla catla), Indian carp (Labeo rohita), catfish (Clarias batrachus) and Nile tilapia (Oreochromis niloticus) sampled in India. To gain a wider comparison, we included 11 whole genome sequences of Aeromonas spp. from different host species in India deposited in the National Center for Biotechnology Information (NCBI). Our findings show that all 15 Aeromonas sequences examined had multiple AMR genes of which the Ambler classes B, C and D β-lactamase genes were the most dominant. The high similarity of AMR genes in the Aeromonas sequences obtained from different host species point to interspecies transmission of AMR genes. Our findings also show that all Aeromonas sequences examined encoded several multidrug efflux-pump proteins. As for genes linked to mobile genetic elements (MBE), only the class I integrase was detected from two fish isolates, while all transposases detected belonged to the insertion sequence (IS) family. Only seven of the 15 Aeromonas sequences examined had plasmids and none of the plasmids encoded AMR genes. In summary, our findings show that Aeromonas spp. isolated from different host species in India carry multiple AMR genes. Thus, we advocate that the control of AMR caused by Aeromonas spp. in India should be based on a One Health approach.

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.