The cytotoxic enterotoxin of Aeromonas hydrophila induces proinflammatory cytokine production and activates arachidonic acid metabolism in macrophages

Exploring Aeromonas dhakensis in Aldabra giant tortoises: a debut report and genetic characterization

Aeromonas dhakensis (A. dhakensis) is becoming an emerging pathogen worldwide, with an increasingly significant role in animals and human health. It is a ubiquitous bacteria found in terrestrial and aquatic milieus. However, there have been few reports of reptile infections. In this study, a bacterial strain isolated from a dead Aldabra giant tortoise was identified as A. dhakensis HN-1 through clinical observation, matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF/MS), and gene sequencing analysis. Subsequently, to evaluate its pathogenicity, the detection of virulence genes and mice infection experiments were performed. A. dhakensis HN-1 was found to contain seven virulence genes, including alt, ela, lip, act, aerA, fla, and hlyA. Mice infected with A. dhakensis HN-1 exhibited hemorrhage of varying degrees in multiple organs. The half-maximal lethal dose (LD50) value of A. dhakensis HN-1 for mice was estimated to be 2.05 × 107 colony forming units (CFU)/mL. The antimicrobial susceptibility test revealed that A. dhakensis HN-1 was resistant to amoxicillin, penicillin, ampicillin and erythromycin. This is the first report of A. dhakensis in Aldabra giant tortoises, expanding the currently known host spectrum. Our findings emphasize the need for One Health surveillance and extensive research to reduce the spread of A. dhakensis across the environment, humans, and animals.

Antimicrobial resistance in aeromonads and new therapies targeting quorum sensing

Aeromonas species (spp.) are well-known fish pathogens, several of which have been recognized as emerging human pathogens. The organism is capable of causing a wide spectrum of diseases in humans, ranging from gastroenteritis, wound infections, and septicemia to devastating necrotizing fasciitis. The systemic form of infection is often fatal, particularly in patients with underlying chronic diseases. Indeed, recent trends demonstrate rising numbers of hospital-acquired Aeromonas infections, especially in immuno-compromised individuals. Additionally, Aeromonas-associated antibiotic resistance is an increasing challenge in combating both fish and human infections. The acquisition of antibiotic resistance is related to Aeromonas' innate transformative properties including its ability to share plasmids and integron-related gene cassettes between species and with the environment. As a result, alternatives to antibiotic treatments are desperately needed. In that vein, many treatments have been proposed and studied extensively in the fish-farming industry, including treatments that target Aeromonas quorum sensing. In this review, we discuss current strategies targeting quorum sensing inhibition and propose that such studies empower the development of novel chemotherapeutic approaches to combat drug-resistant Aeromonas spp. infections in humans. KEY POINTS: • Aeromonas notoriously acquires and maintains antimicrobial resistance, making treatment options limited. • Quorum sensing is an essential virulence mechanism in Aeromonas infections. • Inhibiting quorum sensing can be an effective strategy in combating Aeromonas infections in animals and humans.

Ocular mucosal homeostasis of teleost fish provides insight into the coevolution between microbiome and mucosal immunity

BACKGROUND

The visual organ plays a crucial role in sensing environmental information. However, its mucosal surfaces are constantly exposed to selective pressures from aquatic or airborne pathogens and microbial communities. Although few studies have characterized the conjunctival-associated lymphoid tissue (CALT) in the ocular mucosa (OM) of birds and mammals, little is known regarding the evolutionary origins and functions of immune defense and microbiota homeostasis of the OM in the early vertebrates.

RESULTS

Our study characterized the structure of the OM microbial ecosystem in rainbow trout (Oncorhynchus mykiss) and confirmed for the first time the presence of a diffuse mucosal-associated lymphoid tissue (MALT) in fish OM. Moreover, the microbial communities residing on the ocular mucosal surface contribute to shaping its immune environment. Interestingly, following IHNV infection, we observed robust immune responses, significant tissue damage, and microbial dysbiosis in the trout OM, particularly in the fornix conjunctiva (FC), which is characterized by the increase of pathobionts and a reduction of beneficial taxa in the relative abundance in OM. Critically, we identified a significant correlation between viral-induced immune responses and microbiome homeostasis in the OM, underscoring its key role in mucosal immunity and microbiota homeostasis.

CONCLUSIONS

Our findings suggest that immune defense and microbiota homeostasis in OM occurred concurrently in early vertebrate species, shedding light on the coevolution between microbiota and mucosal immunity. Video Abstract.

Nelumbo nucifera synthesized selenium nanoparticles modulate the immune-antioxidants, biochemical indices, and pro/anti-inflammatory cytokines pathways in Oreochromis niloticus infected with Aeromonas veronii

Bacterial infection is considered one of the major issues in fish culturing that results in economic losses. Metal nanoparticles are a cutting-edge and effective disease management and preventive strategy because of their antibacterial ability. In this investigation, the selenium nanoparticles were prepared by a biological method using Nelumbo nucifera leaves extract. The in-vitro antibacterial activity of N. nucifera synthesized selenium nanoparticles (NN-SeNPs) was tested against Aeromonas veronii. A treatment assay was conducted on 210 Oreochromis niloticus (average body weight: 27 ± 2.00 g). A preliminary approach was conducted on 90 fish for determination of the therapeutic concentration of NN-SeNPs which was found to be 4 mg/L. Fish (n = 120) were categorized into four groups for 10 days; G1 (control) and G2 (NN-SeNPs) were non-challenged and treated with 0 and 4 mg/L NN-SeNPs, respectively. While, G3 and G4 were infected with 2 × 106 CFU/mL of A. veronii and treated with 0 and 4 mg/L NN-SeNPs, respectively. NN-SeNPs exhibited an inhibition zone against A. veronii with a diameter of 16 ± 1.25 mm. The A. veronii infection increased the hepato-renal biomarkers (alanine and aspartate aminotransferases and creatinine) than the control group. An oxidative stress was the consequence of A. veronii infection (higher malondialdehyde and hydrogen peroxide levels with lower glutathione peroxidase superoxide, dismutase, and catalase activity). A. veronii infection resulted in lower immunological biomarker values (immunoglobulin M, lysozyme, and complement 3) with higher expression of the inflammatory cytokines (interleukin-1β and tumor necrosis factor-ɑ) as well as lower expression of the anti-inflammatory cytokines (interleukin-10 and transforming growth factor-β). Therapeutic application with 4 mg/L NN-SeNPs prevented the disease progression; and modulated the hepato-renal function disruptions, oxidant-immune dysfunction, as well as the pro/anti-inflammatory cytokines pathway in the A. veronii-infected fish. These findings suggest that NN-SeNPs, employed as a water therapy, can safeguard fish from the harmful effects of A. veronii and serve as a promising antibacterial agent for sustainable aquaculture.

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.

Surface water quality, public health, and ecological risks in Bangladesh-a systematic review and meta-analysis over the last two decades

Water quality has recently emerged as one of the utmost severe ecological problems being faced by the developing countries all over the world, and Bangladesh is no exception. Both surface and groundwater sources contain different contaminants, which lead to numerous deaths due to water-borne diseases, particularly among children. This study presents one of the most comprehensive reviews on the current status of water quality in Bangladesh with a special emphasis on both conventional pollutants and emerging contaminants. Data show that urban rivers in Bangladesh are in a critical condition, especially Korotoa, Teesta, Rupsha, Pashur, and Padma. The Buriganga River and few locations in the Turag, Balu, Sitalakhya, and Karnaphuli rivers have dissolvable oxygen (DO) levels of almost zero. Many waterways contain traces of NO3, NO2, and PO4-3 pollutants. The majority of the rivers in Bangladesh also have Zn, Cu, Fe, Pb, Cd, Ni, Mn, As, and Cr concentrations that exceed the WHO permissible limits for safe drinking water, while their metal concentrations exceed the safety threshold for irrigation. Mercury poses the greatest hazard with 90.91% of the samples falling into the highest risk category. Mercury is followed by zinc 57.53% and copper 29.16% in terms of the dangers they pose to public health and the ecosystem. Results show that a considerable percentage of the population is at risk, being exposed to contaminated water. Despite hundreds of cryptosporidiosis cases reported, fecal contamination, i.e., Cryptosporidium, is totally ignored and need serious considerations to be regularly monitored in source water.

β-Catenin Elicits Drp1-Mediated Mitochondrial Fission Activating the Pro-Apoptotic Caspase-1/IL-1β Signalosome in Aeromonas hydrophila-Infected Zebrafish Macrophages

Canonical Wnt signaling plays a major role in regulating microbial pathogenesis. However, to date, its involvement in A. hydrophila infection is not well known. Using zebrafish (Danio rerio) kidney macrophages (ZKM), we report that A. hydrophila infection upregulates wnt2, wnt3a, fzd5, lrp6, and β-catenin (ctnnb1) expression, coinciding with the decreased expression of gsk3b and axin. Additionally, increased nuclear β-catenin protein accumulation was observed in infected ZKM, thereby suggesting the activation of canonical Wnt signaling in A. hydrophila infection. Our studies with the β-catenin specific inhibitor JW67 demonstrated β-catenin to be pro-apoptotic, which initiates the apoptosis of A. hydrophila-infected ZKM. β-catenin induces NADPH oxidase (NOX)-mediated ROS production, which orchestrates sustained mitochondrial ROS (mtROS) generation in the infected ZKM. Elevated mtROS favors the dissipation of the mitochondrial membrane potential (ΔΨm) and downstream Drp1-mediated mitochondrial fission, leading to cytochrome c release. We also report that β-catenin-induced mitochondrial fission is an upstream regulator of the caspase-1/IL-1β signalosome, which triggers the caspase-3 mediated apoptosis of the ZKM as well as A. hydrophila clearance. This is the first study suggesting a host-centric role of canonical Wnt signaling pathway in A. hydrophila pathogenesis wherein β-catenin plays a primal role in activating the mitochondrial fission machinery, which actively promotes ZKM apoptosis and helps in containing the bacteria.

Isolation of Aeromonas salmonicida subspecies salmonicida from aquaculture environment in India: Polyphasic identification, virulence characterization, and antibiotic susceptibility

This study reports the polyphasic identification, characterization of virulence potential, and antibiotic susceptibility of Aeromonas salmonicida subspecies salmonicida COFCAU_AS, isolated from an aquaculture system in India. The physiological, biochemical, 16s rRNA gene sequencing and PAAS PCR test identified the strain as Aeromonas salmonicida. The MIY PCR tests established the subspecies as 'salmonicida'. The in vitro tests showed the isolated bacterium as haemolytic with casein, lipid, starch, and gelatin hydrolysis activity, indicating its pathogenic attributes. It also showed the ability to produce slime and biofilm, and additionally, it possessed an A-layer surface protein. In vivo pathogenicity test was performed to determine the LD₅₀ dose of the bacterium in Labeo rohita fingerlings (14.42 ± 1.01 g), which was found to be 10^6.9 cells fish^-1. The bacteria-challenged fingerlings showed skin lesions, erythema at the base of the fins, dropsy, and ulcer. Almost identical clinical signs and mortalities were observed when the same LD₅₀ dose was injected into other Indian major carp species, L. catla and Cirrhinus mrigala. Out of the twelve virulent genes screened, the presence of nine genes viz., aerA, act, ast, alt, hlyA, vapA, exsA, fstA, and lip were detected, whereas ascV, ascC, and ela genes were absent. The A. salmonicida subsp. salmonicida COFCAU_AS was resistant to antibiotics such as penicillin G, rifampicin, ampicillin, and vancomycin while highly sensitive to amoxiclav, nalidixic acid, chloramphenicol, ciprofloxacin, and tetracycline. In summary, we have isolated a virulent A. salmonicida subsp. salmonicida from a tropical aquaculture pond which can cause significant mortality and morbidity in Indian major carp species.

Metabolite features of serum and intestinal microbiota response of largemouth bass (Micropterus salmoides) after Aeromonas hydrophila challenge

The enteric morphology, enteric microbiota structure and serum metabolomics of M. salmoides before and after infected by A. hydrophila were analysed to explore the pathogenic mechanism of A. hydrophila infection in M. salmoides. The results revealed that, after the infection of A. hydrophila, the villus boundary of largemouth bass became less obvious; the relative abundance of Proteobacteria and decreasing relative abundance of Tenericutes were increasing; genera relative abundance of putatively beneficial bacteria (Mycoplasma) were decreasing, whereas the genus Aeromonas increased after infection; serum metabolomic analysis showed that infection with A. hydrophila caused disorder to the metabolic processes of largemouth bass, particularly amino acid metabolism, and caused inflammation; several potential pathogen infection-related and significantly differential intestinal microbiota-related metabolite markers were identified, such as 6-hydroxy-5-methoxyindole glucuronide, zalcitabine, bilirubin, aciclovir. This study may provide new insights into the potential association between enteric microbiota and serum metabolism and the pathogenic mechanism of M. salmoides infected by A. hydrophila, providing a scientific basis for disease control in largemouth bass breeding.

Evaluation of Proteomic and Lipidomic Changes in Aeromonas-Infected Trout Kidney Tissue with the Use of FT-IR Spectroscopy and MALDI Mass Spectrometry Imaging

Aeromonas species are opportunistic bacteria causing a vast spectrum of human diseases, including skin and soft tissue infections, meningitis, endocarditis, peritonitis, gastroenteritis, and finally hemorrhagic septicemia. The aim of our research was to indicate the molecular alterations in proteins and lipids profiles resulting from Aeromonas sobria and A. salmonicida subsp. salmonicida infection in trout kidney tissue samples. We successfully applied FT-IR (Fourier transform infrared) spectroscopy and MALDI-MSI (matrix-assisted laser desorption/ionization mass spectrometry imaging) to monitor changes in the structure and compositions of lipids, secondary conformation of proteins, and provide useful information concerning disease progression. Our findings indicate that the following spectral bands’ absorbance ratios (spectral biomarkers) can be used to discriminate healthy tissue from pathologically altered tissue, for example, lipids (CH2/CH3), amide I/amide II, amide I/CH2 and amide I/CH3. Spectral data obtained from 10 single measurements of each specimen indicate numerous abnormalities concerning proteins, lipids, and phospholipids induced by Aeromonas infection, suggesting significant disruption of the cell membranes. Moreover, the increase in the content of lysolipids such as lysophosphosphatidylcholine was observed. The results of this study suggest the application of both methods MALDI-MSI and FT-IR as accurate methods for profiling biomolecules and identifying biochemical changes in kidney tissue during the progression of Aeromonas infection.

Natterin-Induced Neutrophilia Is Dependent on cGAS/STING Activation via Type I IFN Signaling Pathway

Natterin is a potent pro-inflammatory fish molecule, inducing local and systemic IL-1β/IL-1R1-dependent neutrophilia mediated by non-canonical NLRP6 and NLRC4 inflammasome activation in mice, independent of NLRP3. In this work, we investigated whether Natterin activates mitochondrial damage, resulting in self-DNA leaks into the cytosol, and whether the DNA sensor cGAS and STING pathway participate in triggering the innate immune response. Employing a peritonitis mouse model, we found that the deficiency of the tlr2/tlr4, myd88 and trif results in decreased neutrophil influx to peritoneal cavities of mice, indicative that in addition to MyD88, TRIF contributes to neutrophilia triggered by TLR4 engagement by Natterin. Next, we demonstrated that gpcr91 deficiency in mice abolished the neutrophil recruitment after Natterin injection, but mice pre-treated with 2-deoxy-d-glucose that blocks glycolysis presented similar infiltration than WT Natterin-injected mice. In addition, we observed that, compared with the WT Natterin-injected mice, DPI and cyclosporin A treated mice had a lower number of neutrophils in the peritoneal exudate. The levels of dsDNA in the supernatant of the peritoneal exudate and processed IL-33 in the supernatant of the peritoneal exudate or cytoplasmic supernatant of the peritoneal cell lysate of WT Natterin-injected mice were several folds higher than those of the control mice. The recruitment of neutrophils to peritoneal cavity 2 h post-Natterin injection was intensely impaired in ifnar KO mice and partially in il-28r KO mice, but not in ifnγr KO mice. Finally, using cgas KO, sting KO, or irf3 KO mice we found that recruitment of neutrophils to peritoneal cavities was virtually abolished in response to Natterin. These findings reveal cytosolic DNA sensors as critical regulators for Natterin-induced neutrophilia.

Lethal dose and histopathological alterations induced by Aeromonas salmonicida in experimentally challenged common carp, Cyprinus carpio

Aeromonas salmonicida is the obligate pathogen of fishes having zoonotic potential. It is reported to cause considerable losses in world aquaculture. The current study has successfully demonstrated the induction of histopathological lesions in experimentally infected common carp. In the current study, the lethal concentration (LD₅₀-96 h) of typical A. Salmonicida for common carp was found to be 1.5 × 10⁷CFU mL^-1. About 40% and 60% fish mortalities occurred after 72 h in the groups inoculated with 10⁷ and 10⁸ CFU mL^-1 bacterial suspension, respectively. The fish challenged with A. salmonicida showed symptoms like abnormal swimming behaviour, lethargy, intra-abdominal fluid, haemorrhages on the ventral side of the body, vent and fins. The signs proceeded with the death of fish. In the histological sections, severe pathological alterations were reported in the tissue sections of internal organs. The microscopic observation showed sinusoidal and large blood vessel congestion in the liver, profuse haemorrhage, necrosis and infiltration of blood cells in the internal organs. The tubular architecture was lost with the infiltration of leucocytes in the kidney. In gills, more intense and prominent lamellar fusion was observed with leucocytic infiltration, telangiectasia and hyperplasia of lamellar epithelial cells. In summary, we have experimentally induced the typical A. salmonicida infection in common carp. The study will provide a research foundation for further studies on the host-pathogen interaction, therapeutics and epidemiology of A. salmonicida.

Construction and Characterization of an Aeromonas hydrophila Multi-Gene Deletion Strain and Evaluation of Its Potential as a Live-Attenuated Vaccine in Grass Carp

Aeromonas hydrophila is an important pathogen that causes motile Aeromonas septicemia (MAS) in the aquaculture industry. Aerolysin, hemolysin, serine protease and enterotoxins are considered to be the major virulence factors of A. hydrophila. In this study, we constructed a five-gene (aerA, hly, ahp, alt and ast) deletion mutant strain (named Aeromonas hydrophila five-gene deletion strain, AHFGDS) to observe the biological characteristics and detect its potential as a live-attenuated vaccine candidate. AHFGDS displayed highly attenuated and showed increased susceptibility to fish blood and skin mucus killing, while the wild-type strain ZYAH72 was highly virulent. In zebrafish (Danio rerio), AHFGDS showed a 240-fold higher 50% lethal dose (LD50) than that of the wild-type strain. Immunization with AHFGDS by intracelomic injection or immersion routes both provided grass carp (Ctenopharyngodon idella) significant protection against the challenge of the strain ZYAH72 or J-1 and protected the fish organs from serious injury. Further agglutinating antibody titer test supported that AHFGDS could elicit a host-adaptive immune response. These results suggested the potential of AHFGDS to serve as a live-attenuated vaccine to control A. hydrophila infection in aquaculture.

Isolation and characterization of two virulent Aeromonads associated with haemorrhagic septicaemia and tail-rot disease in farmed climbing perch Anabas testudineus

Diseased Anabas testudineus exhibiting signs of tail-rot and ulcerations on body were collected from a fish farm in Assam, India during the winter season (November 2018 to January 2019). Swabs from the infected body parts were streaked on sterilized nutrient agar. Two dominant bacterial colonies were obtained, which were then isolated and labelled as AM-31 and AM-05. Standard biochemical characterisation and 16S rRNA and rpoB gene sequencing identified AM-31 isolate as Aeromonas hydrophila and AM-05 as Aeromonas jandaei. Symptoms similar to that of natural infection were observed on re-infecting both bacteria to disease-free A. testudineus, which confirmed their virulence. LC50 was determined at 1.3 × 104 (A. hydrophila) and 2.5 × 104 (A. jandaei) CFU per fish in intraperitoneal injection. Further, PCR amplification of specific genes responsible for virulence (aerolysin and enterotoxin) confirmed pathogenicity of both bacteria. Histopathology of kidney and liver in the experimentally-infected fishes revealed haemorrhage, tubular degeneration and vacuolation. Antibiotic profiles were also assessed for both bacteria. To the best of our knowledge, the present work is a first report on the mortality of farmed climbing perch naturally-infected by A. hydrophila as well as A. jandaei, with no records of pathogenicity of the latter in this fish.

Expansion of Necrosis Depending on Hybrid Motor-Driven Motility of Aeromonas hydrophila in a Murine Wound Infection Model

The gram-negative bacterium Aeromonas hydrophila is a cause of fulminant and lethal necrotizing soft tissue infections (NSTIs). Suppressing the rapid proliferation of the pathogen and expansion of the necrosis caused in the host is an important issue in clinical practice, but the pathogenic mechanism for the rapid aggravation has not been clarified. In this study, we characterized the function of two types of motor stators in A. hydrophila and explored the role of motility during wound infection. In vitro analysis showed that the motility was reliably maintained while being complemented by the stators. We created a non-motile strain that lacked genes encoding two types of motor stators and analyzed the role of motility in a murine wound infection model. Examination of the bacterial burden in the local infection site and systemic circulation revealed that motility was not essential for the proliferation of A. hydrophila in the host. However, the extent of necrosis at the lesions was lower, and survival times were prolonged in mice infected with the non-motile strain compared with mice infected with the parent strain. These results provide evidence that the rapid expansion of necrosis and the progression to death within a short time period is dependent on the motility of A. hydrophila.

Pathogenic profile and cytotoxic activity of Aeromonas spp. isolated from Pectinatella magnifica and surrounding water in the South Bohemian aquaculture region

Pectinatella magnifica is an invasive freshwater bryozoan that has expanded in many localities worldwide, including fishing areas. It contains microbial communities, predominantly consisting of Aeromonas bacteria that are frequently associated with fish infections. The objective of this study was to investigate the potential pathogenicity of Aeromonas spp. associated with P. magnifica and evaluate the health risks for fish. Aeromonas strains were isolated from P. magnifica (101 strains) and from surrounding water (29 strains) in the South Bohemian region and investigated for the presence of 14 virulence-associated genes using PCR. We demonstrated high prevalence of phospholipase GCAT, polar flagellin, enolase, DNAse, aerolysin/cytotoxic enterotoxin, serine protease and heat-stable cytotonic enterotoxin-coding genes. Further, all twelve isolates that were analysed for cytotoxicity against intestinal epithelial cells were found to be cytotoxic. Six of the isolates were also tested as co-cultures composed of pairs. Enhanced cytotoxicity was observed when the pair was composed of strains from different species. In conclusion, P. magnifica is colonized by Aeromonas strains that have a relatively high prevalence of virulence-associated genes and the ability to provoke disease. Results also suggest a possibly increased risk arising from mixed infections.

Role of neuroendocrine modulation and biochemistry in the sepsis in Piaractus mesopotamicus

Sepsis is a systemic process with multifactorial pathophysiology that affects most animal species. It is responsible for high rates of morbidity and mortality. This work aimed to study the biochemical and neuroendocrine changes of the sepsis process in Piaractus mesopotamicus after Aeromonas hydrophila inoculation analyzing changes in blood leukocyte and differences in neuroendocrine-biochemical modulation using RNA-seq. Fish showed hypercortisolemia, inhibition of glucose absorption, followed by hypocortisolemia and then hyperglycemia. Thyroid hormones (T₃ and T₄) showed immediate decrease in serum and T₄ increased 6 h post-inoculation (HPI). Sepsis-induced hormonal alterations triggered changes in the metabolic pathways increasing protein and lipid catabolism, use of transient anaerobic glycolysis and liver injury. A reference transcriptome was constructed based on blood leukocytes from P. mesopotamicus. The assembly resulted in total 266,272 contigs with a N50 of 2786 bp. There was a reorganization of plasma membrane of leukocytes at the beginning of the septic process with increased expression of neuroendocrine receptors and with continuous flow of neurotransmitters, hormones and solutes with compensatory regulation at 6 HPI. Three and nine HPI seemed to be critical, the expression of a number of transcription factors was increased, including the modulatory DEGs related to glucocorticoid and thyroid hormones induced and suppressed (FDR < 0.05). Neuroendocrine modulation can regulate leukocytes and biochemical parameters of peripheral blood, being important sources for the study of the pathophysiology of sepsis. These finding highlights the importance of further studies focusing on biochemical-neuroendocrine changes in blood leukocytes and systemic sepsis.

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.

Gut Bacteria of Water Monitor Lizard (Varanus salvator) Are a Potential Source of Antibacterial Compound(s)

For the past few decades, there has been limited progress in the development of novel antibacterials. Previously, we postulated that the gut microbiota of animals residing in polluted environments are a forthcoming supply of antibacterials. Among various species, the water monitor lizard is an interesting species that feeds on organic waste and the carcass of wild animals. Gut microbiota of the water monitor lizard were sequestered, identified and cultivated in RPMI-1640 to produce conditioned medium (CM). Next, the antimicrobial properties of CM were evaluated versus a selection of Gram-negative (Escherichia coli K1, Serratia marcescens, Pseudomonas aeruginosa, Salmonella enterica and Klebsiella pneumoniae) and Gram-positive bacteria (Streptococcus pyogenes, methicillin-resistant Staphylococcus aureus, and Bacillus cereus). CM were partially characterized by heat inactivation at 95°C for 10 min and tested against P. aeruginosa and S. pyogenes. CM were also tested against immortalized human keratinocytes (HaCaT) cells lines. The results demonstrated that gut microbiota isolated from water monitor lizard produced molecules with remarkable bactericidal activities. To determine the identity of the active molecules, CM were subjected to Liquid Chromatography-Mass Spectrometry. Several molecules were identified belonging to the classes of flavonoids, terpenoids, alkaloids, polyhydroxy alkaloids, polyacetylenes, bisphenols, amides, oxylipin and pyrazine derivatives with known broad-spectrum antimicrobial, anti-tumour, anti-oxidant, anti-inflammatory, and analgesic attributes. Furthermore, the detailed analysis of these molecules could lead us to develop effective therapeutic antibacterials.

Morphological, microbiological and ultrastructural aspects of sepsis by Aeromonas hydrophila in Piaractus mesopotamicus

Aeromonas bacteria can cause an infection characterized by septicemia and is one of the most common pathogens in tropical fish. This disease is responsible for high morbidity and mortality rates, causing considerable losses in aquaculture. Thus, the understanding of its pathophysiology is crucial to develop control strategies of this bacterial infection in farmed fish. This study aimed to characterize early pathological aspects of acute sepsis in pacu (Piaractus mesopotamicus) experimentally infected with Aeromonas hydrophila. A total of 160 juvenile pacus were inoculated intraperitoneally with A. hydrophila (1.78 x 10⁹ CFU/mL) and at 0 (control), 1, 3, 6, and 9 hours post-inoculation (hpi), animals were anesthetized and samples were collected for microbiological, light microscopy and transmission electron microscopy (TEM) analyzes. The results showed the occurrence of hemodynamic alterations, such as hemorrhage and congestion, which were observed mainly after 6 and 9 hpi. It was possible to re-isolate Aeromonas at all sampling times except in control group. However, just after 9 hpi it was possible to find the bacteria in all fish and tissues. Light microscopy analyses revealed a degenerative process, necrosis and vascular damage mainly at 6 and 9 hpi. According to the ultrastructural examination, areas of cellular death were identified in all examined tissues, especially at 6 and 9 hpi. However, the most severe, related to necrosis, were observed after 6 and 9 hpi. The findings suggested that this bacterium spreads in the first hpi through the fish organs, mainly affecting spleen, liver and kidney, causing irreversible lesions at the molecular level.

GWAS Analysis Indicated Importance of NF-κB Signaling Pathway in Host Resistance Against Motile Aeromonas Septicemia Disease in Catfish

Motile Aeromonas septicemia (MAS) disease caused by a bacterial pathogen, Aeromonas hydrophila, is an emerging but severe disease of catfish. Genetic enhancement of disease resistance is considered to be effective to control the disease. To provide an insight into the genomic basis of MAS disease resistance, in this study, we conducted a genome-wide association study (GWAS) to identify quantitative trait loci (QTL). A total of 1820 interspecific backcross catfish of 7 families were challenged with A. hydrophila, and 382 phenotypic extremes were selected for genotyping with the catfish 690 K SNP arrays. Three QTL on linkage group (LG) 2, 26 and 29 were identified to be significantly associated with MAS resistance. Within these regions, a total of 24 genes had known functions in immunity, 10 of which were involved in NF-κB signaling pathway, suggesting the importance of NF-κB signaling pathway in MAS resistance. In addition, three suggestively significant QTL were identified on LG 11, 17, and 20. The limited numbers of QTL involved in MAS resistance suggests that marker-assisted selection may be a viable approach for catfish breeding.

Virulence genes contributing to Aeromonas hydrophila pathogenicity in Oreochromis niloticus

Bacterial diseases are the main cause of high economic loss in aquaculture, particularly gram-negative bacteria. This study was conducted for the isolation and identification of Aeromonas and Pseudomonas spp. from diseased fish. Twenty-two Aeromonas and sixteen Pseudomonas isolates were recovered from diseased Nile tilapia (Oreochromis niloticus) raised in eight earthen ponds in Elhox, Metoubes, Kafrelsheikh, Egypt. The recovered isolates were further identified using PCR as 22 Aeromonas hydrophila, 11 Pseudomonas aeruginosa, and 5 Pseudomonas fluorescens isolates. The 22 A. hydrophila isolates were screened for the presence of four virulence genes. Sixteen of the isolates (72.72%) were positive for the aerolysin gene (aer); 4 (18.18%) harbored the cytotoxic enterotoxin gene (act); and 2 (9.09%) carried the hemolysin A gene (hylA) while the cytotonic heat-stable enterotoxin gene (ast) was absent from all the tested isolates. The pathogenicity test indicated the direct relationship between the mortality percentage and the genotype of the tested A. hydrophila isolates as the mortality rates were 63.3 and 73.3% for isolates with two virulence genes (aer⁺ & act⁺, and aer⁺ and hylA⁺, respectively), followed by 40, 53.3, and 56.6% for isolates with only one virulence gene (hylA, act, and aer, respectively) and 20% for isolates lacking virulence genes. Based on the sensitivity test, the multi-antibiotic resistance profiles were as follows: 90.9% of the A. hydrophila isolates were sensitive to florfenicol and doxycycline; then 68.18% were susceptible to oxytetracycline, norfloxacin, and ciprofloxacin; and 63.63% were susceptible to sulfamethoxazole-trimethoprim, while only 27.27 and 4.5% were sensitive to erythromycin and cephradine, respectively, and all the isolates were resistant to amoxicillin and ampicillin.

The Significance of Mesophilic Aeromonas spp. in Minimally Processed Ready-to-Eat Seafood

Minimally processed and ready-to-eat (RTE) seafood products are gaining popularity because of their availability in retail stores and the consumers' perception of convenience. Products that are subjected to mild processing and products that do not require additional heating prior to consumption are eaten by an increasing proportion of the population, including people that are more susceptible to foodborne disease. Worldwide, seafood is an important source of foodborne outbreaks, but the exact burden is not known. The increased interest in seafood products for raw consumption introduces new food safety issues that must be addressed by all actors in the food chain. Bacteria belonging to genus Aeromonas are ubiquitous in marine environments, and Aeromonas spp. has held the title "emerging foodborne pathogen" for more than a decade. Given its high prevalence in seafood and in vegetables included in many RTE seafood meals, the significance of Aeromonas as a potential foodborne pathogen and a food spoilage organism increases. Some Aeromonas spp. can grow relatively uninhibited in food during refrigeration under a broad range of pH and NaCl concentrations, and in various packaging atmospheres. Strains of several Aeromonas species have shown spoilage potential by the production of spoilage associated metabolites in various seafood products, but the knowledge on spoilage in cold water fish species is scarce. The question about the significance of Aeromonas spp. in RTE seafood products is challenged by the limited knowledge on how to identify the truly virulent strains. The limited information on clinically relevant strains is partly due to few registered outbreaks, and to the disputed role as a true foodborne pathogen. However, it is likely that illness caused by Aeromonas might go on undetected due to unreported cases and a lack of adequate identification schemes. A rather confusing taxonomy and inadequate biochemical tests for species identification has led to a biased focus towards some Aeromonas species. Over the last ten years, several housekeeping genes has replaced the 16S rRNA gene as suitable genetic markers for phylogenetic analysis. The result is a more clear and robust taxonomy and updated knowledge on the currently circulating environmental strains. Nevertheless, more knowledge on which factors that contribute to virulence and how to control the potential pathogenic strains of Aeromonas in perishable RTE seafood products are needed.

Effects of inosine 5'-monophosphate supplementation in high fishmeal and high soybean diets on growth, immune-related gene expression in gibel carp (Carassius auratus gibelio var. CAS Ⅲ), and its challenge against Aeromonas hydrophila infection

The present study was conducted to evaluate dietary inosine 5'-monophosphate (5'-IMP) on growth, immune genes expression and disease resistance against Aeromonas hydrophila in juvenile gibel carp (Carassius auratus gibelio var. CAS Ⅲ) (initial body weight: 7.48 g). Six diets were formulated containing exogenous 5'-IMP at three gradient levels (0, 0.1% and 0.2%) in the high dietary fishmeal group (15% fishmeal: D1, D2, D3) and in the high dietary soybean meal group (33% soybean meal: D4, D5, D6). Each diet was randomly allotted to triplicate tanks in a recirculating system. After the feeding trial, fish were exposed to Aeromonas hydrophila challenge. Hematological and immunological responses were analyzed before and after challenge. The results indicated that feeding rate in all 5'-IMP supplemented treatments (D2, D3, D5 and D6) and daily growth coefficient in D5 and D6 were reduced compared with those of respective control treatments (D1 and D4) without 5'-IMP addition (P < 0.05). The cumulative survival rates were numerically improved by dietary 5'-IMP supplementation (P > 0.05). Compared with the respective control treatment, in the high fishmeal group, plasma SOD and MPO were significantly elevated in D3 at the end of feeding trial (P < 0.05), plasma SOD and lysozyme were significantly increased in D3 after bacterial challenge (P < 0.05); in high soybean meal group, plasma lysozyme activity was significantly elevated in D5 post bacterial challenge (P < 0.05). Most of the expression of immune related genes (intelectin, major histocompatibility complex class II β (MHC II β), Complement 3 (C3), Complement component C7-1 (ccC7), lysozyme C, Interleukin 1β (IL-1β), Tumor necrosis factor α1 (TNF-α1), Transforming growth factor-beta (TGF-β) and Interleukin 8 (IL-8)) in spleen, kidney and liver of the fish were significantly affected by supplementation of 5'-IMP at the end of feeding trial and post bacterial challenge. Additionally, adding 5'-IMP in high soybean meal diets exerted further effects of promoting immunity than counterparts in high fishmeal diets. Considering enhanced disease resistance, the immunopotentiation of 5'-IMP was manifested when the addition level was 0.1% in high soybean meal diets and 0.2% in high fishmeal diets.

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.

Comparative Genomics of Aeromonas hydrophila Secretion Systems and Mutational Analysis of hcp1 and vgrG1 Genes From T6SS

Virulent Aeromonas hydrophila causes severe motile Aeromonas septicemia in warmwater fishes. In recent years, channel catfish farming in the U.S.A. and carp farming in China have been affected by virulent A. hydrophila, and genome comparisons revealed that these virulent A. hydrophila strains belong to the same clonal group. Bacterial secretion systems are often important virulence factors; in the current study, we investigated whether secretion systems contribute to the virulent phenotype of these strains. Thus, we conducted comparative secretion system analysis using 55 A. hydrophila genomes, including virulent A. hydrophila strains from U.S.A. and China. Interestingly, tight adherence (TaD) system is consistently encoded in all the vAh strains. The majority of U.S.A. isolates do not possess a complete type VI secretion system, but three core elements [tssD (hcp), tssH, and tssI (vgrG)] are encoded. On the other hand, Chinese isolates have a complete type VI secretion system operon. None of the virulent A. hydrophila isolates have a type III secretion system. Deletion of two genes encoding type VI secretion system proteins (hcp1 and vgrG1) from virulent A. hydrophila isolate ML09-119 reduced virulence 2.24-fold in catfish fingerlings compared to the parent strain ML09-119. By determining the distribution of genes encoding secretion systems in A. hydrophila strains, our study clarifies which systems may contribute to core A. hydrophila functions and which may contribute to more specialized adaptations such as virulence. Our study also clarifies the role of type VI secretion system in A. hydrophila virulence.

Outcome of co-infection with opportunistic and multidrug resistant Aeromonas hydrophila and A. veronii in zebrafish: Identification, characterization, pathogenicity and immune responses

Fish can be potentially co-infected by two or more bacterial strains, which can make synergistic influence on the virulence of infection. In this study, two opportunistic and multidrug resistant Aeromonas strains were isolated from wounds of morbid zebrafish with typical deep skin lesions similar to Motile Aeromonas Septicemia. Isolates were genetically identified as A. hydrophila and A. veronii by 16 S rRNA sequencing and phylogenetic analysis. Both isolates were positive for virulent genes (aerA, lip, ser, exu gcaT) and selected phenotypic tests (DNase, protease, gelatinase, lipase, biofilm production and β-haemolysis). A. hydrophila and A. veronii had strong antibiotic resistance against ampicillin, tetracycline, nalidixic acid, kanamycin, erythromycin, clindamycin and trimethoprim-sulfamethoxazole. Histopathological studies revealed that co-infection causes severe necrosis and hypertrophy in the muscles, kidney and liver of zebrafish. Naturally co-infected zebrafish showed highly induced tnf-α, il-1β, il-6, il-12, ifn, ifn-γ, cxcl18 b and ccl34a.4 at transcription level compared to healthy fish, suggesting virulence factors may activate immune and inflammatory responses of zebrafish. Experimentally infected zebrafish showed significantly higher mortality under co-infection with A. hydrohila and A. veronii (87%), followed by individual challenge of A. hydrophila (72%) or A. veronii (67%) suggesting that virulence of A. hydrophila have greater pathogenicity than A. veronii during co-infection.

Molecular and Phenotypic Analysis of Hemolytic Aeromonas Strains Isolated from Food in Egypt Revealed Clinically Important Multidrug Resistance and Virulence Profiles

The aim of this study was to determine the public health significance of hemolytic Aeromonas species isolated from 213 food samples in Egypt, based on their virulence and antimicrobial-resistance potential. We recovered 63 strains, isolated from fish, raw milk, karish cheeses, and ras cheese in 29 (31.18%) of 93, 10 (25.00%) of 40, 13 (32.50%) of 40, and 11 (27.50%) of 40 samples, respectively. The most prevalent virulence gene was alt (50.79%), followed by aerA (34.92%), asa1 (39.68%), ahh1 (20.63%), act (11.11%), and ast (3.17%). Thirteen strains screened in this study carried no hemolysin gene, but only the alt gene, and another eight hemolytic strains screened, carried no virulence gene. The virulence signatures " ahh1+ aerA" and " alt+ act," in which the genes interact synergistically to induce severe diarrhea, were detected in two and four strains, respectively. Most showed resistance to third-generation cephalosporins, aztreonam, and imipenem, which indicates the complexity of the β-lactamase production in our hemolytic Aeromonas strains. Fourteen (22.22%) of 63 strains carried one or more antimicrobial-resistance markers, including the bla_CTX-M, bla_TEM, tet(A), tet(E), and intI1 genes, which were detected in 6.34, 3.17, 3.17, 4.76, and 14.28% of isolates, respectively. In conclusion, the majority of hemolytic Aeromonas strains isolated from the intestinal contents of healthy fish and naturally contaminated milk and cheeses were not commensal but had developed multidrug-resistance and virulence profiles, indicating an emerging potential health risk. Importantly, screening for certain hemolysin genes may not be reliable in predicting the pathogenic potential of Aeromonas species and, thereby, the safety of analyzed foods. Our findings indicate that specific criteria are required for the phenotypic and molecular analysis of Aeromonas species in food items, particularly those eaten without further treatment, to ensure their safety.

Antibiotic resistance and virulence traits of bacterial pathogens from infected freshwater fish, Labeo rohita

Bacterial infectious diseases are a main dangerous problem in Aquaculture farming. It causes multiple diseases in fish as well as in human being and it has considerable virulence potential. In this connection, the moot of study focus to discriminate bacterial isolates recovered from naturally diseased Labeo rohita fish and their virulent characteristics. Based on the β-haemolysis factor, four isolates (KADR11, KADR12, KADR13 and KADR14) were selected for further delineation. These bacterial isolates showed high similarity with Providencia rettgeri, Aeromonas sp., Aeromonas sp. and Aeromonas enteropelogenes respectively, using partial 16S r-RNA gene amplification and biochemical characterizations were also supported. The further study investigates the virulence characteristics of isolates showed separation of outer membrane proteins (OMPs) and lipopolysaccharides (LPS) which appeared between 19^_80 kDa and 20^_100 kDa in SDS^_PAGE analysis respectively. All the four strains were complete resistant (100%) to β-lactam antibiotics. L. rohita were injected intraperitoneally with 0 (control), 2.0 × 10⁴, 2.0 × 10⁵, 2.0 × 10⁶, 2.0 × 10⁷ and 2.0 × 10⁸ cells/fish of Providencia rettgeri KADR11, Aeromonas sp. KADR12, Aeromonas sp. KADR13 and Aeromonas enteropelogenes KADR14 for the determination of lethal dose 50 (LD₅₀) values, which were 2.4 × 10⁷, 4.1 × 10⁵, 2.7 × 10⁷ and 7.4 × 10⁵ cells/fish respectively. The results indicated that isolated strains were possessed the high pathogenic potential for L. rohita.

Effect of Bacillus subtilis on Aeromonas hydrophila-induced intestinal mucosal barrier function damage and inflammation in grass carp (Ctenopharyngodon idella)

Our study explored the effect of oral intubation of Bacillus subtilis on Aeromonas hydrophila-induced intestinal mucosal barrier function damage and inflammation in grass carp. The mid-intestine mucosal tissue was collected for ATPase activity measurement. Intestinal mucosa was also ultrastructurally examined with transmission electron microscope (TEM), and its permeability was determined using Evans blue (EB) and D-lactic acid. The mid-intestine pro-inflammation cytokine, MyD88 and tight junction (TJ) protein mRNA expression levels were measured using real-time quantitative PCR. The results revealed that B. subtilis was found to prevent the decrease in the activity of Na+, K+-ATPase and Ca2+, Mg2+-ATPase, as well as the increase in EB and D-lactic acid concentration and inflammation induced by A. hydrophila in grass carp. Compared with A. hydrophila groups, B. subtilis safeguarded the integrity of intestinal villi and tight junction structure and restrained A. hydrophila-induced down-regulation of TJ proteins zonula occludens-1 (ZO-1) and occludin. B. subtilis also restrained up-regulation of TJ protein claudin b, pro-inflammation cytokine tumour necrosis factor α (TNF-α), cytokine interleukin 8 (IL-8), IL-1β, and adaptor protein myeloid differentiation factor 88 (MyD88) mRNA levels. Thus, oral intubation of B. subtilis could reduce A. hydrophila-induced intestinal mucosal barrier function damage and inflammation.

Virulence Factors of Aeromonas hydrophila: In the Wake of Reclassification

The ubiquitous "jack-of-all-trades," Aeromonas hydrophila, is a freshwater, Gram-negative bacterial pathogen under revision in regard to its phylogenetic and functional affiliation with other aeromonads. While virulence factors are expectedly diverse across A. hydrophila strains and closely related species, our mechanistic knowledge of the vast majority of these factors is based on the molecular characterization of the strains A. hydrophila AH-3 and SSU, which were reclassified as A. piscicola AH-3 in 2009 and A. dhakensis SSU in 2013. Individually, these reclassifications raise important questions involving the applicability of previous research on A. hydrophila virulence mechanisms; however, this issue is exacerbated by a lack of genomic data on other research strains. Collectively, these changes represent a fundamental gap in the literature on A. hydrophila and confirm the necessity of biochemical, molecular, and morphological techniques in the classification of research strains that are used as a foundation for future research. This review revisits what is known about virulence in A. hydrophila and the feasibility of using comparative genomics in light of this phylogenetic revision. Conflicting data between virulence factors, secretion systems, quorum sensing, and their effect on A. hydrophila pathogenicity appears to be an artifact of inappropriate taxonomic comparisons and/or be due to the fact that these properties are strain-specific. This review audits emerging data on dominant virulence factors that are present in both A. dhakensis and A. hydrophila in order to synthesize existing data with the aim of locating where future research is needed.

Lactobacillus acidophilus attenuates Aeromonas hydrophila induced cytotoxicity in catla thymus macrophages by modulating oxidative stress and inflammation

The pathogenesis of Aeromonas hydrophila, a potent fish pathogen, is attributed to its ability to cause motile aeromonad septicaemia leading to apoptosis in a myriad of fish species, including freshwater carp Catla catla. However, the underlying mechanism of antagonistic activity of probiotics against A. hydrophila induced apoptosis is not elucidated due to lack of appropriate in-vitro models. This study reported that the exposure of catla thymus macrophages (CTM) to A. hydrophila markedly induced cellular injuries as evidenced by elevated levels of reactive oxygen species (ROS), reactive nitrogen species (RNS), increased apoptosis, DNA damage and decreased cellular viability. Flow cytometry analysis and Annexin-V/propidium iodide assay further confirmed increased ROS positive cells leading to cell death after infection. The quantitative real-time PCR analysis, also revealed upregulation of inducible nitric-oxide synthase (iNOS), pro-inflammatory cytokine (TNFα), cyclooxygenase2 (COX-2) and downregulation of anti-inflammatory cytokine (IL-10). Pretreatment of cells with probiotic, Lactobacillus acidophilus attenuated A. hydrophila induced apoptosis as evident from the decrease in the levels of ROS, RNS and DNA damage. Significant increase (P≤0.05) in expression of TNFα and IL-10 and decrease in iNOS and COX-2 was observed on probiotic stimulation. In-vivo study using catla fingerlings confirmed similar pattern of ROS, iNOS, NO production and cytokine expression in thymus. This study provides a comprehensive insight into the mechanistic basis of L. acidophilus induced macrophage mediated inflammatory response against A. hydrophila in CTM cells. Further, it speculates the possibility of using cost-effective in-vitro models for screening probiotic candidates of therapeutic potential in aquaculture industry.

Lipidome and transcriptome profiling of pneumolysin intoxication identifies networks involved in statin-conferred protection of airway epithelial cells

Pneumonia remains one of the leading causes of death in both adults and children worldwide. Despite the adoption of a wide variety of therapeutics, the mortality from community-acquired pneumonia has remained relatively constant. Although viral and fungal acute airway infections can result in pneumonia, bacteria are the most common cause of community-acquired pneumonia, with Streptococcus pneumoniae isolated in nearly 50% of cases. Pneumolysin is a cholesterol-dependent cytolysin or pore-forming toxin produced by Streptococcus pneumonia and has been shown to play a critical role in bacterial pathogenesis. Airway epithelium is the initial site of many bacterial contacts and its barrier and mucosal immunity functions are central to infectious lung diseases. In our studies, we have shown that the prior exposure to statins confers significant resistance of airway epithelial cells to the cytotoxicity of pneumolysin. We decided to take this study one step further, assessing changes in both the transcriptome and lipidome of human airway epithelial cells exposed to toxin, statin or both. Our current work provides the first global view in human airway epithelial cells of both the transcriptome and the lipid interactions that result in cellular protection from pneumolysin.

Effect of starvation on survival and virulence expression of Aeromonas hydrophila from different sources

Aeromonas hydrophila is an aquatic bacterium responsible for several human illnesses. The aim of this work was to investigate the survival ability and virulence expression of two strains from different sources (fish, strain 87 and surface water, strain LS) maintained in a seawater microcosm. The strains were analyzed for the total and viable bacterial counts, adhesion ability to Hep-2 cells and aerA gene expression by qPCR throughout the experiment (35 days). Both strains reached a putative VBNC state and lost adhesive properties but exhibited a different behavior in the expression of aerA. This could be due to the different origin of the two strains; the former adapted to a habitat rich of nutrient and the latter already used to survive in a more hostile environment. Moreover, our results indicate that the quantitative determination of aerA mRNA can be a useful indicator of virulence expression under stress conditions.

Hesperidin inhibits inflammatory response induced by Aeromonas hydrophila infection and alters CD4+/CD8+ T cell ratio

Background. Aeromonas hydrophila is an opportunistic bacterial pathogen that is associated with a number of human diseases. Hesperidin (HES) has been reported to exert antioxidant and anti-inflammatory activities. Objectives. The aim of this study was to investigate the potential effect of HES treatment on inflammatory response induced by A. hydrophila infection in murine. Methods. A. hydrophila-infected mice were treated with HES at 250 mg/kg b.wt./week for 4 consecutive weeks. Phagocytosis, reactive oxygen species production, CD4⁺/CD8⁺ T cell ratio, and CD14 expression on intestinal infiltrating monocytes were evaluated. The expression of E-selectin and intercellular adhesion molecule 1 on stimulated HUVECs and RAW macrophage was evaluated. Results. Percentage of CD4⁺ T cells in the intestinal tissues of infected treated mice was highly significantly increased; however, phagocytic index, ROS production, CD8⁺ T cells percentage, and CD14 expression on monocytes were significantly reduced. On the other hand, HES significantly inhibited A-LPS- and A-ECP-induced E-selectin and ICAM-1 expression on HUVECs and ICAM-1 expression on RAW macrophage. Conclusion. Present data indicated that HES has a potential role in the suppression of inflammatory response induced by A. hydrophila toxins through downmodulation of ROS production and CD14 and adhesion molecules expression, as well as increase of CD4⁺/CD8⁺ cell ratio.

Role of calmodulin-calmodulin kinase II, cAMP/protein kinase A and ERK 1/2 on Aeromonas hydrophila-induced apoptosis of head kidney macrophages

The role of calcium (Ca²⁺) and its dependent protease calpain in Aeromonas hydrophila-induced head kidney macrophage (HKM) apoptosis has been reported. Here, we report the pro-apoptotic involvement of calmodulin (CaM) and calmodulin kinase II gamma (CaMKIIg) in the process. We observed significant increase in CaM levels in A. hydrophila-infected HKM and the inhibitory role of BAPTA/AM, EGTA, nifedipine and verapamil suggested CaM elevation to be Ca²⁺-dependent. Our studies with CaM-specific siRNA and the CaM inhibitor calmidazolium chloride demonstrated CaM to be pro-apoptotic that initiated the downstream expression of CaMKIIg. Using the CaMKIIg-targeted siRNA, specific inhibitor KN-93 and its inactive structural analogue KN-92 we report CaM-CaMKIIg signalling to be critical for apoptosis of A. hydrophila-infected HKM. Inhibitor studies further suggested the role of calpain-2 in CaMKIIg expression. CaMK Kinase (CaMKK), the other CaM dependent kinase exhibited no role in A. hydrophila-induced HKM apoptosis. We report increased production of intracellular cAMP in infected HKM and our results with KN-93 or KN-92 implicate the role of CaMKIIg in cAMP production. Using siRNA to PKACA, the catalytic subunit of PKA, anti-PKACA antibody and H-89, the specific inhibitor for PKA we prove the pro-apoptotic involvement of cAMP/PKA pathway in the pathogenicity of A. hydrophila. Our inhibitor studies coupled with siRNA approach further implicated the role of cAMP/PKA in activation of extracellular signal-regulated kinase 1 and 2 (ERK 1/2). We conclude that the alteration in intracellular Ca²⁺ levels initiated by A. hydrophila activates CaM and calpain-2; both pathways converge on CaMKIIg which in turn induces cAMP/PKA mediated ERK 1/2 phosphorylation leading to caspase-3 mediated apoptosis of infected HKM.

Aeromonas hydrophila is a natural fish pathogen and is known to induce apoptosis of HKM. Head kidney is an important immune-organ in fish and HKM are critical for immunity against the invading pathogen. The mechanisms of cell death induced by A. hydrophila are incompletely characterized. We have studied the role of Ca²⁺-dependent signalling pathways in the induction of A. hydrophila-induced HKM apoptosis. We observed that A. hydrophila infection led to increased CaM expression in infected HKM which was Ca²⁺-dependent. The inhibitor and siRNA studies suggested CaM to be pro-apoptotic and triggered CaMKIIg expression in the infected HKM. Calpain-2 appeared to influence CaMKIIg expression. However, further studies are needed to understand the process. We report that the CaM-CaMKIIg pathway is important for initiating cAMP production within the infected HKM. The pro-apoptotic activation of cAMP dependent PKA was quite evident. The activation of ERK 1/2 was observed in the HKM and results clearly suggested the pro-active role of cAMP/PKA in the process. Thus we conclude that CaM-CaMKIIg initiates the cAMP/PKA pathway that induces ERK 1/2 phosphorylation to promote caspase-3 mediated apoptosis of the A. hydrophila-infected HKM.

Genomic study of polyhydroxyalkanoates producing Aeromonas hydrophila 4AK4

The complete genome of Gram-negative Aeromonas hydrophila 4AK4 that has been used for industrial production of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) was sequenced and annotated. Its chromosome is 4,527,993 bp in size encoding 4,272 genes, including 28 rRNA genes and 104 tRNA genes. Comparative analysis indicated that genome of A. hydrophila 4AK4 was similar to that of the A. hydrophila ATCC 7966(T), an intensively studied aeromonad for its pathogenicity related to its genomic information. Genes possibly coming from other species or even other genus were identified in A. hydrophila 4AK4. A large number of putative virulent genes were predicted. However, a cytotonic enterotoxin (Ast) is absent in A. hydrophila 4AK4, allowing the industrial strain to be different from other A. hydrophila strains, indicating possible reduced virulence of strain 4AK4, which is very important for industrial fermentation. Genes involved in polyhydroxyalkanoate (PHA) metabolism were predicted and analyzed. The resulting genomic information is useful for improved production of PHA via metabolic engineering of A. hydrophila 4AK4.

Characterization of Aeromonas hydrophila wound pathotypes by comparative genomic and functional analyses of virulence genes

Aeromonas hydrophila has increasingly been implicated as a virulent and antibiotic-resistant etiologic agent in various human diseases. In a previously published case report, we described a subject with a polymicrobial wound infection that included a persistent and aggressive strain of A. hydrophila (E1), as well as a more antibiotic-resistant strain of A. hydrophila (E2). To better understand the differences between pathogenic and environmental strains of A. hydrophila, we conducted comparative genomic and functional analyses of virulence-associated genes of these two wound isolates (E1 and E2), the environmental type strain A. hydrophila ATCC 7966^T, and four other isolates belonging to A. aquariorum, A. veronii, A. salmonicida, and A. caviae. Full-genome sequencing of strains E1 and E2 revealed extensive differences between the two and strain ATCC 7966^T. The more persistent wound infection strain, E1, harbored coding sequences for a cytotoxic enterotoxin (Act), a type 3 secretion system (T3SS), flagella, hemolysins, and a homolog of exotoxin A found in Pseudomonas aeruginosa. Corresponding phenotypic analyses with A. hydrophila ATCC 7966^T and SSU as reference strains demonstrated the functionality of these virulence genes, with strain E1 displaying enhanced swimming and swarming motility, lateral flagella on electron microscopy, the presence of T3SS effector AexU, and enhanced lethality in a mouse model of Aeromonas infection. By combining sequence-based analysis and functional assays, we characterized an A. hydrophila pathotype, exemplified by strain E1, that exhibited increased virulence in a mouse model of infection, likely because of encapsulation, enhanced motility, toxin secretion, and cellular toxicity.

Aeromonas hydrophila is a common aquatic bacterium that has increasingly been implicated in serious human infections. While many determinants of virulence have been identified in Aeromonas, rapid identification of pathogenic versus nonpathogenic strains remains a challenge for this genus, as it is for other opportunistic pathogens. This paper demonstrates, by using whole-genome sequencing of clinical Aeromonas strains, followed by corresponding virulence assays, that comparative genomics can be used to identify a virulent subtype of A. hydrophila that is aggressive during human infection and more lethal in a mouse model of infection. This aggressive pathotype contained genes for toxin production, toxin secretion, and bacterial motility that likely enabled its pathogenicity. Our results highlight the potential of whole-genome sequencing to transform microbial diagnostics; with further advances in rapid sequencing and annotation, genomic analysis will be able to provide timely information on the identities and virulence potential of clinically isolated microorganisms.

Evasion of mucosal defenses during Aeromonas hydrophila infection of channel catfish (Ictalurus punctatus) skin

The mucosal surfaces of fish serve as the first line of defense against the myriad of aquatic pathogens present in the aquatic environment. The immune repertoire functioning at these interfaces is still poorly understood. The skin, in particular, must process signals from several fronts, sensing and integrating environmental, nutritional, social, and health cues. Pathogen invasion can disrupt this delicate homeostasis with profound impacts on signaling throughout the organism. Here, we investigated the transcriptional effects of virulent Aeromonas hydrophila infection in channel catfish skin, Ictalurus punctatus. We utilized a new 8 × 60 K Agilent microarray for catfish to examine gene expression profiles at critical early timepoints following challenge--2 h, 8 h, and 12 h. Expression of a total of 2,168 unique genes was significantly perturbed during at least one timepoint. We observed dysregulation of genes involved in antioxidant, cytoskeletal, immune, junctional, and nervous system pathways. In particular, A. hydrophila infection rapidly altered a number of potentially critical lectins, chemokines, interleukins, and other mucosal factors in a manner predicted to enhance its ability to adhere to and invade the catfish host.

Early mucosal responses in blue catfish (Ictalurus furcatus) skin to Aeromonas hydrophila infection

Bacterial pathogens are well-equipped to detect, adhere to, and initiate infection in their finfish hosts. The mucosal surfaces of fish, such as the skin, function as the front line of defense against such bacterial insults that are routinely encountered in the aquatic environment. While recent progress has been made, and despite the obvious importance of mucosal surfaces, the precise molecular events that occur soon after encountering bacterial pathogens remain unclear. Indeed, these early events are critical in mounting appropriate responses that ultimately determine host survival or death. In the present study, we investigated the transcriptional consequences of a virulent Aeromonas hydrophila challenge in the skin of blue catfish, Ictalurus furcatus. We utilized an 8×60K Agilent microarray to examine gene expression profiles at key early timepoints following challenge (2 h, 12 h, and 24 h). A total of 1155 unique genes were significantly altered during at least one timepoint. We observed dysregulation in a number of genes involved in diverse pathways including those involved in antioxidant responses, apoptosis, cytoskeletal rearrangement, immunity, and extracellular matrix protein diversity and regulation. Taken together, A. hydrophila coordinately modulates mucosal factors across numerous cellular pathways in a manner predicted to enhance its ability to adhere to and infect the blue catfish host.

The main Aeromonas pathogenic factors

The members of the Aeromonas genus are ubiquitous, water-borne bacteria. They have been isolated from marine waters, rivers, lakes, swamps, sediments, chlorine water, water distribution systems, drinking water and residual waters; different types of food, such as meat, fish, seafood, vegetables, and processed foods. Aeromonas strains are predominantly pathogenic to poikilothermic animals, and the mesophilic strains are emerging as important pathogens in humans, causing a variety of extraintestinal and systemic infections as well as gastrointestinal infections. The most commonly described disease caused by Aeromonas is the gastroenteritis; however, no adequate animal model is available to reproduce this illness caused by Aeromonas. The main pathogenic factors associated with Aeromonas are: surface polysaccharides (capsule, lipopolysaccharide, and glucan), S-layers, iron-binding systems, exotoxins and extracellular enzymes, secretion systems, fimbriae and other nonfilamentous adhesins, motility and flagella.

Effects on mortality and stress response in European sea bass, Dicentrarchus labrax (L.), fed mannan oligosaccharides (MOS) after Vibrio anguillarum exposure

The effects of dietary mannan oligosaccharides (MOS; 4 g kg(-1) ; Bio-Mos, Alltech Inc, USA) in diets for European sea bass, Dicentrarchus labrax (L.), juveniles in relation to disease and stress resistance, combining intestinal infection with Vibrio anguillarum and stress challenge by confinement, were assessed in this study. After 8 weeks of MOS supplementation, fish were exposed to a pathogen challenge test against V. anguillarum by direct gut inoculation combined with a confinement stressor panel. Cumulative mortality of fish fed MOS caused by anally inoculated V. anguillarum decreased from 66% to 12.5% and from 54.1% to 25% in infected and infected + stressed fish, respectively, compared to fish fed control diet. Results for European sea bass revealed a positive effect of MOS dietary inclusion on disease resistance, in terms of cumulative mortality, against gut inoculated V. anguillarum, as well as reduced effects of stress on microbiota diversity. Both of these findings, together with the enhanced innate immune response and the higher gut mucus production and density of eosinophil granulocytes in gut mucosa obtained in previous studies after MOS supplementation (Torrecillas et al. 2007, 2011a,b) suggest that general reinforcement of the innate immune system, and particularly of the intestinal barrier efficiency, is the main defence mechanism of European sea bass fed MOS against pathogenic microorganisms.

Impact of antibiotic treatments on the expression of the R plasmid tra genes and on the host innate immune activity during pRAS1 bearing Aeromonas hydrophila infection in zebrafish (Danio rerio)

BACKGROUND

The transfer of R plasmids between bacteria has been well studied under laboratory conditions and the transfer frequency has been found to vary between plasmids and under various physical conditions. For the first time, we here study the expression of the selected plasmid mobility genes traD, virB11 and virD4 in the 45 kb IncU plasmid, pRAS1, conferring resistance to tetracycline, trimethoprim and sulphonamide, using an in vivo zebrafish infection- treatment model.

RESULTS

Three days after oral infection of adult zebrafish with Aeromonas hydrophila harboring pRAS1, elevated expression of pro-inflammatory cytokine (TNF α, IL-1β and IL-8) and complement C3 genes in the intestine coincided with disease symptoms. Tetracycline, trimethoprim and an ineffective concentration of flumequine given 48 h prior to sampling, strongly increased expression of plasmid mobility genes, whereas an effective dosage of flumequine resulted in lower levels of mRNA copies of these genes relative to placebo treatment. Following effective treatment with flumequine, and ineffective treatments with a low concentration of flumequine, with trimethoprim or with sulphonamide, the intestinal expression of immune genes was strongly induced compared to placebo treated control fish.

CONCLUSIONS

Treatment of zebrafish infected with an antibiotic resistant (TcR, TmR, SuR) A. hydrophila with ineffective concentrations of flumequine or the ineffective antimicrobials tetracycline and trimethoprim strongly induced expression of genes mediating conjugative transfer of the R-plasmid pRAS1. Simultaneously, there was a strong induction of selected inflammatory and immune response genes, which was again evident in fish subjected to ineffective treatment protocols. Our findings point to the essential role of therapeutic practices in escalation or control of antibiotic resistance transfer, and suggest that antibiotic substances, even in sub-inhibitory concentrations, may stimulate innate defenses against bacterial infections.

Multifaceted interactions of bacterial toxins with the gastrointestinal mucosa

The digestive tract is one of the ecosystems that harbors the largest number and greatest variety of bacteria. Among them, certain bacteria have developed various strategies, including the synthesis of virulence factors such as toxins, to interact with the intestinal mucosa, and are responsible for various pathologies. A large variety of bacterial toxins of different sizes, structures and modes of action are able to interact with the gastrointestinal mucosa. Some toxins, termed enterotoxins, directly stimulate fluid secretion in enterocytes or cause their death, whereas other toxins pass through the intestinal barrier and disseminate by the general circulation to remote organs or tissues, where they are active. After recognition of a membrane receptor on target cells, toxins can act at the cell membrane by transducing a signal across the membrane in a hormone-like manner, by pore formation or by damaging membrane compounds. Other toxins can enter the cells and modify an intracellular target leading to a disregulation of certain physiological processes or disorganization of some structural architectures and cell death. Toxins are fascinating molecules, which mimic or interfere with eukaryotic physiological processes. Thereby, they have permitted the identification and characterization of new natural hormones or regulatory pathways. Besides use as protective antigens in vaccines, toxins offer multiple possibilities in pharmacology, such as immune modulation or specific delivery of a protein of interest into target cells.

The two-component QseBC signalling system regulates in vitro and in vivo virulence of Aeromonas hydrophila

We recently demonstrated that the N-acyl-homoserine lactone [autoinducer (AI)-1] and LuxS (AI-2)-based quorum-sensing (QS) systems exerted positive and negative regulation, respectively, on the virulence of a diarrhoeal isolate SSU of Aeromonas hydrophila. However, the role of a newly identified, two-component-based QseBC QS system in the regulation of bacterial virulence in general is not well understood, with only a limited number of studies showing its function in bacterial pathogenesis. In this report, we identified and characterized the QseBC QS system in A. hydrophila SSU and found that, as was the case with enterohaemorrhagic Escherichia coli, the open reading frames for the qseB (the response regulator) and qseC (the sensor histidine kinase) genes overlapped by 4 bp at the ATGA motif. Our data provide evidence that deletion of the qseB gene from A. hydrophila resulted in attenuation of bacterial virulence in a septicaemic mouse model of infection and diminished swimming and swarming motility, and the mutant bacteria formed denser biofilms compared with those from the parental strain of A. hydrophila. The decrease in the virulence of the A. hydrophila ΔqseB mutant correlated with reduced production of protease and the cytotoxic enterotoxin, which has associated haemolytic activity. The swimming and swarming motility, haemolytic activity, protease production and biofilm formation were restored in the qseBC-complemented strain to a level similar to that of the wild-type A. hydrophila SSU. Our study is the first, to our knowledge, to report a functional QseBC QS system in A. hydrophila which may be linked to AI-1 and AI-2 QS systems in modulating bacterial virulence, possibly through the cyclic diguanosine monophosphate.

Immunomodulatory and protective roles of quorum-sensing signaling molecules N-acyl homoserine lactones during infection of mice with Aeromonas hydrophila

Aeromonas hydrophila leads to both intestinal and extraintestinal infections in animals and humans, and the underlying mechanisms leading to mortality are largely unknown. By using a septicemic mouse model of infection, we showed that animals challenged with A. hydrophila die because of kidney and liver damage, hypoglycemia, and thrombocytopenia. Pretreatment of animals with quorum-sensing-associated signaling molecules N-acyl homoserine lactones (AHLs), such as butanoyl and hexanoyl homoserine lactones (C(4)- and C(6)-HSLs), as well as N-3-oxododecanoyl (3-oxo-C(12))-HSL, prevented clinical sequelae, resulting in increased survivability of mice. Since little is known as to how different AHLs modulate the immune response during infection, we treated mice with the above AHLs prior to lethal A. hydrophila infection. When we compared results in such animals to those in controls, the treated animals exhibited a significantly reduced bacterial load in the blood and other mouse organs, as well as various levels of cytokines/chemokines. Importantly, neutrophil numbers were significantly elevated in the blood of C(6)-HSL-treated mice compared to those in animals given phosphate-buffered saline and then infected with the bacteria. These findings coincided with the fact that neutropenic animals were more susceptible to A. hydrophila infection than normal mice. Our data suggested that neutrophils quickly cleared bacteria by either phagocytosis or possibly another mechanism(s) during infection. In a parallel study, we indeed showed that other predominant immune cells inflicted during A. hydrophila infections, such as murine macrophages, when they were pretreated with AHLs, rapidly phagocytosed bacteria, whereas untreated cells phagocytosed fewer bacteria. This study is the first to report that AHL pretreatment modulates the innate immune response in mice and enhances their survivability during A. hydrophila infection.

Pore-forming toxins induce multiple cellular responses promoting survival

Pore-forming toxins (PFTs) are secreted proteins that contribute to the virulence of a great variety of bacterial pathogens. They inflict one of the more disastrous damages a target cell can be exposed to: disruption of plasma membrane integrity. Since this is an ancient form of attack, which bears similarities to mechanical membrane damage, cells have evolved response pathways to these perturbations. Here, it is reported that PFTs trigger very diverse yet specific response pathways. Many are triggered by the decrease in cytoplasmic potassium, which thus emerges as a central regulator. Upon plasma membrane damage, cells activate signalling pathways aimed at restoring plasma membrane integrity and ion homeostasis. Interestingly these pathways do not require protein synthesis. Cells also trigger signalling cascades that allow them to enter a quiescent-like state, where minimal energy is consumed while waiting for plasma membrane damage to be repaired. More specifically, protein synthesis is arrested, cytosolic constituents are recycled by autophagy and energy is stored in lipid droplets.