Secreted enzymes of Aeromonas

PACAP sequence modifications modulate the peptide antimicrobial activity against bacterial pathogens affecting aquaculture

The global aquaculture industry has significant losses each year due to disease outbreaks. Antibiotics are one of the common methods to treat fish infections, but prolonged use can lead to the emergence of resistant strains. Aeromonas spp. Infections are a common and problematic disease in fish, and members of this genera can produce antibiotic resistant strains. Antimicrobial peptides (AMPs) have emerged as an alternative method to treat and prevent infections and pituitary adenylate cyclase activating polypeptide (PACAP) is a prominent member of this family. The objective of this research was to study PACAP's direct antimicrobial activity and its toxicity in fish cells. Four synthetic variants of the natural PACAP from Clarias gariepinus were tested in addition to the natural variant. The experimental results show a different antimicrobial activity against A. salmonicida and A. hydrophila of each PACAP variant, and for the first time show dependence on the culture broth used. Furthermore, the results suggest that the underlying mechanism of PACAP antimicrobial activity includes a bacterial membrane permeabilizing effect, classifying PACAP as a membrane disruptive AMP. This study also demonstrated that the five PACAP variants evaluated showed low toxicity in vitro, at concentrations relevant for in vivo applications. Therefore, PACAP could be a promising alternative to antibiotics in the aquaculture sector.

Redox profile of silver catfish challenged with Aeromonas hydrophila and treated with hexane extract of Hesperozygis ringens (Benth.) Epling through immersion bath

The growing increase in the fish farming sector has favored the establishment of bacterial outbreaks caused by Aeromonas hydrophila in several species. The hexane extract of Hesperozygis ringens (HEHR) (Lamiaceae) leaves increased the survival rate of silver catfish (Rhamdia quelen) experimentally infected by A. hydrophila. However, it is noteworthy that no reports have been found on the possible mechanisms of action of this extract in infected fish. This study aimed to evaluate the effect of the HEHR, administered through single immersion bath, on lipid peroxidation and antioxidant defenses in muscle and liver tissue of silver catfish challenged with A. hydrophila. The results showed that the oxidative status of silver catfish was altered, although oxidative stress was not triggered during the experiment. HEHR at 30 mg/L (HEHR30) was not characterized as a pro-oxidant agent in the presence of infection, unlike florfenicol and HEHR at 15 mg/L treatments in some cases. In short, HEHR30 provided an important increase in hepatic catalase activity, characterizing one of the possible mechanisms involved in the greater survival of fish experimentally infected by A. hydrophila. Additionally, HEHR30 did not induce lipid peroxidation, nor reduced antioxidant defenses of silver catfish infected or not by A. hydrophila.

Initial Post-Release Performance of Cultured Cyprinus chilia Juveniles in a Shallow Lake in Southwestern China

The post-release performance of cultured fish is crucial for understanding the viability of cultured fish and assessing the effects of stock enhancement programs. This study aimed to investigate the initial post-release performance of cultured Cyprinus chilia juveniles by examining their movement, spatial distribution, gut fullness, and gut microbiota in nature. In July 2022, a total of 20,000 C. chilia juveniles, tagged with visible implant fluorescence (VIE), were released into Qilu Lake, a shallow lake in southwestern China. Subsequently, continuous recapture was conducted at fixed recapture sites using trap nets during the first 7 days, one month and three months after release. Out of the released fish, 512 were recaptured, resulting in a recapture rate of 2.56%. The recaptured fish had a 100% tag retention rate. The majority (98.05%) of the recaptured fish were found in the recapture sites located on the eastern or western lakeshore, while only 10 fish were recaptured from the recapture sites in the northern lake area. The water depth range where the recaptured fish were found ranged from 190 to 350 cm, with most fish preferring depths less than 300 cm. The majority of the released fish migrated towards the eastern and western lakeshore, with long-distance movement (greater than 100 m) primarily occurring within the first four days after release. The level of gut fullness in the released fish initially decreased and then increased over time following release. Regarding gut microbiota, the dominant phyla observed in most samples were Firmicutes, Proteobacteria, Cyanobacteria, and Fusobacteria. Furthermore, significant variations in the dominant genera were observed across different samples. Principal coordinates analysis (PCoA) revealed clear separation between the microbial communities of pre-release and post-release C. chilia juveniles. This study demonstrated that VIE tagging was a suitable method for short-term marking of C. chilia juveniles. Lakeshores with water depths less than 300 cm were identified as preferred habitats for C. chilia juveniles. The primary adaptation period for cultured C. chilia juveniles released into the natural environment was found to be approximately 4-5 days. These findings contribute to our understanding of the post-release performance of cultured fish and may provide guidance for the management and evaluation of relevant stock enhancement programs.

Chronic Diarrhea Due to Aeromonas hydrophila in an Immunosuppressed Patient with a Pancreas-Kidney Transplant

The genus Aeromonas belongs to the Aeromonadaceae family. A patient with a pancreas-kidney transplant had multiple episodes of abdominal sepsis after surgery. Aeromonas hydrophila was isolated in the ascitic and biliary fluid drains. After discharge, the patient had several diarrhea episodes, and A. hydrophila was isolated in four stool samples. We decided to test whether the one strain that we initially isolated in ascitic fluid was the same that appeared in the successive stool samples. Five isolates of A. hydrophila were found in the patient. Identification was performed using the MALDI-TOF system and confirmed via multiplex PCR. The analysis of the REP-PCR fingerprint patterns showed one cluster and confirmed that all isolates were related. We also demonstrated the virulent character of this species associated with genes encoding different toxins (act, alt, ast, hlyA, and aerA). The virulence of this species is associated with the expression of genes that encode different toxins, structural proteins, and metal-associated proteins. This case report highlights the severity of this disease, especially in immunocompromised patients, and its adequate treatment.

Screening of host gut-derived probiotics and effects of feeding probiotics on growth, immunity, and antioxidant enzyme activity of hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂)

In recent years, the widespread use of antibiotics in intensive grouper mariculture has resulted in the ineffectiveness of antibiotic treatment, leading to an increasing incidence of diseases caused by bacteria, viruses, and parasites, causing serious economic losses. Hence, it is crucial to develop alternative strategies to antibiotics for healthy and sustainable development of the mariculture industry. Here, we aimed to screen host gut-derived probiotics and evaluate its effects on growth and immunity of grouper. In this study, 43 bacterial strains were isolated from the intestine of the hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂), and a potential probiotic strain G1-26, which can efficiently secrete amylase, protease, and lipase, was obtained using different screening mediums. Based on 16S rDNA sequencing, the potential probiotic strain G1-26 was identified as Vibrio fluvialis. The results of a biological characteristic evaluation showed that V. fluvialis G1-26 could grow at 25-45 °C, pH 5.5-7.5, salinity 10-40, and bile salt concentration 0-0.030%, and produce amylase, lipase, and protease under different culture conditions. Additionally, V. fluvialis G1-26 is sensitive to many antibiotics and does not exhibit aquatic biotoxicity. Subsequently, hybrid groupers were fed diets containing V. fluvialis G1-26 at different concentrations (0, 10⁶, 10⁸, and 10¹⁰ CFU/g) for 60 d. The results showed that V. fluvialis G1-26 at 10⁸ CFU/g did not significantly affect the growth performance of the hybrid grouper (P > 0.05). V. fluvialis G1-26 supplementation at 10⁸ and 10¹⁰ CFU/g significantly promoted the relative expression of immune-related genes in hybrid groupers (TLR3, TLR5, IL-1β, IL-8, IL-10, CTL, LysC, TNF-2, and MHC-2) and improved the activities of alkaline phosphatase, acid phosphatase, total superoxide dismutase, and total protein in the liver. In conclusion, V. fluvialis G1-26, a potential probiotic strain isolated from the intestine of the hybrid grouper, can be used as an effective immunopotentiator at an optimal dose of 10⁸ CFU/g diet. Our results provide a scientific basis for the development and utilization of probiotics in the grouper mariculture industry.

Hesperidin methyl chalcone, a citrus flavonoid, inhibits Aeromonas hydrophila infection mediated by quorum sensing

Plant-derived phytocompounds are effective in treating a variety of ailments and disorders, the most common of which are bacterial infections in humans, which are a major public health concern. Flavonoids, one of the groups of phytocompounds, are known to have significant antimicrobial and anti-infective properties. Hence, the current study investigates the efficacy of the citrus flavonoid hesperidin methylchalcone (HMC) in addressing this major issue. The results of this study indicate that the anti-quorum sensing (anti-QS) action against Aeromonas hydrophila infections is exhibited with a decrease in biofilm development and virulence factors production through in vitro and in silico analyses. In addition, the qPCR findings indicate that HMC has antivirulence action on A. hydrophila by reducing the expression of QS-related virulence genes, including ahyR, ahyB, ahh1, aerA, and lip. Interestingly, HMC significantly rescued the A. hydrophila-infected zebrafish by reducing the internal colonization, demonstrating the in vivo anti-infective potential of HMC against A. hydrophila infection. Based on these results, this study recommends that HMC could be employed as a possible therapeutic agent to treat A. hydrophila-related infections in humans.

Time-dependent microbial shifts during crayfish decomposition in freshwater and sediment under different environmental conditions

Fossilization processes and especially the role of bacterial activity during the preservation of organic material has not yet been well understood. Here, we report the results of controlled taphonomic experiments with crayfish in freshwater and sediment. 16S rRNA amplicon analyzes showed that the development of the bacterial community composition over time was correlated with different stages of decay and preservation. Three dominating genera, Aeromonas, Clostridium and Acetobacteroides were identified as the main drivers in the decomposition of crayfish in freshwater. Using micro-computed tomography (µ-CT), scanning electron microscopy (SEM) and confocal Raman spectroscopy (CRS), calcite clusters were detected after 3-4 days inside crayfish carcasses during their decomposition in freshwater at 24 °C. The precipitation of calcite clusters during the decomposition process was increased in the presence of the bacterial genus Proteocatella. Consequently, Proteocatella might be one of the bacterial genera responsible for fossilization.

Exploring the interactions between the gut microbiome and the shifting surrounding aquatic environment in fisheries and aquaculture: A review

The rise of "new" sequencing technologies and the development of sophisticated bioinformatics tools have dramatically increased the study of the aquaculture microbiome. Microbial communities exist in complex and dynamic communities that play a vital role in the stability of healthy ecosystems. The gut microbiome contributes to multiple aspects of the host's physiological health status, ranging from nutritional regulation to immune modulation. Although studies of the gut microbiome in aquaculture are growing rapidly, the interrelationships between the aquaculture microbiome and its aquatic environment have not been discussed and summarized. In particular, few reviews have focused on the potential mechanisms driving the alteration of the gut microbiome by surrounding aquatic environmental factors. Here, we review current knowledge on the host gut microbiome and its interrelationship with the microbiome of the surrounding environment, mainly including the main methods for characterizing the gut microbiome, the composition and function of microbial communities, the dynamics of microbial interactions, and the relationship between the gut microbiome and the surrounding water/sediment microbiome. Our review highlights two potential mechanisms for how surrounding aquatic environmental factors drive the gut microbiome. This may deepen the understanding of the interactions between the microbiome and environmental factors. Lastly, we also briefly describe the research gaps in current knowledge and prospects for the future orientation of research. This review provides a framework for studying the complex relationship between the host gut microbiome and environmental stresses to better facilitate the widespread application of microbiome technologies in fisheries and aquaculture.

Detection of Virulence-Associated Genes and in vitro Gene Transfer From Aeromonas sp. Isolated From Aquatic Environments of Sub-himalayan West Bengal

Aeromonas is omnipresent in aquatic environments and cause disease within a wide host range. A total of thirty-four isolates from water samples of small fish farms were identified as Aeromonas based on biochemical characteristics and 16S rRNA gene sequence. A total of six virulent factors were analyzed which indicated 100% of isolates as beta-haemolytic and proteolytic, whereas 44.1, 38.2, and 70.6% of isolates produced DNAse, siderophore, and amylase, respectively. Studies on the occurrence of four genetic determinants of virulence factors revealed that aer/haem (haemolytic toxin) and flaA (polar flagella) genes were present in 44.1% of strains whereas ascV (type 3 secretion system) and aspA (serine protease) genes were detected in 21.5 and 8.82% of strains, respectively. Fish (Anabas testudineus) challenge studies showed that the isolate GP3 (Aeromonas veronii) bearing five virulent factors with the combination of aer/haem⁺/ascV⁺/fla⁺ genes induced severe lesions leading to 100% of mortality. In contrast, RB7 possessing four virulence factors and three genes (aer/haem⁺/ascV⁺/aspA⁺) could not produce severe lesions and any mortality indicating the absence of correlation between the virulence factors, its genes, and the pathogenicity in fishes. GP3 was cytotoxic to human liver cell line (WRL-68) in trypan blue dye exclusion assay. The 431 bp aer/haem gene of GP3 was transferable to E. coli Dh5α with a conjugational efficiency of 0.394 × 10^–4 transconjugants per recipient cell. The transfer was confirmed by PCR and by the presence of 23-kb plasmids in both donor and transconjugants. Therefore, the occurrence of mobile genetic elements bearing virulence-associated genes in Aeromonas indicates the need for periodic monitoring of the aquatic habitat to prevent disease outbreaks.

Aeromonas and Human Health Disorders: Clinical Approaches

The genus Aeromonas comprises more than 30 Gram-negative bacterial species and naturally inhabitants from aquatic environments. These microorganisms, commonly regarded as pathogens of fish and several other animals, have been gaining prominence on medical trial due to its ability to colonize and infect human beings. Besides water, Aeromonas are widely spreaded on most varied sources like soil, vegetables, and food; Although its opportunistic nature, they are able to cause infections on immunocompromised or immunocompetent patients. Aeromonas species regarded as potential human pathogens are usually A. hydrophila, A. caviae, and A. veronii biovar sobria. The main clinical manifestations are gastrointestinal tract disorders, wound, and soft tissue infections, as well as septicemia. Regarding to antibiotic responses, the bacteria present a diversified susceptibility profile and show inherence resistance to ampicillin. Aeromonas, as an ascending genus in microbiology, has been carefully studied aiming comprehension and development of methods for detection and medical intervention of infectious processes, not fully elucidated in medicine. This review focuses on current clinical knowledge related to human health disorders caused by Aeromonas to contribute on development of efficient approaches able to recognize and impair the pathological processes.

Extremophilic lipases for industrial applications: A general review

With industrialization and development in modern science enzymes and their applications increased widely. There is always a hunt for new proficient enzymes with novel properties to meet specific needs of various industrial sectors. Along with the high efficiency, the green and eco-friendly side of enzymes attracts human attention, as they form a true answer to counter the hazardous and toxic conventional industrial catalyst. Lipases have always earned industrial attention due to the broad range of hydrolytic and synthetic reactions they catalyse. When these catalytic properties get accompanied by features like temperature stability, pH stability, and solvent stability lipases becomes an appropriate tool for use in many industrial processes. Extremophilic lipases offer the same, thermostable: hot and cold active thermophilic and psychrophilic lipases, acid and alkali resistant and active acidophilic and alkaliphilic lipases, and salt tolerant halophilic lipases form excellent biocatalyst for detergent formulations, biofuel synthesis, ester synthesis, food processing, pharmaceuticals, leather, and paper industry. An interesting application of these lipases is in the bioremediation of lipid waste in harsh environments. The review gives a brief account on various extremophilic lipases with emphasis on thermophilic, psychrophilic, halophilic, alkaliphilic, and acidophilic lipases, their sources, biochemical properties, and potential applications in recent decades.

Comparison of gut microbiome in the Chinese mud snail (Cipangopaludina chinensis) and the invasive golden apple snail (Pomacea canaliculata)

Background

Gut microbiota play a critical role in nutrition absorption and environmental adaptation and can affect the biological characteristics of host animals. The invasive golden apple snail (Pomacea canaliculata) and native Chinese mud snail (Cipangopaludina chinensis) are two sympatric freshwater snails with similar ecological niche in southern China. However, gut microbiota comparison of interspecies remains unclear. Comparing the difference of gut microbiota between the invasive snail P. canaliculata and native snail C. chinensis could provide new insight into the invasion mechanism of P.canaliculata at the microbial level.

Methods

Gut samples from 20 golden apple snails and 20 Chinese mud snails from wild freshwater habitats were collected and isolated. The 16S rRNA gene V3-V4 region of the gut microbiota was analyzed using high throughput Illumina sequencing.

Results

The gut microbiota dominantly composed of Proteobacteria, Bacteroidetes, Firmicutes and Epsilonbacteraeota at phylum level in golden apple snail. Only Proteobacteria was the dominant phylum in Chinese mud snail. Alpha diversity analysis (Shannon and Simpson indices) showed there were no significant differences in gut microbial diversity, but relative abundances of the two groups differed significantly (P < 0.05). Beta diversity analysis (Bray Curtis and weighted UniFrac distance) showed marked differences in the gut microbiota structure (P < 0.05). Unique or high abundance microbial taxa were more abundant in the invasive snail compared to the native form. Functional prediction analysis indicated that the relative abundances of functions differed significantly regarding cofactor prosthetic group electron carrier and vitamin biosynthesis, amino acid biosynthesis, and nucleoside and nucleotide biosynthesis (P < 0.05). These results suggest an enhanced potential to adapt to new habitats in the invasive snail.

Adapting to Novel Environments Together: Evolutionary and Ecological Correlates of the Bacterial Microbiome of the World's Largest Cavefish Diversification (Cyprinidae, Sinocyclocheilus)

The symbiosis between a host and its microbiome is essential for host fitness, and this association is a consequence of the host’s physiology and habitat. Sinocyclocheilus, the largest cavefish diversification of the world, an emerging multi-species model system for evolutionary novelty, provides an excellent opportunity for examining correlates of host evolutionary history, habitat, and gut-microbial community diversity. From the diversification-scale patterns of habitat occupation, major phylogenetic clades (A–D), geographic distribution, and knowledge from captive-maintained Sinocyclocheilus populations, we hypothesize habitat to be the major determinant of microbiome diversity, with phylogeny playing a lesser role. For this, we subject environmental water samples and fecal samples (representative of gut-microbiome) from 24 Sinocyclocheilus species, both from the wild and after being in captivity for 6 months, to bacterial 16S rRNA gene profiling using Illumina sequencing. We see significant differences in the gut microbiota structure of Sinocyclocheilus, reflective of the three habitat types; gut microbiomes too, were influenced by host-related factors. There is no significant association between the gut microbiomes and host phylogeny. However, there is some microbiome related structure at the clade level, with the most geographically distant clades (A and D) being the most distinct, and the two overlapping clades (B and C) showing similarities. Microbes inhabiting water were not a cause for significant differences in fish-gut microbiota, but water quality parameters were. Transferring from wild to captivity, the fish microbiomes changed significantly and became homogenized, signifying plastic changes and highlighting the importance of environmental factors (habitat) in microbiome community assembly. The core microbiome of this group, at higher taxonomic scale, resembled that of other teleost fishes. Our results suggest that divergent natural environments giving rise to evolutionary novelties underlying host adaptations, also includes the microbiome of these fishes.

Aeromonas sobria Serine Protease Degrades Several Protein Components of Tight Junctions and Assists Bacterial Translocation Across the T84 Monolayer

Aeromonas sobria is a Gram-negative pathogen that causes food-borne illness. In immunocompromised patients and the elderly, A. sobria opportunistically leads to severe extraintestinal diseases including sepsis, peritonitis, and meningitis. If A. sobria that infects the intestinal tract causes such an extraintestinal infection, the pathogen must pass through the intestinal epithelial barrier. In our earlier study using intestinal cultured cells (T84 cells), we observed that an A. sobria strain with higher A. sobria serine protease (ASP) production caused a marked level of bacterial translocation across the T84 intestinal epithelial monolayer. Herein, we investigated the effect of ASP on tight junctions (TJs) in T84 cells. We observed that ASP acts on TJs and causes the destruction of ZO-1, ZO-2, ZO-3, and claudin-7 (i.e., some of the protein components constituting TJs), especially in the strains with high ASP productivity. Based on the present results together with those of our earlier study, we propose that ASP may cause a disruption of the barrier function of the intestinal epithelium as a whole due to the destruction of TJs (in addition to the destruction of adherens junctions) and that ASP may assist invasion of the pathogens from the intestinal epithelium into deep sites in the human body.

Dynamics, Diversity, and Virulence of Aeromonas spp. in Homestead Pond Water in Coastal Bangladesh

Aeromonads are aquatic bacteria associated with frequent outbreaks of diarrhea in coastal Bangladesh, but their potential risks from environmental sources have remained largely unexplored. This study, over 2 years, examined homestead pond waters in the region for monthly dynamics and diversity of Aeromonas spp. The bacterial counts showed bi-modal annual growth peak, pre- and post-monsoon, strongly correlating (p < 0.0005) with temperature. Of 200 isolates characterized, Aeromonas veronii bv. sobria (27%) was predominant among co-existent Aeromonas schubertii (20%), Aeromonas hydrophila (17%), Aeromonas caviae (13%), and three more. PCR screening of virulence-related genes identified 15 genotypes (I to XV), however, enterotoxigenicity in animal model was observed for five genotypes, ca. 18% (nine of 50) strains, prevalent in A. veronii bv. sobria, A. hydrophila, and A. caviae. Pathogenic strains were distinguishable by possessing at least three of the major virulence genes: ascV, hlyA, ela, ast, and alt, together with accessory virulence factors. PFGE of XbaI-digested genomic DNA revealed high genetic diversity and distant lineage of potentially toxigenic clones. Therefore, along with increased global warming, Aeromonas spp. having multi-factorial virulence potential in coastal ponds that serve as drinking water sources pose a potential health risk, and underscores the need for routine monitoring.

Virulence and antimicrobial resistance potential of Aeromonas spp. associated with shellfish

Aeromonas spp. are associated with seafood-related outbreaks worldwide. In seafood industry, shellfish play a major role in global seafood production. With this emerging trend of shellfish consumption, shellfish-related bacterial infections are being reported frequently. Aeromonas spp. are natural contaminants found in shellfish. Although 36 species have been identified, some species including Aeromonas hydrophila, Aeromonas caviae and Aeromonas veronii biotype sobria have dragged major attention as foodborne pathogenic bacteria. The ability to elaborate a variety of virulence factors of Aeromonas spp. contributes to the pathogenic activities. Also, emerging antimicrobial resistance in Aeromonas spp. has become a huge challenge in seafood industry. Furthermore, multidrug resistance increases the risk of consumer health. Studies have supplied pieces of evidence about the emerging health risk of Aeromonas spp. isolated from seafood. Therefore, the present review was intended to highlight the prevalence, virulence and antimicrobial resistance of Aeromonas spp. isolated from various types of shellfish.

Modulation of the intestinal barrier adaptive functions in red-eared slider (Trachemys scripta elegans) invading brackish waters

Globally, the increase in sea levels is leading to salinization of freshwater, which might influence the freshwater organisms such as red-eared slider, Trachemys scripta elegans. The turtle can invade brackish water environments, in which it must deal with elevated salinity in the gastrointestinal tract that could impact the intestinal function. The intestinal barrier provides a front-line of organismal defense against the chemical and biological environmental insults. In this study, the adaptive functions of the intestinal barrier including intestinal histomorphology, genes involved in intestinal barrier functions, and the intestinal micro-ecosystem were analyzed in the turtles exposed to freshwater (S0), 5‰ salinity (S5) and 15‰ salinity (S15) water for 30 days. The results showed that the intestine of T. s. elegans maintained normal histomorphological structure in the S5 group, whereas the villus height, crypt depth and the number of goblet cells in the S15 group were lower than that in the S5 and S0 groups. In addition, the relative expression levels of epithelial tight junction-related genes and intestinal immune-related genes in the gut were significantly upregulated in the S15 group, compared to the freshwater group. Mucin-2 gene expression was downregulated, but mucin-1 transcript levels were upregulated in salinity-treated groups. Furthermore, the abundances of phylum Proteobacteria, and genera Morganella and Aeromonas in the intestine were particularly enhanced in the S15 group than the S0 and S5 groups. Taken together, these results indicate that the intestinal barrier plays a protective role in T. s. elegans adaptation to brackish water environments. Our results provide a perspective on the evolution of salinity tolerance and help to evaluate the potential danger of the turtle to other species, and understand the challenges that other species must meet with rising sea levels.

Outer Membrane Vesicles Released From Aeromonas Strains Are Involved in the Biofilm Formation

Aeromonas spp. are Gram-negative rod-shaped bacteria ubiquitously distributed in diverse water sources. Several Aeromonas spp. are known as human and fish pathogens. Recently, attention has been focused on the relationship between bacterial biofilm formation and pathogenicity or drug resistance. However, there have been few reports on biofilm formation by Aeromonas. This study is the first to examine the in vitro formation and components of the biofilm of several Aeromonas clinical and environmental strains. A biofilm formation assay using 1% crystal violet on a polystyrene plate revealed that most Aeromonas strains used in this study formed biofilms but one strain did not. Analysis of the basic components contained in the biofilms formed by Aeromonas strains confirmed that they contained polysaccharides containing GlcNAc, extracellular nucleic acids, and proteins, as previously reported for the biofilms of other bacterial species. Among these components, we focused on several proteins fractionated by SDS-PAGE and determined their amino acid sequences. The results showed that some proteins existing in the Aeromonas biofilms have amino acid sequences homologous to functional proteins present in the outer membrane of Gram-negative bacteria. This result suggests that outer membrane components may affect the biofilm formation of Aeromonas strains. It is known that Gram-negative bacteria often release extracellular membrane vesicles from the outer membrane, so we think that the outer membrane-derived proteins found in the Aeromonas biofilms may be derived from such membrane vesicles. To examine this idea, we next investigated the ability of Aeromonas strains to form outer membrane vesicles (OMVs). Electron microscopic analysis revealed that most Aeromonas strains released OMVs outside the cells. Finally, we purified OMVs from several Aeromonas strains and examined their effect on the biofilm formation. We found that the addition of OMVs dose-dependently promoted biofilm formation, except for one strain that did not form biofilms. These results suggest that the OMVs released from the bacterial cells are closely related to the biofilm formation of Aeromonas strains.

Dietary Azomite, a natural trace mineral complex, improved the growth, immunity response, intestine health and resistance against bacterial infection in largemouth bass (Micropterus salmoides)

Azomite is a hydrated calcium sodium aluminosilicat rich in rare earth elements. To investigate the dietary effects of Azomite on growth, intestine microbiota and morphology, immunohematological changes and disease resistance, seven diets with Azomite supplementation of 0 (the control), 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 g/kg (A0, A1, A2, A3, A4, A5, A6), were prepared and fed to largemouth bass, Micropterus salmoides (7.96 ± 0.19) for 60 days. The results revealed that the weight gain (WG) increased first and then decreased with the increasing dietary Azomite, and the A2 group presented the highest WG and lowest feed conversion ratio among all the groups. The supplementation of 2.0 g/kg Azomite significantly increased the intestine protease activity, the crude protein of whole body and protein retention (P < 0.05), and high inclusion of Azomite (6.0 g/kg) significantly reduced the lipid retention (P < 0.05). The amounts of red blood cells in A5, A6 groups, white blood cells in A3, A5, A6 groups and lymphocyte in A2-A6 groups were all significantly higher than those in the control group (P < 0.05). In addition, serum superoxide dismutase and catalase activities in A5, A6 groups, and serum alkaline phosphatase and lysozyme activities in A2-A4 groups showed significantly higher values than the control group (P < 0.05). Intestinal microbiota analysis indicated that the Tenericutes abundance was increased, whereas Proteobacteria abundance was decreased in all Azomite supplemented groups. The villus height in A2-A4 groups, and the villus width in A2 group were significantly higher than those of the control group (P < 0.05). The cumulative mortality was reduced by the addition of 2.0-5.0 g/kg Azomite after challenging with A. hydrophila (P < 0.05). In conclusion, proper addition of Azomite in diets improved the growth, intestine morphology, immune response and disease resistance in largemouth bass, and the optimal inclusion was estimated to be 2.0-3.0 g/kg diet.

Ornamental fish: a potential source of pathogenic and multidrug-resistant motile Aeromonas spp

Aeromonas spp. are ubiquitous bacteria that cause diseases in fish and other aquatic animals. They are the natural inhabitants of different aquatic environments, such as freshwater, brackishwater and marinewater. Extrinsic stressors, such as crowding, unhygienic handling, poor water quality, polluted feeding and inadequate nutrition, can predispose fish to Aeromonas infection. In ornamental fish, motile Aeromonas spp. are known as aetiological agents of motile aeromonad infections, which cause significant mortality in fish and economic loss in the ornamental fish industry. The existence of different virulence factors leads to the virulence potential of motile Aeromonas spp. There are several antimicrobials used to treat bacterial infections in ornamental fish. However, the extensive use of antimicrobials in the ornamental fish industry causes multidrug resistance. This article reviewed a multitude of virulence factors that are related to the ornamental fish-borne Aeromonas pathogenicity and the antimicrobial resistance determinants related to the multidrug resistance phenotypes of motile Aeromonas spp. in ornamental fish.

Diversity of virulence-associated genes in pathogenic Aeromonas hydrophila isolates and their in vivo modulation at varied water temperatures

Most environmental parameters have no consistent effect on the expression of bacterial genes responsible for their virulence. However, as fish are poikilothermic, the possibility of temperature variation having a pronounced effect on the expression of virulence-associated gene(s) of bacteria infecting the host needs to be investigated. In this study, the diversity of virulence genes in seven Aeromonas hydrophila isolates collected from diseased fish from different parts of India was characterized, and the effect of temperature variation on the extent of expression of their virulence was investigated. All bacterial isolates were screened for a total of nine bacterial virulent genes {aerolysin, hemolysin, cytoen, outer membrane protein TS (Omp TS), elastase, flagellin, lipase, β hemolysin and type 3 secretion system}, and the diversity in their presence or absence were marked at a particular in vitro condition. Three bacterial isolates (nos. 1, 7 and 2) were selected for further study, based on their ability to cause varied mortalities (20-100%) in Labeo rohita juveniles in intraperitoneal challenge study. Further, three isolates were injected intraperitoneally into L. rohita fingerlings at three different temperatures (i.e., 20, 28 and 37 °C) and at 6 h post-challenge, the kidney samples were collected to measure the levels of all nine bacterial virulence genes using semi-quantitative PCR. The maximum level of amplicons of virulence genes in all three A. hydrophila isolates was noticed at 28 °C as compared to 37 °C and 20 °C. It was also observed that haemolysin played a more prominent role in the expression of virulence, when compared to cytoen gene. Hence, it was concluded that water temperature does play a crucial role in governing virulence gene expression, and a temperature of 28 °C would be considered as suitable for looking into the pathogenicity of A. hydrophila for conducting any challenge study with this organism in tropical environment.

Identification of a new effector-immunity pair of Aeromonas hydrophila type VI secretion system

The type VI secretion system (T6SS) is a multiprotein weapon that kills eukaryotic predators or prokaryotic competitors by delivering toxic effectors. Despite the importance of T6SS in bacterial environmental adaptation, it is still challenging to systematically identify T6SS effectors because of their high diversity and lack of conserved domains. In this report, we discovered a putative effector gene, U876-17730, in the whole genome of Aeromonas hydrophila NJ-35 based on the reported conservative domain DUF4123 (domain of unknown function), with two cognate immunity proteins encoded downstream. Phylogenetic tree analysis of amino acids indicates that AH17730 belongs to the Tle1 (type VI lipase effector) family, and therefore was named Tle1^AH. The deletion of tle1^AH resulted in significantly decreased biofilm formation, antibacterial competition ability and virulence in zebrafish (Danio rerio) when compared to the wild-type strain. Only when the two immunity proteins coexist can bacteria protect themselves from the toxicity of Tle1^AH. Further study shows that Tle1^AH is a kind of phospholipase that possesses a conserved lipase motif, Gly-X-Ser-X-Gly (X is for any amino acid). Tle1^AH is secreted by T6SS, and this secretion requires its interaction with an associated VgrG (valine-glycine repeat protein G). In conclusion, we identified a T6SS effector-immunity pair and verified its function, which lays the foundation for future research on the role of T6SS in the pathogenic mechanism of A. hydrophila.

A Metagenomic Study of Intestinal Microbial Diversity in Relation to Feeding Habits of Surface and Cave-Dwelling Sinocyclocheilus Species

Light is completely absent in cave habitats, causing a shortage or lack of autochthonous photosynthesis. Thus, understanding the mechanisms underlying the ability of organisms to adapt to the unique cave habitat is of great interest. We used high-throughput sequencing of the 16S ribosomal RNA gene of intestinal microorganisms from 11 Sinocyclocheilus (Cypriniformes: Cyprinidae) species, to explore the characteristics of intestinal microorganisms and the adaptive mechanisms of Sinocyclocheilus cavefish and surface fish. We found that the α-diversity and richness of the intestinal microbiome were much higher in cavefish than in surface fish. Principal coordinate analysis showed that cavefish and surface fish formed three clusters because of different dominant gut microorganisms which are generated by different habitats. Based on PICRUSt-predicted functions, harmful substance degradation pathways were much more common in cavefish intestinal microorganisms than in those from surface fish. The intestinal microbiota of surface fish group 1 had a higher capacity for carbohydrate metabolism, whereas protein and amino acid metabolism and digestive pathways were more abundant in microorganisms from the cavefish group and surface fish group 2. Combined analysis of the intestinal microbial composition and functional predictions further revealed the structures and functions of intestinal microbial communities in Sinocyclocheilus cave and surface species. Moreover, based on their habits and intestinal microbial composition and intestinal microbial functional predictions, we inferred that the three fish groups were all omnivorous; however, surface fish group 1 preferred feeding on plants, while surface fish group 2 and cavefish preferred meat. This study improves our understanding of mechanisms of adaptation in cave habitats and may contribute to the protection of these habitats from water pollution.

Species identification, virulence markers and antimicrobial resistance profiles of Aeromonas sp. isolated from marketed hard-shelled mussel (Mytilus coruscus) in Korea

Hard-shelled mussel (Mytilus coruscus) is a popular seafood in Korea. This study aimed to determine the virulence markers and antimicrobial resistance patterns of 33 Aeromonas strains isolated from mussels. The isolates were identified as A. salmonicida (n = 14), A. veronii (n = 9), A. enteropelogenes (n = 4), A. caviae (n = 3), A. allosaccharophila (n = 2) and A. bivalvium (n = 1) by gyrB gene sequencing. The sequence divergence between and within the species ranged from 3·70 to 10·40% and 0-1·50% respectively. Every species formed a distinct group in a neighbour-joining phylogenetic tree. The DNase, gelatinase, caseinase, β-haemolysis, biofilm and lipase activities were observed in 33 (100·00%), 31 (93·93%), 30 (90·90%), 27 (81·81%), 21 (63·63%) and 17 (51·51%) isolates respectively. The virulence genes were detected by PCR in the following frequencies: fla (90·09%), aer (87·88%), hlyA (87·88%), ahyB (81·19%), gcaT (75·76%), ser (69·70%), lip (66·67%), alt (57·58%), ast (51·51%) and act (21·21%). Every isolate was resistant to at least three of 18 antimicrobials in the disk diffusion test. The multiple antimicrobial resistance index values ranged from 0·11 to 0·44 among the isolates. Our study suggests that mussels can be a potential reservoir of virulent and multidrug-resistant Aeromonas sp. SIGNIFICANCE AND IMPACT OF THE STUDY: Aeromonas sp. are known as common pathogenic bacteria isolated from seafood. The virulence factors and antimicrobial resistance profiles of mussel-borne Aeromonas sp. are poorly understood. This study demonstrated for the first time the existence of virulence markers and antimicrobial resistance of Aeromonas sp. from mussels in Korea. Majority of the isolates were positive for phenotypic virulence characteristics and harboured several virulence genes which reveal the potential virulence of mussel-borne Aeromonas sp. Multiple antimicrobial resistance was also observed among the isolates. Our study highlights the importance of food safety standards in mussel consumption.

Prevalence of virulence and antimicrobial resistance genes in Aeromonas species isolated from marketed cockles (Tegillarca granosa) in Korea

This study aimed to examine incidence, virulence and antimicrobial properties in Aeromonas spp. isolated from cockles (Tegillarca granosa) in Korea. Firstly, genomic DNA was extracted from 32 Aeromonas spp. isolates, and PCR screening for virulence, antimicrobial resistance genes was carried out. The disk diffusion assay was used to examine antimicrobial susceptibility. Aeromonas spp. isolates comprised, A. hydrophila (n = 8), A. veronii (n = 15), A. media (n = 2), A. salmonicida (n = 2), A. allosaccharophila (n = 1), A. bestiarum (n = 1), A. culicicola (n = 1), A. enteropelogenes (n = 1) and A. rivipollensis (n = 1). High prevalence of virulence-related genes reported as; act (69%), alt (47%), ast (41%), aerA (56%), lip (50%), ahyB (47%), ser (28%), fla (66%), gcat (44%), ascV (50%) and hlyA (72%). All isolates were multidrug resistant, while highest resistance level observed for ampicillin (100%), followed by imipenem (81%), rifampicin (78%), cephalothin (72%), piperacillin (47%) and Colistin sulfate (31%). The presence of bla_SHV , bla_CTX , tetE, aac(6')-Ib, strA-strB, qnrS, qnrB and IntI1 genes were reported in varying combinations. Nevertheless, bla_TEM , bla_IMP , tetA, tetB, qnrA, qnrB and aphAI-IAB genes and the class1 integron were not detected. The high occurrence of virulence and antimicrobial resistance genes in cockles reveals that it can be a potential health risk source for consumers.

Integrated selection and identification of bacteria from polluted sites for biodegradation of lipids

Wastewater treatment plants face major social concern towards removal of problematic pollutants such as fat oils and grease (FOG). In this context, the main objective of the present work was to select natural bacterial isolates from different polluted sites and evaluate them comparatively to isolates from commercial products, for improved bioremediation strategies and bioaugmentation. In total, 196 isolates were analysed for genomic diversity by two PCR-fingerprinting methods and screened for biodegradation potential with pollutants as sole carbon source. The net area under curve (NAUC) was used for preliminary evaluation of growth ability in M9 medium supplemented with oleic acid and triolein. A principal component analysis of all NAUC data showed that natural isolates presented higher overall biodegradation ability and enabled the selection of 11 natural isolates for lipid degradation assays. Selected isolates were identified by 16S rRNA gene sequencing as members of genera with previously described degradative strains, namely, Acinetobacter (1), Aeromonas (2), Bacillus (1), Pseudomonas (1) and Staphylococcus (6). Best biodegradation results in 7-days assay of FOG content removal were 37.9% for oleic acid and 19.1% for triolein by an Aeromonas sp. isolate and a Staphylococcus cohnii isolate, respectively. A respirometry approach confirmed their higher oxygen uptake rates, although longer adaptation phases where required by the Aeromonas sp. isolate. Consequently, these isolates showed great potential for future bioaugmentation products, to promote FOG degradation, for both in situ and ex situ approaches.

Isolation, Identification and Characteristics of Aeromonas veronii From Diseased Crucian Carp (Carassius auratus gibelio)

Aeromonas species often cause disease in farmed fish. In the present study, dominant bacteria were isolated from diseased crucian carp (Carassius auratus gibelio). Based on this, a bacterial isolate was tentatively named CFJY-623. This isolate was identified as Aeromonas veronii based on analysis of its morphological, physiological, and biochemical features, as well as 16S rRNA and gyrB gene sequences. Six virulence genes related to pathogenicity including aerolysin, cytotonic enterotoxins, elastase, glycerophospholipid: cholesterol acyltransferase, lipase, and serine protease were identified in this A. veronii isolate. The median lethal dosage (LD50) of the CFJY-623 isolate for crucian carp was determined as 1.31 × 107 CFU/mL. Artificial experimental infection showed that the CFJY-623 isolate caused considerable histological lesions in the fish, including tissue cell degeneration, necrosis, and inflammatory cell infiltrating. Drug sensitivity testing showed that the isolate was susceptible to aminoglycosides, carbapenemes, and nitrofurans. Exploring its growing features showed that this isolate exhibited a high level of environmental adaptability. These results provided a scientific basis for the identification of A. veronii and treatment for fish infected by this pathogen.

Phylogenetic characteristics, virulence properties and antibiogram profile of motile Aeromonas spp. isolated from ornamental guppy (Poecilia reticulata)

Aeromonas spp. are opportunistic pathogenic bacteria related to an assembly of infectious diseases in ornamental fish. In the present study, virulence properties and antibiotic susceptibility of 52 guppy-borne Aeromonas spp. were investigated. The isolates were identified as A. veronii (n = 34), A. dhakensis (n = 10), A. hydrophila (n = 3), A. caviae (n = 3) and A. enteropelogenes (n = 2) by gyrB gene sequencing. The gyrB sequence deviation within and among the species ranged from 0 to 2.6% and 2.7-9.2%. Each species formed a distinct group in the unrooted neighbor-joining phylogenetic tree. The phenotypic virulence factors such as β-hemolysis, slime, caseinase, DNase, gelatinase and lipase production were observed in 28 (53.9%), 33 (63.5%), 28 (53.9%), 42 (80.8%), 37 (71.2%) and 42 (80.8%) isolates, respectively. The virulence genes were detected by PCR assay in the following proportions- act (84.6%), hly (80.8%), aer (73.1%), lip (73.1%), gcaT (73.1%), ascV (53.8%), ahyB (53.8%) fla (51.9%), alt (48.1%), ast (36.5%) and ser (34.6%), respectively. The amoxicillin, ampicillin, imipenem, nalidixic acid, oxytetracycline and rifampicin were resistant to more than 70.0% of the isolates in antibiotic susceptibility test. Our study suggests that the ornamental guppy can be a potential reservoir of virulent and multi-drug resistant Aeromonas spp.

Antimicrobial resistance of pathogenic Aeromonas spp. isolated from marketed Pacific abalone (Haliotis discus hannai) in Korea

AIMS

The object of this study was to identify potential health concerns of the Aeromons spp. isolated from marketed Pacific abalone (Haliotis discus hannai) with respect to their virulence and antimicrobial resistance patterns.

METHODS AND RESULTS

We identified 29 strains of aeromonads consisting of five species; Aeromonas hydrophila (n = 9), Aeromonas enteropelogenes (n = 14), Aeromonas veronii (n = 3), Aeromonas salmonicida (n = 2) and Aeromonas sobria (n = 1), by employing series of biochemical tests and gene sequencing. In the phenotypic virulence assays, all isolates showed gelatinase and caseinase activities, while lipase formation (69%), phospholipase production (90%), DNase formation (82%), slime production (49%) and haemolysis activity (α = 18% and β = 82%) were also detected among isolates. Prevalence of virulence genes; aerA (100%), fla (66%), ahyB (73%), act (52%), alt (42%), ast (35%), ser (52%), gcat (69%), ascV (43%), hlyA (83%), lip (52%) and exu (59%) were detected by PCR assays. In disc diffusion test, 100% resistance was detected against ampicillin while cephalothin, rifampicin, oxytetracycline, colistine sulphate, nalidixic acid and piperaciliin were resisted by 86, 73, 42, 35, 28, 20 and 20% of the isolates respectively. Thirteen (45%) of the isolates showed multiple antimicrobial resistance (MAR) indices ≥ 0·2.

CONCLUSIONS

Our findings suggest that the potential health risk posed by the abalone-borne Aeromonas spp. should not be underestimated.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first time to evaluate possible public health risks upon consumption of abalone harbored Aeromonas spp. and also to isolate potential pathogenic and multidrug-resistant Aeromonas spp. from Pacific abalone in Korea.

Diverse effects of nitric oxide reductase NorV on Aeromonas hydrophila virulence-associated traits under aerobic and anaerobic conditions

NorV has been known to be an anaerobic nitric oxide reductase associated with nitric oxide (NO) detoxification. Recently, we showed that the norV gene of Aeromonas hydrophila was highly upregulated after co-culturing with Tetrahymena thermophila. Here, we demonstrated that the transcription and expression levels of norV were upregulated in a dose-dependent manner after exposure to NO under aerobic and anaerobic conditions. To investigate the roles of norV in resisting predatory protists and virulence of A. hydrophila, we constructed the norV gene-deletion mutant (ΔnorV). Compared to the wild type, the ΔnorV mutant showed no significant difference in growth at various NO concentrations under aerobic conditions but significantly stronger NO-mediated growth inhibition under anaerobic conditions. The deletion of norV exhibited markedly decreased cytotoxicity, hemolytic and protease activities under aerobic and anaerobic conditions. Also, the hemolysin co-regulated protein (Hcp) in the ΔnorV mutant showed increased secretion under aerobic conditions but decreased secretion under anaerobic conditions as compared to the wild-type. Moreover, the inactivation of norV led to reduced resistance to predation by T. thermophila, decreased survival within macrophages and highly attenuated virulence in zebrafish. Our data indicate a diverse role for norV in the expression of A. hydrophila virulence-associated traits that is not completely dependent on its function as a nitric oxide reductase. This study provides insights into an unexplored area of NorV, which will contribute to our understanding of bacterial pathogenesis and the development of new control strategies for A. hydrophila infection.

Aeromonas sobria serine protease decreases epithelial barrier function in T84 cells and accelerates bacterial translocation across the T84 monolayer in vitro

Aeromonas sobria is a pathogen causing food-borne illness. In immunocompromised patients and the elderly, A. sobria can leave the intestinal tract, and this opportunistically leads to severe extraintestinal diseases including sepsis, peritonitis, and meningitis. To cause such extraintestinal diseases, A. sobria must pass through the intestinal epithelial barrier. The mechanism of such bacterial translocation has not been established. Herein we used intestinal (T84) cultured cells to investigate the effect of A. sobria serine protease (ASP) on junctional complexes that maintain the intercellular adhesion of the intestinal epithelium. When several A. sobria strains were inoculated into T84 monolayer grown on Transwell inserts, the strain with higher ASP production largely decreased the value of transepithelial electrical resistance exhibited by the T84 monolayer and markedly caused bacterial translocation from the apical surface into the basolateral side of T84 monolayer. Further experiments revealed that ASP acts on adherens junctions (AJs) and causes the destruction of both nectin-2 and afadin, which are protein components constituting AJs. Other studies have not revealed the bacterial pathogenic factors that cause the destruction of both nectin-2 and afadin, and our present results thus provide the first report that the bacterial extracellular protease ASP affects these molecules. We speculate that the destruction of nectin-2 and afadin by the action of ASP increases the ability of A. sobria to pass through intestinal epithelial tissue and contributes to the severity of pathological conditions.

Extracts of Hesperozygis ringens (Benth.) Epling: in vitro and in vivo antibacterial activity against fish pathogenic bacteria

AIMS

This study investigated the in vitro antibacterial activity of Hesperozygis ringens (Benth.) Epling leaf extracts against fish pathogenic bacteria, as well as the in vivo activity of the most active extract in silver catfish (Rhamdia quelen) experimentally infected with Aeromonas hydrophila. Moreover, the chemical composition of the extract used in the survival assay was evaluated.

METHODS AND RESULTS

Only hexane extract (HEHR) showed in vitro antibacterial activity (MIC and MBC ranging from 1600 to 3200 μg ml^-1 ) against clinical isolates of A. hydrophila, Raoultella ornithinolytica and Citrobacter freundii, obtained from naturally infected silver catfish, and A. hydrophilaATCC 7966. The major compound of the volatile fraction of HEHR was determined as pulegone. HEHR promoted a 93·33% relative survival rate of silver catfish experimentally infected with A. hydrophila 7 days after a single therapeutic bath at 30 mg l^-1 , while florfenicol at 4 mg l^-1 , which promoted a 60% relative survival rate.

CONCLUSIONS

The antibacterial activity of H. ringens (Benth.) Epling leaf extracts seems to be related to phytochemicals of apolar character, since HEHR promoted better survival rate of infected animals than florfenicol.

SIGNIFICANCE AND IMPACT OF THE STUDY

The HEHR has potential to be used in the control and treatment of bacterial infections in organic aquaculture.

The genus Aeromonas: A general approach

The genus Aeromonas comprises more than thirty Gram-negative bacterial species which mostly act as opportunistic microorganisms. These bacteria are distributed naturally in diverse aquatic ecosystems, where they are easily isolated from animals such as fish and crustaceans. A capacity for adaptation also makes Aeromonas able to colonize terrestrial environments and their inhabitants, so these microorganisms can be identified from different sources, such as soils, plants, fruits, vegetables, birds, reptiles, amphibians, among others. Infectious processes usually develop in immunocompromised humans; in fish and other marine animals this process occurs under conditions of stress. Such events are most often associated with incorrect practices in aquaculture. Aeromonas has element diverse ranges, denominated virulence factors, which promote adhesion, colonization and invasion into host cells. These virulence factors, such as membrane components, enzymes and toxins, for example, are differentially expressed among species, making some strains more virulent than others. Due to their diversity, no single virulence factor was considered determinant in the infectious process generated by these microorganisms. Unlike other genera, Aeromonas species are erroneously differentiated by conventional biochemical tests. Therefore, molecular assays are necessary for this purpose. Nevertheless, new means of identification have been considered in order to generate methods that, like molecular tests, can correctly identify these microorganisms. The main objectives of this review are to explain environmental and structural characteristics of the Aeromonas genus and to discuss virulence mechanisms that these bacteria use to infect aquatic organisms and humans, which are important aspects for aquaculture and public health, respectively. In addition, this review aims to clarify new tests for the precise identification of the species of Aeromonas, contributing to the exact and specific diagnosis of infections by these microorganisms and consequently the treatment.

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.

Detecção de Aeromonas spp. e do gene de virulência aerolisina em tilápias do Nilo (Oreochromis niloticus) com a técnica de mPCR

RESUMO: As infecções causadas por bactérias do gênero Aeromonas estão entre as doenças mais comuns em peixes cultivados em todo o mundo, com ocorrência de aeromoniose em todos os países que possuem cultivo de tilápia do Nilo (Oreochromis niloticus). O presente trabalho descreve o desenvolvimento de uma nova multiplex PCR (mPCR) para diagnóstico de Aeromonas spp. e identificação do gene aerolisina (aerA). Para padronização da mPCR foram utilizadas cepas de referência de várias espécies do gênero Aeromonas e de outros gêneros. Também foram usadas cepas de campo de A. hydrophila oriundas de cultivos de peixes pacamãs (Lophiosilurus alexandri) e Aeromonas spp. de tilápias do Nilo. Os primers foram desenhados com base na região 16S rRNA e aerA. Para verificar a melhor temperatura de anelamento foram utilizados gradientes entre 59°C a 61°C com 40ng de DNA molde. Os produtos da amplificação da região 16S rRNA e do gene aerA apresentaram 786 e 550pb, respectivamente. A mPCR apresentou melhor temperatura de anelamento a 57,6°C com limite de detecção das concentrações de DNA em ambos genes (16S rRNA and aerA) de 10-10g/μL. A mPCR padronizada é rápida, sensível e específica no diagnóstico de Aeromonas spp. e identificação do gene aerolisina. Esta metodologia apresenta vantagens quando comparada aos métodos de diagnóstico convencionais, podendo ser utilizada em cultivos comerciais de tilápias do Nilo ou outros peixes. A identificação do gene aerolisina é uma importante ferramenta na determinação do potencial patogênico dos isolados de Aeromonas spp. estudados.

Tetrahymena thermophila Predation Enhances Environmental Adaptation of the Carp Pathogenic Strain Aeromonas hydrophila NJ-35

Persistence of Aeromonas hydrophila in aquatic environments is the principle cause of fish hemorrhagic septicemia. Protistan predation has been considered to be a strong driving force for the evolution of bacterial defense strategies. In this study, we investigated the adaptive traits of A. hydrophila NJ-35, a carp pathogenic strain, in response to Tetrahymena thermophila predation. After subculturing with Tetrahymena, over 70% of A. hydrophila colonies were small colony variants (SCVs). The SCVs displayed enhanced biofilm formation, adhesion, fitness, and resistance to bacteriophage infection and oxidative stress as compared to the non-Tetrahymena-exposed strains. In contrast, the SCVs exhibited decreased intracellular bacterial number in RAW264.7 macrophages and were highly attenuated for virulence in zebrafish. Considering the outer membrane proteins (OMPs) are directly involved in bacterial interaction with the external surroundings, we investigated the roles of OMPs in the antipredator fitness behaviors of A. hydrophila. A total of 38 differentially expressed proteins were identified in the SCVs by quantitative proteomics. Among them, three lipoproteins including SurA, Slp, and LpoB, and a serine/threonine protein kinase (Stpk) were evidenced to be associated with environmental adaptation of the SCVs. Also, the three lipoproteins were involved in attenuated virulence of SCVs through the proinflammatory immune response mediated by TLR2. This study provides an important contribution to the understanding of the defensive traits of A. hydrophila against protistan predators.

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.

Inhibition of Autolysis by Lipase LipA in Streptococcus pneumoniae Sepsis

More than 50% of sepsis cases are associated with pneumonia. Sepsis is caused by infiltration of bacteria into the blood via inflammation, which is triggered by the release of cell wall components following lysis. However, the regulatory mechanism of lysis during infection is not well defined. Mice were infected with Streptococcus pneumoniae D39 wild-type (WT) and lipase mutant (ΔlipA) intranasally (pneumonia model) or intraperitoneally (sepsis model), and survival rate and pneumococcal colonization were determined. LipA and autolysin (LytA) levels were determined by qPCR and western blotting. S. pneumoniae Spd_1447 in the D39 (type 2) strain was identified as a lipase (LipA). In the sepsis model, but not in the pneumonia model, mice infected with the ΔlipA displayed higher mortality rates than did the D39 WT-infected mice. Treatment of pneumococci with serum induced LipA expression at both the mRNA and protein levels. In the presence of serum, the ΔlipA displayed faster lysis rates and higher LytA expression than the WT, both in vitro and in vivo. These results indicate that a pneumococcal lipase (LipA) represses autolysis via inhibition of LytA in a sepsis model.

Involvement of the Arg566 residue of Aeromonas sobria serine protease in substrate specificity

Aeromonas sobria serine protease (ASP) is an extracellular serine protease secreted by the organism. Here, we identified the amino acid residue of ASP that contributes to substrate specificity by using both synthetic peptides and biological protein components. The results showed that the arginine residue at position 566 (Arg-566) of ASP, which is located in the extra occluding region of ASP close to an entrance of the catalytic cavity, is involved in the substrate specificity. A substitutional point mutation of the Arg-566 residue of ASP to Ala residue (ASP[R566A]) caused a decrease of the proteolytic efficiency for a certain substrate. In addition, ASP lost the ability to recognize the primary substrate by such a point mutation, and ASP[R566A] reacted to a wide range of synthetic substrates. It is likely that Arg-566 causes an interaction with the amino acid residue at position P3 of the substrate, which is the third amino acid residue upstream from the cleavage site. Another study using ORF2 protein, a chaperone protein of ASP, further suggested that Arg-566 could also play an important role in interaction with ORF2. We therefore conclude that the Arg-566 residue of ASP is likely responsible for the selection of substrates.

Sanitary evaluation of domestic water supply facilities with storage tanks and detection of Aeromonas, enteric and related bacteria in domestic water facilities in Okinawa Prefecture of Japan

To provide for temporary restrictions of the public water supply system, storage tanks are commonly installed in the domestic water systems of houses and apartment buildings in Okinawa Prefecture of Japan. To learn more about the sanitary condition and management of these water supply facilities with storage tanks (hereafter called "storage tank water systems") and the extent of bacterial contamination of water from these facilities, we investigated their usage and the existence of Aeromonas, enteric and related bacteria. Verbal interviews concerning the use and management of the storage tank water systems were carried out in each randomly sampled household. A total of 54 water samples were collected for bacteriological and physicochemical examinations. Conventional methods were used for total viable count, fecal coliforms, identification of bacteria such as Aeromonas, Enterobacteriaceae and non-fermentative Gram-negative rods (NF-GNR), and measurement of residual chlorine. On Aeromonas species, tests for putative virulence factor and an identification using 16S rRNA and rpoB genes were also performed. Water from the water storage systems was reported to be consumed directly without boiling in 22 of the 54 houses (40.7%). 31 of the sampled houses had installed water storage tanks of more than 1 cubic meter (m³) per inhabitant, and in 21 of the sampled houses, the tank had never been cleaned. In all samples, the total viable count and fecal coliforms did not exceed quality levels prescribed by Japanese waterworks law. Although the quantity of bacteria detected was not high, 23 NF-GNR, 14 Enterobacteriaceae and 5 Aeromonas were isolated in 42.6%, 7.4% and 3.7% of samples respectively. One isolated A. hydrophila and four A. caviae possessed various putative virulence factors, especially A. hydrophila which had diverse putative pathogenic genes such as aer, hlyA, act, alt, ast, ser, and dam. Many bacteria were isolated when the concentration of residual chlorine was below 0.1 mg/l and the water temperature was above 20 °C. These results suggest that elevated water temperature and mismatch between tank size and water demand lead to loss of residual chlorine in tap water. Therefore, to minimize growth of aquatic bacteria such as Aeromonas spp. and Pseudomonas spp., we recommend that an appropriate size tank and/or volume of stored water is always used, and also suggest installation of some means of reducing water temperature such as shading.