Impact of stress on Aeromonas diversity in tambaqui (Colossoma macropomum) and lectin level change towards a bacterial challenge

The Importance of Baseline Health Surveillance Efforts in Freshwater Fish Conservation Using the Threatened Iberian Leuciscids as an Example

Freshwater fish species are experiencing the highest decline among vertebrates in this century. Although a great effort has been made to identify and tackle threats to the conservation of this taxa, several knowledge gaps still exist particularly for noncommercial endangered species, including considerations regarding fish health status. These species face deteriorating environmental conditions in their natural habitats that may lead to stress and increased risk for infectious disease outbreaks. Establishing health surveillance is crucial to identify and predict physiologic disruption in fish populations. Additionally, information retrieved may be used to direct targeted efforts to contribute to improving the conservation status of these species. We used threatened Iberian leuciscids as a case study to discuss the current knowledge regarding their health surveillance and to suggest recommendations for the establishment of practical health assessments that can benefit conservation plans for these species and be implemented in threatened or endangered freshwater fish species plans globally.

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

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

Molecular characterization and evaluation of virulence traits of Aeromonas spp. isolated from the tambaqui fish (Colossoma macropomum)

The present study was developed to characterize, at the species level, 34 strains of Aeromonas spp., previously isolated from stressed tambaqui fish (Colossoma macropomum), to elucidate virulence factors, as well as their antibiotic resistance profile. Amplification of the gyrB gene identified the strains as A. hydrophila, A. dhakensis, A. caviae, A. veronii and A. jandaei. Bacterial virulence was evaluated by enzymatic assays for phenotypical production of hemolysins, proteases and lipases followed by the search for genes codifying the enzymes β-hemolysin, serine protease and lipase. Phenotypical production of virulence factors was diversified and proteolytic activity demonstrated to be a common expression among the strains. On the other hand, the lip gene encoding extracellular lipase was the most expressed. Furthermore, A. hydrophila was the most prevalent species isolated from tambaqui in our work.

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.

Purinergic signalling displays an anti-inflammatory profile in the spleen of fish experimentally infected with Aeromonas caviae: Modulation of the immune response

Extracellular adenosine triphosphate (ATP) and its metabolite adenosine (Ado) are recognized as key mediators of immune and inflammatory responses. Depending on its concentration, ATP may act as an immunostimulant or immunodepressant, while Ado levels display an anti-inflammatory profile. The aim of this study was to evaluate whether splenic purinergic signalling is capable of modulating immune and inflammatory responses in fish experimentally infected with Aeromonas caviae. Triphosphate diphosphohydrolase (NTPDase) and 5'-nucleotidase activities increased in the spleen of silver catfish (Rhamdia quelen) experimentally infected with A. caviae compared with the uninfected control group. Moreover, splenic Ado levels increased in the infected animals relative to the uninfected control group. Based on these lines of evidence, our findings revealed that adenine nucleotide hydrolysis is modified in the spleen of fish infected with A. caviae attempting to restrict the inflammatory process through the upregulation of NTPDase and 5'-nucleotidase activities, which occurs in an attempt to hydrolyse the excessive ATP in the extracellular environment and rapidly hydrolyse AMP to form Ado. In summary, purinergic signalling can modulate immune and inflammatory responses during A. caviae infection.

Aeromonas caviae inhibits hepatic enzymes of the phosphotransfer network in experimentally infected silver catfish: Impairment on bioenergetics

Several studies have been demonstrated that phosphotransfer network, through the adenylate kinase (AK) and pyruvate kinase (PK) activities, allows for new perspectives leading to understanding of disease conditions associated with disturbances in energy metabolism, metabolic monitoring and signalling. In this sense, the aim of this study was to evaluate whether experimental infection by Aeromonas caviae alters hepatic AK and PK activities of silver catfish Rhamdia quelen. Hepatic AK and PK activities decreased in infected animals compared to uninfected animals, as well as the hepatic adenosine triphosphate (ATP) levels. Also, a severe hepatic damage was observed in the infected animals due to the presence of dilation and congestion of vessels, degeneration of hepatocytes and loss of liver parenchyma architecture and sinusoidal structure. Therefore, we have demonstrated, for the first time, that experimental infection by A. caviae inhibits key enzymes linked to the communication between sites of ATP generation and ATP utilization. Moreover, the absence of a reciprocal compensatory mechanism between these enzymes contributes directly to hepatic damage and for a severe energetic imbalance, which may contribute to disease pathophysiology.

Aeromonas caviae alters the cytosolic and mitochondrial creatine kinase activities in experimentally infected silver catfish: Impairment on renal bioenergetics

Cytosolic and mitochondrial creatine kinases (CK), through the creatine kinase-phosphocreatine (CK/PCr) system, provide a temporal and spatial energy buffer to maintain cellular energy homeostasis. However, the effects of bacterial infections on the kidney remain poorly understood and are limited only to histopathological analyses. Thus, the aim of this study was to investigate the involvement of cytosolic and mitochondrial CK activities in renal energetic homeostasis in silver catfish experimentally infected with Aeromonas caviae. Cytosolic CK activity decreased in infected animals, while mitochondrial CK activity increased compared to uninfected animals. Moreover, the activity of the sodium-potassium pump (Na⁺, K⁺-ATPase) decreased in infected animals compared to uninfected animals. Based on this evidence, it can be concluded that the inhibition of cytosolic CK activity by A. caviae causes an impairment on renal energy homeostasis through the depletion of adenosine triphosphate (ATP) levels. This contributes to the inhibition of Na⁺, K⁺-ATPase activity, although the mitochondrial CK activity acted in an attempt to restore the cytosolic ATP levels through a feedback mechanism. In summary, A. caviae infection causes a severe energetic imbalance in infected silver catfish, which may contribute to disease pathogenesis.