Inhibition of frog antimicrobial peptides by extracellular products of the bacterial pathogenAeromonas hydrophila

The importance of antimicrobial peptides (AMPs) in amphibian skin defense

Antimicrobial peptides (AMPs) are produced for defense in nearly all taxa from simple bacteria to complex mammalian species. Some amphibian families have developed this defensive strategy to a high level of sophistication by loading the AMPs into specialized granular glands within the dermis. Enervated by the sympathetic nervous system, the granular glands are poised to deliver an array of AMPs to cleanse the wound and facilitate healing. There have been a number of excellent review publications in recent years that describe amphibian AMPs with an emphasis on their possible uses for human medicine. Instead, my aim here is to review what is known about the nature of amphibian AMPs, the diversity of amphibian AMPs, regulation of their production, and to provide the accumulated evidence that they do, indeed, play an important role in the protection of amphibian skin, vital for survival. While much has been learned about amphibian AMPs, there are still important gaps in our understanding of peptide synthesis, storage, and functions.

Changes in the gut microbiota diversity of brown frogs (Rana dybowskii) after an antibiotic bath

Background

Captive amphibians frequently receive antibiotic baths to control bacterial diseases. The potential collateral effect of these antibiotics on the microbiota of frogs is largely unknown. To date, studies have mainly relied on oral administration to examine the effects of antibiotics on the gut microbiota; in contrast, little is known regarding the effects of bath-applied antibiotics on the gut microbiota. The gut microbiota compositions of the gentamicin, recovery, and control groups were compared by Illumina high-throughput sequencing, and the functional profiles were analysed using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt). Furthermore, the relationship between the structure and predicted functional composition of the gut microbiota was determined.

Results

The alpha diversity indices were significantly reduced by the gentamicin bath, illustrating that this treatment significantly changed the composition of the gut microbiota. After 7 days, the gut microbiota of the recovery group was not significantly different from that of the gentamicin group. Forty-four indicator taxa were selected at the genus level, comprising 42 indicators representing the control group and 2 indicators representing the gentamicin and recovery groups. Potential pathogenic bacteria of the genera Aeromonas, Citrobacter, and Chryseobacterium were significantly depleted after the gentamicin bath. There was no significant positive association between the community composition and functional composition of the gut microbiota in the gentamicin or control frogs, indicating that the functional redundancy of the gut bacterial community was high.

Conclusions

Gentamicin significantly changed the structure of the gut microbiota of R. dybowskii, and the gut microbiota exhibited weak resilience. However, the gentamicin bath did not change the functional composition of the gut microbiota of R. dybowskii, and there was no significant correlation between the structural composition and the functional composition of the gut microbiota.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-03044-z.

Comparison of Gut Microbiota Diversity and Predicted Functions Between Healthy and Diseased Captive Rana dybowskii

The gut microbiota plays a key role in host health, and disruptions to gut bacterial homeostasis can cause disease. However, the effect of disease on gut microbiota assembly remains unclear and gut microbiota-based predictions of health status is a promising yet poorly established field. Using Illumina high-throughput sequencing technology, we compared the gut microbiota between healthy (HA and HB) and diarrhoeic (DS) Rana dybowskii groups and analyzed the functional profiles through a phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) analysis. In addition, we estimated the correlation between gut microbiota structures and predicted the functional compositions. The results showed significant differences in the phylogenetic diversity (Pd), Shannon, and observed richness (Sobs) indices between the DS and HB groups, with significant differences observed in the gut microbiota composition between the DS group and the HA and HB groups. Linear discriminant analysis (LDA) effect size (LEfSe) results revealed that Proteobacteria were significantly enriched in the DS group; Bacteroidetes were significantly enriched in the HA and HB groups; and Aeromonas, Citrobacter, Enterococcus, Hafnia-Obesumbacterium, Morganella, Lactococcus, Providencia, Vagococcus, and Staphylococcus were significantly enriched in the DS group. Venn diagrams revealed that there were many more unique genera in the DS group than the HA and HB groups. Among 102 sensitive species selected using the indicator method, 33 indicated a healthy status and 69 (e.g., Acinetobacter, Aeromonas, Legionella, Morganella, Proteus, Providencia, Staphylococcus, and Vagococcus) indicated a diseased status. There was a significant and positive association between the composition and functional composition of the gut microbiota, thus indicating low functional redundancy of the frog gut bacterial community. Rana dybowskii disease was associated with changes in the gut microbiota, which subsequently disrupted bacterial-mediated functions. The results of this study can aid in revealing the effect of the R. dybowskii gut microbiota on host health and provide a basis for elucidating the mechanism of the occurrence of R. dybowskii disease.

Probiotics Modulate a Novel Amphibian Skin Defense Peptide That Is Antifungal and Facilitates Growth of Antifungal Bacteria

Probiotics can ameliorate diseases of humans and wildlife, but the mechanisms remain unclear. Host responses to interventions that change their microbiota are largely uncharacterized. We applied a consortium of four natural antifungal bacteria to the skin of endangered Sierra Nevada yellow-legged frogs, Rana sierrae, before experimental exposure to the pathogenic fungus Batrachochytrium dendrobatidis (Bd). The probiotic microbes did not persist, nor did they protect hosts, and skin peptide sampling indicated immune modulation. We characterized a novel skin defense peptide brevinin-1Ma (FLPILAGLAANLVPKLICSITKKC) that was downregulated by the probiotic treatment. Brevinin-1Ma was tested against a range of amphibian skin cultures and found to inhibit growth of fungal pathogens Bd and B. salamandrivorans, but enhanced the growth of probiotic bacteria including Janthinobacterium lividum, Chryseobacterium ureilyticum, Serratia grimesii, and Pseudomonas sp. While commonly thought of as antimicrobial peptides, here brevinin-1Ma showed promicrobial function, facilitating microbial growth. Thus, skin exposure to probiotic bacterial cultures induced a shift in skin defense peptide profiles that appeared to act as an immune response functioning to regulate the microbiome. In addition to direct microbial antagonism, probiotic-host interactions may be a critical mechanism affecting disease resistance.

Effects of Captivity and Season on the Gut Microbiota of the Brown Frog (Rana dybowskii)

The gut microbiota of amphibians is affected by exogenous and endogenous factors. We performed a comprehensive analysis using high-throughput sequencing technology and functional predictions and observed general changes in the gut microbiota of frogs in different growth stages, seasons, and growth environments. There were no significant differences in microbial richness and diversity between juvenile and adult wild frogs, between the summer and autumn groups of captive frogs, or between wild and captive frogs. There were significant differences in the gut microbiota community structure of Rana dybowskii between the summer and autumn groups of captive frogs and between wild and captive R. dybowskii, whereas the differences between juvenile and adult wild frogs were not significant. The dominant gut bacterial phyla in frogs from both captive and wild environments included Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria. Linear discriminant effect size (LEfSe) analysis showed that Bacteroidetes and Firmicutes were significantly enriched in captive and wild R. dybowskii, respectively linear discriminant analysis (LDA > 4). The core operational taxonomical units (OTUs) that were found in >90% of all frogs tested encompassed 15 core OTUs. The captive frogs exhibited 15 core OTUs in addition to the above overall core microbiota, whereas the wild frogs exhibited 19 core OTUs in addition to the above overall core microbiota. Predictions made using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) suggested that eleven KEGG pathways, such as infectious diseases, immune system diseases, metabolism, metabolism of other amino acids, metabolism of cofactors and vitamins, metabolism of terpenoids and polyketides, neurodegenerative diseases, and transport and catabolism, were enriched in captive frogs. The relative abundance of several red-leg-syndrome-related pathogens increased significantly in captive frogs compared with that in wild frogs. To our knowledge, this is the first study on the effects of individual seasons and captivity on the gut microbiota of frogs.

Immunogenicity of extracellular products from an inactivated vaccine against Aeromonas veronii TH0426 in koi, Cyprinus carpio

Aeromonas veronii is a type of human-livestock-aquatic animal pathogen; it is widely found in nature and causes many deaths among aquatic animals. Extracellular products (ECPs) are secreted by the pathogen during growth and reproduction. These products are considered effective protective antigens that can induce the host to produce an immune response. In this study, the ECPs of A.veronii TH0426 were prepared by ultrafiltration, and then the pathogenicity and enzymatic activity of the ECPs were determined. All the groups were injected intraperitoneally, as follows: group one: ECP protein with an equal volume of Freund's adjuvant; group two: ECPs and formalin-killed cells (FKC) of A.veronii combined with an equal volume of Freund's adjuvant (FKC + ECPs); group three: formalin-killed cells (FKC) of A.veronii combined with an equal volume of Freund's adjuvant (FKC); and, group four: sterile PBS as the control group. The expression levels of IgM, IL-1β, and TNF-α and the lysozyme activity in blood were examined at 7, 14, and 21 days after the immunizations. The results show that the ECPs can produce protease, lipase, amylase and hemolyase, and there was no lecithinase, urease, or gelatinase activity. The results indicate that the ECPs were clearly pathogenic to koi fish, and the LD₅₀ dose was 391.6 μg/fish. Throughout this study, the RPS of the three experimental groups were 75%, 50%, and 70%. This study indicates that the ECPs of A.veronii can effectively enhance the ability of kio fish to resist bacterial invasion.

Sequestered Alkaloid Defenses in the Dendrobatid Poison Frog Oophaga pumilio Provide Variable Protection from Microbial Pathogens

Most amphibians produce their own defensive chemicals; however, poison frogs sequester their alkaloid-based defenses from dietary arthropods. Alkaloids function as a defense against predators, and certain types appear to inhibit microbial growth. Alkaloid defenses vary considerably among populations of poison frogs, reflecting geographic differences in availability of dietary arthropods. Consequently, environmentally driven differences in frog defenses may have significant implications regarding their protection against pathogens. While natural alkaloid mixtures in dendrobatid poison frogs have recently been shown to inhibit growth of non-pathogenic microbes, no studies have examined the effectiveness of alkaloids against microbes that infect these frogs. Herein, we examined how alkaloid defenses in the dendrobatid poison frog, Oophaga pumilio, affect growth of the known anuran pathogens Aeromonas hydrophila and Klebsiella pneumoniae. Frogs were collected from five locations throughout Costa Rica that are known to vary in their alkaloid profiles. Alkaloids were isolated from individual skins, and extracts were assayed against both pathogens. Microbe subcultures were inoculated with extracted alkaloids to create dose-response curves. Subsequent spectrophotometry and cell counting assays were used to assess growth inhibition. GC-MS was used to characterize and quantify alkaloids in frog extracts, and our results suggest that variation in alkaloid defenses lead to differences in inhibition of these pathogens. The present study provides the first evidence that alkaloid variation in a dendrobatid poison frog is associated with differences in inhibition of anuran pathogens, and offers further support that alkaloid defenses in poison frogs confer protection against both pathogens and predators.

Functional analysis for gut microbes of the brown tree frog (Polypedates megacephalus) in artificial hibernation

BACKGROUND

Annual hibernation is an adaptation that helps many animals conserve energy during food shortage in winter. This natural cycle is also accompanied by a remodeling of the intestinal immune system, which is an aspect of host biology that is both influenced by, and can itself influence, the microbiota. In amphibians, the bacteria in the intestinal tract show a drop in bacterial counts. The proportion of pathogenic bacteria is greater in hibernating frogs than that found in nonhibernating frogs. This suggests that some intestinal gut microbes in amphibians can be maintained and may contribute to the functions in this closed ecosystem during hibernation. However, these results were derived from culture-based approaches that only covered a small portion of bacteria in the intestinal tract.

METHODS

In this study, we use a more comprehensive analysis, including bacterial appearance and functional prediction, to reveal the global changes in gut microbiota during artificial hibernation via high-throughput sequencing technology.

RESULTS

Our results suggest that artificial hibernation in the brown tree frog (Polypedates megacephalus) could reduce microbial diversity, and artificially hibernating frogs tend to harbor core operational taxonomic units that are rarely distributed among nonhibernating frogs. In addition, artificial hibernation increased significantly the relative abundance of the red-leg syndrome-related pathogenic genus Citrobacter. Furthermore, functional predictions via PICRUSt and Tax4Fun suggested that artificial hibernation has effects on metabolism, disease, signal transduction, bacterial infection, and primary immunodeficiency.

CONCLUSIONS

We infer that artificial hibernation may impose potential effects on primary immunodeficiency and increase the risk of bacterial infections in the brown tree frog.

RESAMA: A Network for Monitoring Health and Husbandry Practices in Aquatic Research Facilities

Health monitoring is a crucial aspect of the management of any research animal house. RESAMA is a network strong of 60 academic and private partners acting in France since the end of 2012. The network aims to increase awareness of animal caretakers and researchers on health management issues in facilities holding aquatic model species (zebrafish, Xenopus, medaka, Mexican tetra). To do so, each partner research facility will be visited at least once. The visiting team is composed at least of one veterinarian and one zootechnician specialized in aquatic species. The visit results in a health-monitoring assessment of the facility, which includes a sampling for histo-pathological, bacteriological, and molecular pathogen detection. During the visit, rearing practices are also reviewed through an interview of animal caretakers. However, the present report essentially focuses on the health-monitoring aspect. The ultimate goal of the project is to provide a network-wide picture of health issues in aquatic facilities. Performed in parallel, the rearing practice assessment will ultimately help to establish rational relationship between handling practices and animal health in aquatic facilities. The study is still in progress. Here, we describe the results to be drawn from an analysis of the 23 facilities that had been visited so far. We sampled 720 fish and 127 amphibians and performed a little less than 1400 individual tests.

Life history linked to immune investment in developing amphibians

The broad diversity of amphibian developmental strategies has been shaped, in part, by pathogen pressure, yet trade-offs between the rate of larval development and immune investment remain poorly understood. The expression of antimicrobial peptides (AMPs) in skin secretions is a crucial defense against emerging amphibian pathogens and can also indirectly affect host defense by influencing the composition of skin microbiota. We examined the constitutive or induced expression of AMPs in 17 species at multiple life-history stages. We found that AMP defenses in tadpoles of species with short larval periods (fast pace of life) were reduced in comparison with species that overwinter as tadpoles and grow to a large size. A complete set of defensive peptides emerged soon after metamorphosis. These findings support the hypothesis that species with a slow pace of life invest energy in AMP production to resist potential pathogens encountered during the long larval period, whereas species with a fast pace of life trade this investment in defense for more rapid growth and development.

Mortality of therapeutic fish Garra rufa caused by Aeromonas sobria

OBJECTIVE

To investigate a case of mass mortality of Garra rufa (G. rufa) from a fish hatchery farm in Slovakia.

METHODS

Causative bacterial agent was swabbing out of affected fish skin area and subsequently identified using commercial test system. Antibiotic susceptibility was determined by the disk diffusion method.

RESULTS

Infected G. rufa was characterized by abnormal swimming behaviour, bleeding of skin lesions and local haemorrhages. Despite of using recommended aquatic antibiotic treatment no improvement was achieved and Aeromonas sobria (A. sobria) was identified as a causative agent of fish mortality. Due to massive fish mortality, antibiotic susceptibility of pure isolated culture of A. sobria was evaluated employing eight antibiotics against human infections. A. sobria was resistant only against one antibiotic, namely ampicilin.

CONCLUSIONS

These results indicate that A. sobria can act as a primary pathogen of G. rufa and may be a potential risk factor for immunodeficient or immunoincompetent patients during the ichthyotherapy.

Interacting symbionts and immunity in the amphibian skin mucosome predict disease risk and probiotic effectiveness

Pathogenesis is strongly dependent on microbial context, but development of probiotic therapies has neglected the impact of ecological interactions. Dynamics among microbial communities, host immune responses, and environmental conditions may alter the effect of probiotics in human and veterinary medicine, agriculture and aquaculture, and the proposed treatment of emerging wildlife and zoonotic diseases such as those occurring on amphibians or vectored by mosquitoes. Here we use a holistic measure of amphibian mucosal defenses to test the effects of probiotic treatments and to assess disease risk under different ecological contexts. We developed a non-invasive assay for antifungal function of the skin mucosal ecosystem (mucosome function) integrating host immune factors and the microbial community as an alternative to pathogen exposure experiments. From approximately 8500 amphibians sampled across Europe, we compared field infection prevalence with mucosome function against the emerging fungal pathogen Batrachochytrium dendrobatidis. Four species were tested with laboratory exposure experiments, and a highly susceptible species, Alytes obstetricans, was treated with a variety of temperature and microbial conditions to test the effects of probiotic therapies and environmental conditions on mucosome function. We found that antifungal function of the amphibian skin mucosome predicts the prevalence of infection with the fungal pathogen in natural populations, and is linked to survival in laboratory exposure experiments. When altered by probiotic therapy, the mucosome increased antifungal capacity, while previous exposure to the pathogen was suppressive. In culture, antifungal properties of probiotics depended strongly on immunological and environmental context including temperature, competition, and pathogen presence. Functional changes in microbiota with shifts in temperature provide an alternative mechanistic explanation for patterns of disease susceptibility related to climate beyond direct impact on host or pathogen. This nonlethal management tool can be used to optimize and quickly assess the relative benefits of probiotic therapies under different climatic, microbial, or host conditions.

Analysis of skin and secretions of Dybowski's frogs (Rana dybowskii) exposed to Staphylococcus aureus or Escherichia coli identifies immune response proteins

The aim of the present study was to investigate responses in Dybowski's frogs (Rana dybowskii) exposed to bacteria, using proteomic and transcriptomic approaches. Staphylococcus aureus and Escherichia coli were used as representative Gram-positive and Gram-negative bacteria, respectively, in an infectious challenge model. Frog skin and skin secretions were collected and protein expression in infected frogs compared to control frogs by two-dimensional gel electrophoresis, silver staining, and image analysis. Proteins that demonstrated differential expression were analysed by mass spectrometry and identified by searching protein databases. More than 180 protein spots demonstrated differential expression in E. coli- or S. aureus-challenged groups and, of these, more than 55 spots were up- or down-regulated at least sixfold, post-infection. Proteins with a potential function in the immune response were identified, such as stathmin 1a, annexin A1, superoxide dismutase A, C-type lectin, lysozyme, antimicrobial peptides, cofilin-1-B, mannose receptor, histone H4, prohormone convertase 1, carbonyl reductase 1 and some components of the Toll-like receptor (TLR) signalling pathway. These molecules are potential candidates for further investigation of immune mechanisms in R. dybowskii; in particular, TLR-mediated responses, which might be activated in frogs exposed to pathogenic bacteria as part of innate immune defence, but which might also impact on adaptive immunity to infection.

Functional characterization of codCath, the mature cathelicidin antimicrobial peptide from Atlantic cod (Gadus morhua)

Cathelicidins are among the best characterized antimicrobial peptides and have been shown to have an important role in mammalian innate immunity. We recently isolated a novel mature cathelicidin peptide (codCath) from Atlantic cod and in the present study we functionally characterized codCath. The peptide demonstrated salt sensitivity with abrogation of activity at physiological salt concentrations. In low ionic strength medium we found activity against marine and non-marine Gram-negative bacteria with an average MIC of 10 μM, weak activity against a Gram-positive bacterium (MIC 80 μM), and pronounced antifungal activity (MIC 2.5 μM). The results suggest the kinetics and mode of action of codCath to be fast killing accompanied by pronounced cell lysis. Extracellular products (ECPs) of three marine bacteria caused breakdown of the peptide into smaller fragments and the cleaved peptide lost its antibacterial activity. Proteolysis of the peptide on the other hand was abolished by prior heat-treatment of the ECPs, suggesting a protease involvement. We observed no cytotoxicity of the peptide in fish cells up to a concentration of 40 μM and the selectivity of activity was confirmed with bacterial and mammalian membrane mimetics. We conclude that the potent broad-spectrum activity of codCath hints at a role of the peptide in cod immune defense.

Skin peptides of different life stages of Ewing's tree frog

In frogs, an important mechanism of skin innate immunity against invading microbial pathogens is secretion of antimicrobial peptides from the specialized granular glands. Since these glands develop fully in skin dermis after completion of metamorphosis, they are small and immature in skin of larvae (tadpoles). Skin secretions vary among different life stages. Antimicrobial activity and peptide composition of natural mixture of skin peptides of three different life stages of New Zealand Ewing's Tree Frog (Litoria ewingii), tadpoles, metamorphs and adults were analyzed. The peptide mixtures were collected from skin secretions and analyzed for activity against the standard reference bacterium, Escherichia coli (ATCC 25922). Their peptide components were analyzed using liquid chromatography mass spectrometry (LC-MS). The peptide mixture from adults and metamorphs contained the species-specific antimicrobial peptide uperin 7.1 and inhibited the growth of E. coli (ATCC 25922). In contrast, the peptide mixture of tadpoles did not inhibit the growth of E. coli (ATCC 25922). This peptide mixture did not contain uperin 7.1 but had peptides whose molecular masses did not correspond to molecular masses of any known frog antimicrobial peptides.