Source or Sink: Examining the Role of Biofilms in Transmission of Mycobacterium spp. in Laboratory Zebrafish

Ecological and evolutionary perspectives advance understanding of mycobacterial diseases

Predicting the outbreak of infectious diseases and designing appropriate preventive health actions require interdisciplinary research into the processes that drive exposure to and transmission of disease agents. In the case of mycobacterial diseases, the epidemiological understanding of the scientific community hitherto was based on the clinical studies of infections in vertebrates. To evaluate the information gained by comprehensively accounting for the ecological and evolutionary constraints, we conducted literature searches assessing the role of mycobacteria interactions with non-vertebrate species in the origin of their pathogenicity and variations in disease risk. The reviewed literature challenges the current theory of person-to-person transmission for several mycobacterial infections. Furthermore, the findings suggest that diverse non-vertebrate organisms influence virulence, mediate transmission, and contribute to pathogen abundance in relation to vertebrate exposure. We advocate that an ecological and evolutionary framework provides novel insights to support a more comprehensive understanding of the prevention and management of diseases in vertebrates.

Amphibians as a model to study the role of immune cell heterogeneity in host and mycobacterial interactions

Mycobacterial infections represent major concerns for aquatic and terrestrial vertebrates including humans. Although our current knowledge is mostly restricted to Mycobacterium tuberculosis and mammalian host interactions, increasing evidence suggests common features in endo- and ectothermic animals infected with non-tuberculous mycobacteria (NTMs) like those described for M. tuberculosis. Importantly, most of the pathogenic and non-pathogenic NTMs detected in amphibians from wild, farmed, and research facilities represent, in addition to the potential economic loss, a rising concern for human health. Upon mycobacterial infection in mammals, the protective immune responses involving the innate and adaptive immune systems are highly complex and therefore not fully understood. This complexity results from the versatility and resilience of mycobacteria to hostile conditions as well as from the immune cell heterogeneity arising from the distinct developmental origins according with the concept of layered immunity. Similar to the differing responses of neonates versus adults during tuberculosis development, the pathogenesis and inflammatory responses are stage-specific in Xenopus laevis during infection by the NTM M. marinum. That is, both in human fetal and neonatal development and in tadpole development, responses are characterized by hypo-responsiveness and a lower capacity to contain mycobacterial infections. Similar to a mammalian fetus and neonates, T cells and myeloid cells in Xenopus tadpoles and axolotls are different from the adult immune cells. Fetal and amphibian larval T cells, which are characterized by a lower T cell receptor (TCR) repertoire diversity, are biased toward regulatory function, and they have distinct progenitor origins from those of the adult immune cells. Some early developing T cells and likely macrophage subpopulations are conserved in adult anurans and mammals, and therefore, they likely play an important role in the host-pathogen interactions from early stages of development to adulthood. Thus, we propose the use of developing amphibians, which have the advantage of being free-living early in their development, as an alternative and complementary model to study the role of immune cell heterogeneity in host-mycobacteria interactions.

FELASA-AALAS Recommendations for Biosecurity in an Aquatic Facility, Including Prevention of Zoonosis, Introduction of New Fish Colonies, and Quarantine

FELASA and AALAS established a joint working group to advise on good practices for the exchange of fish for research. In a first manuscript, the working group made recommendations for health monitoring and reporting of monitoring results. The focus of this second related manuscript is biosecurity in fish facilities. First, we define the risk of contamination of personnel by zoonotic pathogens from fish or from system water, including human mycobacteriosis. Preventive measures are recommended, such as wearing task-specific personal protective equipment. Then we discuss biosecurity, highlighting the establishment of biosecurity barriers to preserve the health status of a facility. A functional biosecurity program relies on integration of the entire animal facility organization, including the flow of staff and animals, water treatments, and equipment sanitation. Finally, we propose 4 steps for introducing new fish colonies: consideration of international trade and national restrictions; assessing risk according to fish source and developmental stage; establishing quarantine barriers; and the triage, screening, and treatment of newly imported fish. We then provide 3 realistic sample scenarios to illustrate practical biosecurity risk assessments and mitigation measures based on considerations of health status and quarantine conditions.

Differences in susceptibility to Mycobacterium chelonae in zebrafish (Danio rerio) lines commonly used in scientific research

Mycobacteriosis is one of the most common diseases encountered in laboratory zebrafish. These infections can present a problem to researchers using zebrafish because they may introduce unknown experimental variables. Whilst differences in severity of infections between species of Mycobacterium infecting zebrafish have been well documented, little is known about differences in susceptibility between zebrafish lines. Previous surveys have found higher prevalence in the TU zebrafish line relative to other lines, suggesting that there may be underlying genetic differences in susceptibility. This study investigates Mycobacterium chelonae H1E2-GFP infections in four different zebrafish lines commonly used in research (AB, 5D, casper and TU). Fish were exposed to a labelled (green-fluorescent protein (GFP)) strain of M. chelonae by intraperitoneal injection, and infection status was evaluated after 10 weeks. Visualization of GFP in euthanized fish and histology were used as endpoints. In GFP images, severity was assessed by image analysis, and in histological sections, counts of granulomas containing acid-fast bacteria were used. Results indicated differences in severity of infections between lines, but no significant differences in prevalence.

The Effects of Water Volume and Bacterial Concentration on the Water Filtration Assay Used in Zebrafish Health Surveillance

The number of zebrafish in biomedical research has increased exponentially over the past decades, leading to pressure onthe laboratory animal community to develop and refine techniques to monitor zebrafish health so that suitable stocks can be maintained for research. The water filtration assay is a promising technique in which water from a zebrafish system is filtered, and the filter analyzed by PCR. In the present report, we studied how the volume of water tested and the concentration of bacterial pathogens affected test results. To do so, we used stock solutions of 3 zebrafish pathogens: Edwardsiella ictaluri, Aeromonas hydrophila, and Mycobacterium marinum. We used these stocks to create solutions with known concentrations of each pathogen, ranging between 102 and 107 Colony Forming Units (CFU) per ml. One, 2, and 3 L of each solution was filtered using positive pressure, and the filters were submitted to a commercial lab for PCR testing. Results were fit with a logistic regression model, and the probability of obtaining a positive result were calculated. Test sensitivity varied by organism, but in general, test results were positively correlated with the volume of the water filtered and with the concentration of bacteria in solution. We conclude that a positive result can be expected for E. ictaluri at 105 CFU per mL, A. hydrophila at 106 CFU perml, and M. marinum at 106 CFU per mL, when 3 L of solution are filtered.

Histopathology of laboratory-reared Nothobranchius fishes: Mycobacterial infections versus neoplastic lesions

Short-lived killifishes of the genus Nothobranchius Peters, 1868 (Cyprinodontiformes) are considered promising model organisms for biomedical research on ageing and tumorigenesis. We conducted histopathological analysis of 411 adult individuals from three Nothobranchius species to study details on spontaneous age-related neoplastic lesions. Light microscopy based on H&E and toluidine blue-stained sections revealed (a) non-proliferative liver changes with pronounced vacuolation of hepatocytes; (b) proliferation of kidney haemopoietic tissue contributing to excretory system damage; (c) proliferation of splenic mononuclear haemoblasts accompanied by reduced erythropoiesis; (d) proliferation of mononuclear cell aggregates in the liver parenchyma; and (e) rare occurrence of hepatocellular adenomas. Ziehl-Neelsen (ZN) staining revealed that the proliferative lesions are a host defence response to mycobacterial infections manifested by activation of the mononuclear phagocytic system and atypical granulomatous inflammatory reaction. 16S rRNA analysis identified three species of Mycobacterium in our samples. Our findings turn attention to lesions which mimic neoplasms by their gross appearance and question the light microscopic interpretation of lesions unless differential ZN staining is included. Beyond the limitations of our morphological approach, the intensity of mycobacterial infections is a challenging opportunity for research into the molecular-genetic background of the mononuclear phagocytic system reaction in Nothobranchius killifish.

Aquatic Models: Water Quality and Stability and Other Environmental Factors

The use of aquatic animals in ecotoxicology, genetic, and biomedical research has grown immensely in recent years, especially due to the increased use of zebrafish in the laboratory setting. Because water is the primary environment of most aquatic species, the composition and management of this water is paramount to ensuring their health and welfare. In this publication, we will describe the important variables in water quality that can influence animal health and research results, using the zebrafish model for detailed specifics of optimal conditions. Wherever possible, recommendations are provided to reduce the potential impact of poor or highly variable water quality, and standards are given which can be used as institutional goals to maximize animal health and welfare and reduce research variability. It is increasingly important that authors of publications describing work done using aquatic models characterize water quality and other environmental conditions of the animal environment so that the work can be repeated and understood in context of these important factors. It is clear that there are a great many extrinsic factors which may influence research outcomes in the aquatics model laboratory setting, and consequently, an increased level of funding will be essential to support continued research exploring these and other important husbandry conditions. References from a large body of literature on this subject are provided.

The Influence of Behavioral, Social, and Environmental Factors on Reproducibility and Replicability in Aquatic Animal Models

The publication of reproducible, replicable, and translatable data in studies utilizing animal models is a scientific, practical, and ethical necessity. This requires careful planning and execution of experiments and accurate reporting of results. Recognition that numerous developmental, environmental, and test-related factors can affect experimental outcomes is essential for a quality study design. Factors commonly considered when designing studies utilizing aquatic animal species include strain, sex, or age of the animal; water quality; temperature; and acoustic and light conditions. However, in the aquatic environment, it is equally important to consider normal species behavior, group dynamics, stocking density, and environmental complexity, including tank design and structural enrichment. Here, we will outline normal species and social behavior of 2 commonly used aquatic species: zebrafish (Danio rerio) and Xenopus (X. laevis and X. tropicalis). We also provide examples as to how these behaviors and the complexity of the tank environment can influence research results and provide general recommendations to assist with improvement of reproducibility and replicability, particularly as it pertains to behavior and environmental complexity, when utilizing these popular aquatic models.

The Stability and Efficacy of Tricaine Methanesulfonate (MS222) Solution After Long-Term Storage

Tricaine methanesulfonate (MS222) is widely used for the anesthesia and euthanasia of laboratory zebrafish. Fresh solutions have been recommended for each use; however, researchers often mix and store concentrated stock solutions for convenience and to reduce occupational exposure and environmental waste. While this is common practice, published guidelines are often inconsistent. Thus, the objective of this study was to evaluate the stability and anesthetic efficacy of MS222 after long-term storage and to develop specific storage parameters. Stock solutions (100 mg/mL MS222) were mixed and stored in amber jars at 4 °C and -20 °C for 2- and 6-mo. Stability of the solutions was analyzed using liquid chromatography-ion trapmass spectrometry and compared with fresh MS222. Fifty adult (30 male, 20 female) wildtype AB zebrafish (Danio rerio) wererandomly anesthetized with 150 mg/L of one of the following MS222 solutions to evaluate anesthetic efficacy: 1) freshly prepared(0m); 2) 2 mo at 4 °C (2m4); 3) 2 mo at -20 °C (2m-20); 4) 6 mo at 4 °C (6m4); 5) 6 mo at -20 °C (6m-20). Time to cessation of swimming, loss of equilibrium, lack of response to von Frey (VF) stimulation, return of equilibrium, and resumption of swimming were compared between groups. Two fish from each group were euthanized at 24-h and 2-wk after anesthesia, and histopathology was performed. All solutions were determined to be stable under all storage conditions. No clinically significant differences were observed between the fresh and stored stock groups during anesthetic testing. No evidence ofanesthetic-related histologic changes were noted in the gills, skin, kidneys, muscle, and central nervous system. Hepatic megalocytosis and a reduction in hepatic vacuolation were seen to varying degrees across all groups, but did not follow a treatment-related trend. Therefore, 100 mg/mL solutions of MS222 can be stored in amber jars at 4 °C or -20 °C for 6 mo and still used to effectively anesthetize zebrafish.

Transmission of Mycobacterium chelonae and Mycobacterium marinum in laboratory zebrafish through live feeds

The zebrafish (Danio rerio) is a popular vertebrate model organism used in a wide range of research fields. Mycobacteriosis, caused by Mycobacterium species, is particularly concerning because it is a common disease associated with chronic infections in these fish. Infections are also a source of uncontrolled experimental variance that may influence research results. Live feeds for zebrafish are common and include paramecia (Paramecium caudatum), brine shrimp (Artemia franciscana) and rotifers (Branchionus spp.). Although nutritionally beneficial, live feeds may pose a biosecurity risk. In this study, we investigate transmission of Mycobacterium chelonae and Mycobacterium marinum through these three live feeds. We show that all three live feeds ingest both M. marinum and M. chelonae and can transmit mycobacterial infections to zebrafish. This observation emphasizes the need for live feeds to be included in the consideration of potential biosecurity risks. This study is of importance to other beyond the zebrafish community, including those of additional aquatic models and those using live feeds for other types of aquaculture.

Transmission of Mycobacterium chelonae and Mycobacterium marinum in laboratory zebrafish through live feeds

The zebrafish (Danio rerio) is a popular vertebrate model organism used in a wide range of research fields. Mycobacteriosis, caused by Mycobacterium species, is particularly concerning because it is a common disease associated with chronic infections in these fish. Infections are also a source of uncontrolled experimental variance that may influence research results. Live feeds for zebrafish are common and include paramecia (Paramecium caudatum), brine shrimp (Artemia franciscana) and rotifers (Branchionus spp.). Although nutritionally beneficial, live feeds may pose a biosecurity risk. In this study, we investigate transmission of Mycobacterium chelonae and Mycobacterium marinum through these three live feeds. We show that all three live feeds ingest both M. marinum and M. chelonae and can transmit mycobacterial infections to zebrafish. This observation emphasizes the need for live feeds to be included in the consideration of potential biosecurity risks. This study is of importance to other beyond the zebrafish community, including those of additional aquatic models and those using live feeds for other types of aquaculture.