Effect of Algal-Derived Compounds on Growth and Survival of The Fish Pathogen Francisella noatunensis subsp. orientalis

Piscine francisellosis, caused by Francisella noatuenensis subsp orientalis (Fno), is an emerging infectious disease in the tilapia industry, but no effective commercial treatments or vaccines are available. The use of immunostimulants is a promising method to control diseases in aquaculture, and various algae and algal-derived compounds are potent immunostimulants for improving immune status. Algae produce a great variety of secondary metabolites that exert a broad spectrum of biological activities. The aim of this thesis was to evaluate the effectiveness of algal compounds against Fno in vitro and in vivo and determine their potential to control francisellosis infection in Nile tilapia Oreochromis niloticus L. under experimental conditions, and in an alternative host, namely the greater wax moth Galeria mellonella. Some of the algae and their compounds (Chlorella sp., alginic acid, and ß-glucan) exerted antimicrobial activity in vitro against Fno, Aeromonas hydrophila and Streptococcus agalactiae and stimulated responses of Nile tilapia macrophages (Chapter 2). An immersion challenge model for Fno STIR-GUS-F2f7 was developed in two genetic groups of Nile tilapia, and the homo gold strain was more susceptible to infection than wild type (Chapter 3). In vivo trials were conducted in Nile tilapia homo gold where fish were fed diets supplemented with 10% Scenedesmus quaricauda, 10% Haematococcus pluvialis, and 0.1% or 0.2% alginic acid or ß-glucan, and then challenged with Fno and co-infected with S. agalactiae (Chapter 4). The Fno challenge failed to produce mortality; however, co-infection resulted in high mortalities in all groups. As the in vivo trial in tilapia could not be to repeated, a G. mellonella model for Fno was validated. Fno doses between 0.7–1.7 x 108 CFU mL-1 killed G. mellonella, while tetracycline, alginic acid and ß-glucan rescued the wax moth from lethal doses of bacteria (Chapter 5)

Larva of greater wax moth Galleria mellonella is a suitable alternative host for the fish pathogen Francisella noatunensis subsp. orientalis

Background Francisella noatunensis subsp. orientalis (Fno) is the etiological agent of francisellosis in cultured warm water fish, such as tilapia. Antibiotics are administered to treat the disease but a better understanding of Fno infection biology will inform improved treatment and prevention measures. However, studies with native hosts are costly and considerable benefits would derive from access to a practical alternative host. Here, larvae of Galleria mellonella were assessed for suitability to study Fno virulence. Results Larvae were killed by Fno in a dose-dependent manner but the insects could be rescued from lethal doses of bacteria by antibiotic therapy. Infection progression was assessed by histopathology (haematoxylin and eosin staining, Gram Twort and immunohistochemistry) and enumeration of bacteria recovered from the larval haemolymph on selective agar. Fno was phagocytosed and could survive intracellularly, which is consistent with observations in fish. Virulence of five Fno isolates showed strong agreement between G. mellonella and red Nile tilapia hosts. Conclusions This study shows that an alternative host, G. mellonella, can be applied to understand Fno infections, which will assist efforts to identify solutions to piscine francisellosis thus securing the livelihoods of tilapia farmers worldwide and ensuring the production of this important food source