Abstract
BackgroundFrancisella 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.ResultsLarvae 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.ConclusionsThis 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.
Highlights
Francisella noatunensis subsp. orientalis (Fno) is the etiological agent of francisellosis in cultured warm water fish, such as tilapia
Piscine francisellosis is a global disease caused by the bacterium Francisella noatunensis, with F. noatunensis subsp. orientalis (Fno) infecting warm water fish and F. noatunensis subsp. noatunensis (Fnn) affecting cold water species [1]
Effect of temperature on G. mellonella survival after injection with Fno In the initial experiment to determine the effect of temperature on survival of G. mellonella larvae after injection with ca. 1 × 109 colony-forming units (CFU)/mL of Fno STIR-GUS-F2f7, the group of larvae incubated at 28 °C appeared to have lowest survival while the group kept at 15 °C had greatest survival (Fig. 1), so in all subsequent experiments larvae were incubated at 28 °C
Summary
Francisella noatunensis subsp. orientalis (Fno) is the etiological agent of francisellosis in cultured warm water fish, such as tilapia. Fno is a particular concern for tilapia producers as it can cause up to 95% mortality [12, 13] and Relatively little is known of the infection biology of Fno and a deeper fundamental understanding of virulence and pathogenicity may inform new and improved treatments, prevention measures and farm management practices. To this end, experimental studies have been performed in the native fish hosts and, though these trials have extended our knowledge of francisellosis, such an approach is costly, requires specialist infrastructure (2020) 20:8 such as aquaria, raises ethical questions and can be constrained by legal statutes. More practical alternative hosts that offer insights into the biology of F. noatunensis infections have been explored including zebrafish and their embryos [6, 15]; this fish model suffers similar drawbacks to native hosts and best practice in research seeks adherence to the principles of the 3Rs, i.e. the replacement, reduction and refinement of the use of animals in experiments [16, 17]
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