Preliminary comparative assessment of Vibrio harveyi virulence in Galleria mellonella larvae and European sea bass

Vibrio harveyi is a major bacterial pathogen in Mediterranean aquaculture, causing high mortality and economic losses, particularly in European sea bass ( Dicentrarchus labrax ). Despite extensive efforts to develop vaccines, no effective commercial option exists, highlighting the need to better understand infection mechanisms. Studies using fish hosts are labor-intensive, costly, and ethically demanding, emphasizing the value of alternative in vivo models. This study evaluated larvae of the greater wax moth ( Galleria mellonella ) as an alternative model for investigating V. harveyi virulence. Parallel infection trials were conducted in G. mellonella larvae and European sea bass to compare infection outcomes between the alternative and native hosts. Two V. harveyi isolates were tested; one obtained from a clinically diseased fish (94/17) and the other from the intestine of a clinically healthy fish (A27). Larvae were injected with bacterial suspensions (10⁵–10⁸ CFU/mL), bacterial supernatants, and heat-killed cells, with PBS-injected and untreated larvae as controls. Both isolates successfully established infection and caused dose-dependent mortality in larvae, irrespective of their origin, confirming the opportunistic nature of V. harveyi . Injection of bacterial supernatants resulted in 53.3% mortality for isolate 94/17 and 56.7% for isolate A27, demonstrating the toxicity of extracellular products. Similar mortality patterns were observed in European sea bass, with rapid death following high bacterial doses and dose-dependent survival at lower inocula. The LD₅₀ for both isolates was approximately 10⁵ CFU/mL in larvae and fish. This preliminary study suggests that Galleria mellonella may serve as a reliable and reproducible alternative model for studying V. harveyi virulence, offering practical advantages while supporting the 3Rs principles (replacement, reduction, and refinement).