Streptococcus suis is an encapsulated Gram-positive bacterium and the leading cause of sepsis and meningitis in young pigs, resulting in considerable economic losses in the porcine industry. S. suis is considered an emerging zoonotic agent with increasing numbers of human cases over the last years. In the environment, both avirulent and virulent strains occur in pigs, with no evidence for consistent adapatation of virulent strains to the human host. Currently, there is an urgent need for a convenient, reliable and standardised animal model to rapidly assess S. suis virulence. Wax moth (Galleria mellonella) larvae have successfully been used in human and animal infectious disease studies. Here, we developed G. mellonella larvae as a model to assess virulence of S. suis strains.
Fourteen isolates of S. suis belonging to different serotypes killed G. mellonella larvae in a dose-dependent manner. Larvae infected with the virulent serotype 2 strain, S. suis S3881/S10, were rescued by antibiotic therapy. Crucially, the observed virulence of the different serotypes and mutants was in agreement with virulence observed in piglets (Sus scrofa) and the zebrafish larval infection model. Infection with heat-inactivated bacteria or bacteria-free culture supernatants showed that in most cases live bacteria are needed to cause mortality in G. mellonella.
The G. mellonella model is simple, cost-efficient, and raises less ethical issues than experiments on vertebrates and reduces infrastructure requirements. Furthermore, it allows experiments to be performed at the host temperature (37 °C). The results reported here, indicate that the G. mellonella model may aid our understanding of veterinary microbial pathogens such as the emerging zoonotic pathogen S. suis and generate hypotheses for testing in the target animal host. Ultimately, this might lead to the timely introduction of new effective remedies for infectious diseases. Last but not least, use of the G. mellonella infection model to study S. suis virulence adheres to the principles of replacement, reduction and refinement (3Rs) and can potentially reduce the number of vertebrates used for experimental infection studies.