Pathogenesis and in vivo interactions of human Streptococcus agalactiae isolates in the Galleria mellonella invertebrate model

Abstract

Group B Streptococcus (GBS) is a gram positive bacterium colonizing the gastrointestinal and urogenital tracts in humans. However under certain conditions GBS invades leading to severe infections in neonates, pregnant women, immunocompromised patients and the elderly people. The precise mechanisms involved in the transition from colonizer to pathogen remain to be elucidated, however it has been suggested that environmental determinants may regulate gene expression resulting in GBS invasion. We have assessed the potential of the moth Galleria mellonella as a model to study the in vivo virulence and GBS interactions of invasive and noninvasive human isolates from our population. Temperature, pH and bacterial competition effects were examined in the model as well as the response of Galleria hemocytes to GBS infection. GBS strains were able to effectively grow and infect G. mellonella in a dose dependent manner with a (half-lethal dose) LD₅₀ 1 × 10⁷ CFU after 24 h. GBS infection induced larva melanization with hemocyte vacuolation and depletion. Larval killing increased with environmental conditions such as temperature (37 °C) and pH (≥5.5-7.2). Bacterial interference assays showed a remarkable antagonistic effect of Lactobacillus gasseri (cells and filtrates) on GBS infection and significantly improved Galleria survival. The protective effect of L. gasseri was observed even at ratios similar to those of GBS colonization suggesting that L. gasseri modulation by its metabolic products is relevant. Exposure to L. gasseri acidic filtrates induced growth inhibition and prevented larva killing after infection with the hypervirulent GBS clone (a multiresistant clinical ST 17 strain). We showed that mechanisms mediating these effects are mainly pH dependent, however other mechanisms may have a role depending on inocula. We also found that G. mellonella infected with invasive human GBS isolates showed differential killing curves with higher killing rates after 24 h when compared to those considered colonizers or noninvasive isolates. Overall it has been shown that G. mellonella may be a representative in vivo model for baseline GBS studies. Given the potential effects over the hypervirulent strain, our findings support the use of L. gasseri in the GBS control strategies based on Lactobacillus formulations.

Keywords: Galleria mellonella; Group B Streptococcus; Lactobacillus gasseri; Virulence.