Interactions between <i>Streptococcus agalactiae</i> and <i>Candida albicans</i> affect persistence and virulence

<i>Streptococcus agalactiae</i> (Group B Streptococcus or GBS), a Gram-positive bacterium, and <i>Candida albicans</i>, a polymorphic fungus, are commensal microbes in most of the population they colonize. However, for certain patients, they can cause severe and sometimes fatal infections. Previous research has indicated that GBS and <i>C. albicans</i> can synergize to enhance the colonization of GBS in the bladders of mice, but not much was known prior to this study about how interactions between GBS and <i>C. albicans</i> alter treatment effectiveness and infection outcome <i>in vivo</i>. Results showed that interactions between the two opportunistic pathogens were influenced by media nutrient availability and that the presence of <i>C. albicans</i> in a culture reduces the effectiveness of certain antibiotics against GBS <i>in vitro</i>. This study also utilized a larval zebrafish model to investigate differences in virulence in solo infections vs co-infections with both pathogens <i>in vivo</i>. Co-infections of GBS and <i>C. albicans</i> into the otic vesicle were found to have increased virulence compared to solo infections of either pathogen. Co-infection also led to an increased GBS burden compared to solo GBS infections. Co-infections of GBS and <i>C. albicans</i> by yolk sac injection were not more virulent than solo infections with either pathogen. However, the antibiotic clindamycin was less effective in preventing mortality in co-infections compared to solo GBS infections. Overall, these findings highlight how interactions between GBS and <i>C. albicans</i> can influence treatment effectiveness and virulence during infection.