Bordetella bronchiseptica promotes adherence, colonization, and cytotoxicity of Streptococcus suis in a porcine precision-cut lung slice model
<i>Bordetella (B.) bronchiseptica</i> and <i>Streptococcus (S.) suis</i> are major pathogens in pigs, which are frequently isolated from co-infections in the respiratory tract and contribute to the porcine respiratory disease complex (PRDC). Despite the high impact of co-infections on respiratory diseases of swine (and other hosts), very little is known about pathogen-pathogen-host interactions and the mechanisms of pathogenesis. In the present study, we established a porcine precision-cut lung slice (PCLS) model to analyze the effects of <i>B. bronchiseptica</i> infection on adherence, colonization, and cytotoxic effects of <i>S. suis</i>. We hypothesized that induction of ciliostasis by a clinical isolate of <i>B. bronchiseptica</i> may promote subsequent infection with a virulent <i>S. suis</i> serotype 2 strain. To investigate this theory, we monitored the ciliary activity by light microscopy, measured the release of lactate dehydrogenase, and calculated the number of PCLS-associated bacteria. To study the role of the pore-forming toxin suilysin (SLY) in <i>S. suis</i>-induced cytotoxicity, we included a SLY-negative isogenic mutant and the complemented mutant strain. Furthermore, we analyzed infected PCLS by histopathology, immunofluorescence microscopy, and field emission scanning electron microscopy. Our results showed that pre-infection with <i>B. bronchiseptica</i> promoted adherence, colonization, and, as a consequence of the increased colonization, the cytotoxic effects of <i>S. suis</i>, probably by reduction of the ciliary activity. Moreover, cytotoxicity induced by <i>S. suis</i> is strictly dependent on the presence of SLY. Though the underlying molecular mechanisms remain to be fully clarified, our results clearly support the hypothesis that <i>B. bronchiseptica</i> paves the way for <i>S. suis</i> infection.