Mono- and co-infections of primary porcine respiratory cells with <i>Bordetella bronchiseptica</i> and <i>Streptococcus suis</i> are not affected by the dermonecrotic toxin

<i>Bordetella bronchiseptica</i> is a gram-negative bacterium contributing to respiratory diseases in many different animal species. In the swine population, it occurs frequently and plays a role in the Porcine Respiratory Disease Complex as well as in the pathogenesis of atrophic rhinitis. The dermonecrotic toxin (DNT) is involved in the destruction of the nasal conchae, a hallmark of atrophic rhinitis, and several studies have shown the effects of DNT on osteoblastic cells. Surprisingly, only little is known about the interactions of DNT and respiratory epithelial cells. Thus, we investigated the influence of DNT on porcine respiratory epithelial cells during mono- and co-infections <i>in vitro</i>. For this, we infected porcine precision-cut lung slices and air-liquid interface cultures with a DNT-positive <i>B. bronchiseptica</i> wild-type strain and its isogenic DNT-deficient mutant strain. For co-infection experiments, a <i>Streptococcus suis</i> serotype 2 wild-type strain was used. We evaluated cytotoxic effects and colonization of both pathogens, as well as the pro-inflammatory cytokine response of the host cells. Remarkably, DNT neither contributed to the cytotoxic effects of <i>B. bronchiseptica</i> nor did it affect bacterial colonization. Regarding the cytokine response, pro-inflammatory cytokines were expressed mainly upon infection with <i>B. bronchiseptica</i> but hardly after infection with <i>S. suis</i>, whereas co-infection with both pathogens had an amplifying effect on cytokine expression after prolonged infection, independently of DNT. Concluding, we found no evidence that DNT contributes to the early stages of infection with <i>B. bronchiseptica</i> and <i>S. suis</i> in <i>in vitro</i> models of the porcine respiratory tract.