Limited Interactions between Streptococcus Suis and Haemophilus Parasuis in In Vitro Co-Infection Studies

Abstract

<i>Streptococcus suis</i> and <i>Haemophilus parasuis</i> are normal inhabitants of the porcine upper respiratory tract but are also among the most frequent causes of disease in weaned piglets worldwide, causing inflammatory diseases such as septicemia, meningitis and pneumonia. Using an in vitro model of infection with tracheal epithelial cells or primary alveolar macrophages (PAMs), it was possible to determine the interaction between <i>S. suis</i> serotype 2 and <i>H. parasuis</i> strains with different level of virulence. Within <i>H. parasuis</i> strains, the low-virulence F9 strain showed higher adhesion levels to respiratory epithelial cells and greater association levels to PAMs than the high-virulence Nagasaki strain. Accordingly, the low-virulence F9 strain induced, in general, higher levels of pro-inflammatory cytokines than the virulent Nagasaki strain from both cell types. In general, <i>S. suis</i> adhesion levels to respiratory epithelial cells were similar to <i>H. parasuis</i> Nagasaki strain. Yet, <i>S. suis</i> strains induced a significantly lower level of pro-inflammatory cytokine expression from epithelial cells and PAMs than those observed with both <i>H. parasuis</i> strains. Finally, this study has shown that, overall and under the conditions used in the present study, <i>S. suis</i> and <i>H. parasuis</i> have limited in vitro interactions between them and use probably different host receptors, regardless to their level of virulence.