Pre-exposure to Streptococcus suis improved survival of influenza virus co-infection in mice.

The synergism of the influenza virus and respiratory tract pathogens is known to exacerbate diseases in both humans and animals. The mechanism of the co-infection of associated respiratory tract pathogens is explored in this study. Co-infection has a directional effect when influenza virus or other pathogens occur in a different order. In the present study, we used a mouse animal model to study the synergism of influenza virus and Streptococcus suis co-infection in different orders of administration. We found that the group infected with bacteria alone did not show any clinical symptoms, but the group infected with the virus alone showed 100 % mortality and clinical signs typical in infected mice. In the bacteria infected following virus pre-exposure group, the mice died before the virus-infected group and showed severer clinical signs. When the influenza virus was administered after the bacteria, the infected mice showed reduced mortality compared with mice administered the influenza virus alone. The results indicated that the order of infection significantly affected the outcome of the co-infection of these two pathogens in the mice. However, the underlying mechanism was unclear. Therefore, a transcriptome analysis of mouse lungs was conducted to explore the potential mechanism. The results showed that inflammation and cell damage signaling pathways were upregulated, which may have contributed to the increased mortality in the secondary bacterial infection group. Upregulated innate immunity may have been a major cause of reduced mortality when the bacteria were inoculated before the virus infection.