Streptococcus suis is an important zoonotic pathogen which can infect humans and pigs worldwide, posing a potential risk to global public health. Suilysin, a pore-forming cholesterol-dependent cytolysin, is considered to play an important role in the pathogenesis of S. suis infections. It is known that infection with influenza A viruses may favor susceptibility to secondary bacterial infection, resulting in more severe disease and increased mortality. However, the molecular mechanisms underlying these coinfections are incompletely understood. Applying highly differentiated primary porcine respiratory epithelial cells grown under air-liquid interface (ALI) conditions, we analyzed the contribution of swine influenza viruses (SIV) to the virulence of S. suis, with a special focus on its cytolytic toxin, suilysin. We found that during secondary bacterial infection, suilysin of S. suis contributed to the damage of well-differentiated respiratory epithelial cells in the early stage of infection, whereas the cytotoxic effects induced by SIV became prominent at later stages of infection. Prior infection by SIV enhanced the adherence to and colonization of porcine airway epithelial cells by a wild-type (wt) S. suis strain and a suilysin-negative S. suis mutant in a sialic acid-dependent manner. A striking difference was observed with respect to bacterial invasion. After bacterial monoinfection, only the wt S. suis strain showed an invasive phenotype, whereas the mutant remained adherent. When the epithelial cells were preinfected with SIV, the suilysin-negative mutant also showed an invasion capacity. Therefore, we propose that coinfection with SIV may compensate for the lack of suilysin in the adherence and invasion process of suilysin-negative S. suis.
FIG 4Invasion of the bronchiolar epithelium by S. suis after preinfection with SIV. PBEC were preinfected with 5 ϫ 10 4 TCID 50 /filter H3N2 for 24 h, subsequently inoculated with S. suis wt strain 10 or the 10Δsly mutant at 2.5 ϫ 10 7 /filter from the apical compartment for 4 h, and then washed thoroughly to remove nonadherent bacteria and further incubated under ALI conditions until 48 hpi. Before fixation, the infected cells were washed with PBS to remove nonadherent bacteria. PBEC monoinfected with S. suis wt strain 10, the 10Δsly mutant, or H3N2 and mock-infected cells served as controls. (A) Immunostaining detection for colocalization of streptococci with H3N2-infected cells in the bronchiolar epithelium. Streptococci are labeled in green, nucleoproteins of H3N2 are stained in red, and nuclei are shown in blue (DAPI). The areas in the squares in the center of the panels in the bottom row are magnified in the insets at the top left of those panels. Bars, 25 m. (B) Immunostaining detection for invasion of streptococci into the bronchiolar epithelium, with vertical sections of PBEC being shown. Streptococci are labeled in green, nucleoproteins of H3N2 are stained in red, and nuclei are shown in blue (DAPI). White arrows indicate tissue-invasive bacteria, and the dashed...