Efficient suilysin-mediated invasion and apoptosis in porcine respiratory epithelial cells after streptococcal infection under air-liquid interface conditions

Meng, Fandan; Wu, Nai-Huei; Seitz, Maren; Herrler, Georg; Valentin-Weigand, Peter

To further dissect to what extent suilysin-induced cytotoxic effects may contribute to deeper invasion of S. suis, we co-stained streptococci and ciliated cells of PBEC at 48 hpi and 96hpi. In these experiments the suilysin-positive wt strain and the complemented strain cS148 induced damage to the PBEC resulting in the loss of a large proportion of ciliated cells at 48hpi (Fig. 5A ). Vertical sections of these samples illustrate that the loss of the ciliated cells enables the wt and cS148 bacteria to invade to deeper areas of the cell layer (Fig. 5B) . Strain cW461F was able to invade PBEC to a certain extent and caused only moderate local lesions at 48 hpi. In striking contrast, the suilysin-deficient mutants 10Δ sly and 10Δ cpsΔ sly remained adherent to ciliated cells and were unable to cross this barrier. This effect was more pronounced at 96 hpi ( Fig. 5C ), when just a few cilia were detectable and invasive S. suis wt even reached the basal area of the epithelial cell layer. Similar results were obtained with strain cS148, i. e. many bacteria crossed the epithelial barrier. Accordingly, despite large numbers of adherent bacteria at 96 hpi, the suilysin-deficient nonencapsulated mutant 10Δ cpsΔ sly was unable to invade the ciliated epithelium. Interestingly, infection of PBEC by the point-mutated suilysin cW461F for 96h revealed many intracellular bacteria but hardly any damaged ciliated epithelial cells or bacteria invading deeper areas (Fig. 5) .