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

A striking result was the high capacity of S. suis to invade differentiated airway epithelial cells, as reflected by invasion rates ranging between 28% and 40% determined for PTEC and PBEC, respectively. This result was unexpected since our previous study on host cell invasion of S. suis using the human cell line HEp2 revealed a rather low invasive capacity 39 . However, we cannot directly compare invasiveness of S. suis in both culture systems, since quantification was done by different methods due to the models applied in the present and the previous study. Because of the very low invasion of immortalized cells, encapsulated S. suis is considered as an extracellular bacterium 31, 39, 42 . It has been suggested that invasion by this streptococcus requires the down-regulation of the capsule to achieve efficient adherence and subsequent invasion. Following invasion, the capsule would have to be upregulated again to ensure bacterial survival in the blood stream. However, this has never been demonstrated in vivo. Our data show that encapsulated S. suis is very efficient in invasion suggesting that differential expression of the capsule may not be essential for efficient invasion.