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

Interestingly, both suilysin-negative mutants, 10Δ sly and 10Δ cpsΔ sly, were hardly found intracellularly ( Fig. 4B ,C). Quantification of the microscopic data revealed that more than 40%, 18% and 35% of all detected bacteria were located intracellularly in PBEC in the case of the suilysin-positive wt strain, strain cW461F and strain cS148, respectively (Fig. 4F ). In contrast, invasion rates of both suilysin-deficient mutants were significantly lower (2.6% for 10Δ sly, below detection limit for 10Δ cpsΔ sly). The clear difference between wt and mutant S. suis was also found in the invasion of PTEC, for which the rate of intracellular wt bacteria was 28% (data not shown). We conclude that suilysin is crucial for efficient invasion of S. suis into airway epithelial cells, and that cytolytic activity of suilysin contributes significantly to this effect, since strain cW461F was significantly less invasive than wt strain and strain cS148. Nevertheless, there was a significant difference between invasion rates of both suilysin-negative mutants and strain cW461F (Fig. 4F ), suggesting that there may be invasion-promoting effects of suilysin which do not depend on cytolytic activity.