Fluorescence analysis of apoptosis induced by Pseudomonas aeruginosa in endothelial cells

Intracellular invasion of professional phagocytic cells like monocytes and macrophages by a pathogen usually triggers the apoptosis of the host cells. The aim of this study was to evaluate if Pseudomonas aeruginosa, although not considered a classic intracellular pathogen, could adhere to endothelial cell surface, invade the intracellular compartment and subsequently induce apoptosis of the cells. The adherence and invasion capacity of P. aeruginosa to endothelial cells was monitored using Cravioto's adapted method. The apoptotic cells were evidenced by staining with Acridine orange/Ethidium bromide. The qualitative assay of bacterial adherence to the cellular substrate revealed that all tested strains adhered to endothelial cells surface, exhibiting a diffuse, aggregative or mixed (diffuse-aggregative or localized-aggregative) pattern and 20-70% adherence rates. The adherence of P. aeruginosa induced the reorganization of cytoskeleton filaments and formation of endocytic membrane expansions. Cell free P. aeruginosa culture supernatants did not induce any cell death response, as noticed in case of whole bacterial culture, showing the capacity to induce apoptosis of endothelial cells. The fluorescence microscopy examination revealed chromatin condensation, fragmented nuclei, and membrane blebbing and apoptotic bodies in pathogen invaded cells.