Pseudomonas aeruginosa selective adherence to and entry into human endothelial cells

The pathogenesis of Pseudomonas aeruginosa disseminated infections depends on bacterial interaction with blood vessels. We have hypothesized that in order to traverse the endothelial barrier, bacteria would have to adhere to and damage endothelial cells. To test this hypothesis, we studied the adherence to human endothelial cells in primary culture of the piliated P. aeruginosa strain PAK and of two isogenic nonpiliated strains: PAK/p-, which carries a mutation in the pilin structural gene, and PAK-N1, a mutant defective in the regulatory rpoN gene. PAK adhered significantly more than did the pilus-lacking strains. P. aeruginosa was also taken up by endothelial cells, as determined by quantitative bacteriologic assays and by transmission electron microscopy. This internalization of P. aeruginosa seems to be a selective process, since the piliated strain was taken up significantly more than the nonpiliated bacteria and the avirulent Escherichia coli DH5 alpha, even following bacterial centrifugation onto the cell monolayers. A significant fraction of the internalized P. aeruginosa PAK was recovered in a viable form after 6 h of residence within endothelial cells. Progressive endothelial cell damage resulted from PAK intracellular harboring, as indicated by the release of lactate dehydrogenase. An increasing concentration of PAK cells was recovered from the extracellular medium with time, suggesting that ingested bacteria were released from endothelial cells and multiplied freely. We speculate that in vivo the ability of some P. aeruginosa strains to resist intracellular residence would afford protection from host defenses and antibiotics and that the release of viable bacteria into bloodstream may represent a central feature of the pathogenesis of bacteremia in compromised patients.