Staphylococcal alpha-toxin is not sufficient to mediate escape from phagolysosomes in upper-airway epithelial cells

Intracellular Staphylococcus aureus has been implicated in the establishment of chronic infections. It is therefore imperative to understand by what means S. aureus is able to survive within cells. Here we use two expression systems with a fluorescent readout to assay alpha-toxin expression and function within phagolysosomes of infected upper-airway epithelial cells: avirulent Staphylococcus carnosus TM300 and phenotypically alpha-toxin-negative S. aureus laboratory strains. Data from CFU recovery assays suggest that the presence of alpha-toxin is not beneficial for the intracellular survival of recombinant Staphylococcus strains. This finding was corroborated by immunofluorescence studies: whereas S. carnosus and S. aureus are able to deliver S. aureus alpha-toxin to lumina of host cell phagolysosomes, the membrane integrity of these organelles was not affected. Alpha-toxin-expressing strains were detected exclusively within lysosome-associated membrane protein 1 (LAMP1)-yellow fluorescent protein (YFP)-positive vesicles. Measurements of intraphagosomal pH illustrated that all infected phagolysosomes acidified regardless of alpha-toxin expression. In contrast, S. aureus expressing Listeria monocytogenes listeriolysin O leads to the breakdown of the phagolysosomal membrane, as indicated by staphylococci that are not associated with LAMP1-YFP-decorated vesicles and that do not reside within an acidic cellular environment. Thus, our results suggest that staphylococcal alpha-toxin is not sufficient to mediate phagolysosomal escape in upper-airway epithelial cells.