Staphylococcus aureus as a proficient intracellular pathogen inducing cell death in different human host cell lines

The opportunistic pathogen Staphylococcus aureus is a well-known cause of life-threatening infections in humans. Many isolated strains are multidrug resistant, making the treatment of infections challenging. Formerly designated a merely extracellular pathogen, S. aureus is now recognized to invade and survive within both phagocytic and non-phagocytic host cells, providing a protective hiding niche from antibiotic and immune control. Once inside the cell, the pathogen employs an arsenal of regulatory mechanisms and virulence factors to adjust to the hostile environment, to orchestrate survival in and escape out of intracellular compartments, and to eventually disseminate within the host. However, many studies focus partially or exclusively on non-human models and observations obtained from human cell lines still vary extensively, considering the variety of staphylococcal strains and the host cell types analyzed.

Our aim is to unravel and compare post-invasion events and strategies of host cell remodeling to promote intracellular survival of four different S. aureus isolates, of distinct infection backgrounds and virulence patterns. Intracellular fates are analyzed in three different human cell lines, which reflect host tissues affected upon clinical manifestations. Accordingly, strains were assessed microscopically for intracellular uptake, their co-localization with markers of lysosomal activity, their ability to escape from autophago-/ endolysosomal compartments into the cytosol, and cytotoxicity.

A key strategy for S. aureus to escape the intracellular compartment and disseminate within the host is the manipulation of both timing and mode of host cell death. The extent, pathway, and kinetics of host cell death induction were investigated using several techniques, including flow cytometry, western blot, and xCELLigence RTCA.
We present data derived from S. aureus-infected human U937 monocytes, A549 lung epithelial and SaOS-2 osteoblast-like cells, showing that S. aureus isolates are taken up and reside intracellularly both membrane-enclosed and/or cytosolically. Intracellular S. aureus induces host cell death of different kinetics and magnitude in a strain- and cell type-dependent manner, and includes (but is likely not limited to) apoptotic signaling, as shown by the activation of apoptosis key effectors.

Altogether, we show distinct host cell responses upon intracellular infection of different S. aureus isolates in selected human host cell lines.