Staphylococcus aureus Internalization in Non-Professional Human Phagocytes

Although S. aureus is generally considered as an extracellular pathogen, it has been showed that it can invade a variety of non-professional phagocytes, such as human endothelial cells, bovine epithelial cells and murine fibroblasts. Chronic infections like osteomyelitis, the frequent failure of antibiotic treatments to overcome staphylococcal infections or their recurrence might be related to the ability of S. aureus to invade and persist within certain types of host cells, providing a protected niche. However, the precise role of cellular invasion in S. aureus pathology in vivo is still controversial.

We and others have previously elucidated molecular mechanisms of the internalization step of S. aureus cellular invasion. The entry into the host-cell proceeds by a "zipper-type" mechanism, an F-actin dependent process which resembles professional phagocytosis. Bacterial internalization requires the interaction between bacterial surface adhesins and host-cell receptors. An essential interaction is the fibronectin bridging between S. aureus fibronectin binding proteins (FnBPs) and the host fibronectin receptor integrin α5ß1.

Currently, we study in detail the post-invasion events:

1. The intracellular fate of S. aureus, relying on microscopic techniques,
2. Gene expression changes of S. aureus during internalisation, using microarray technology.

We have provided the first in vivo evidence that S. aureus can survive in human epithelial cells for prolonged periods of time (J Infect Dis. 2005 Sep 15;192(6):1023-8). Data were obtained from endonasal mucosa specimens from patients suffering recurrent S. aureus rhinosinusitis. This supports the concept that S. aureus has to be also considered as an intracellular pathogen. This unusual survival strategy provides a niche, protecting bacteria from humoral and phagocytic responses as well as from the majority of currently used antimicrobials. Intracellular survival might also explain long-term carriage and the challenge of decolonizing carriers. This work was recognized by the 2006 Pfizer award in Infectious Diseases.

Recently, we have developed an in vitro model using human lung epithelial cells that shows intracellular bacterial persistence for up to 2 weeks. Using an original approach we successfully collected and amplified low amounts of bacterial RNA recovered from infected eukaryotic cells. Transcriptomic analysis using an oligoarray covering the whole S. aureus genome was performed at two post-internalization times and compared to gene expression of non-internalized bacteria (BMC Genomics. 2007 Jun 14;8(1):171). No signs of cellular death were observed after prolonged internalization of Staphylococcus aureus 6850 in epithelial cells. Following internalization, extensive alterations of bacterial gene expression were observed. Whereas major metabolic pathways including cell division, nutrient transport and regulatory processes were drastically down regulated, numerous genes involved in iron scavenging and virulence were up regulated. This initial adaptation was followed by a transcriptional increase in several metabolic functions. However, expression of several toxin genes known to affect host cell integrity appeared strictly limited.