Different capacity of host cell types to take up and degrade S. aureus

M. Strobel, S. Niemann, E. Vogel, L. Tuchscherr, C.A. Kreis, G. Peters, B. Löffler

Abstract

Introduction: Host cell invasion and intracellular persistence are major features of Staphylococcus aureus, which contribute to virulence and disease development. The uptake of bacteria is an active process of host cells, which is dependent on the cytoskeleton, and has been described for various cell types. Not only professional, but also non-professional phagocytes have the capacity to degrade ingested bacteria which helps to clear an infection. The aim of our study is to overview on different host cell types to take up and digest S. aureus. Materials and Methods: Initially, we compared S. aureus invasion in different types of human cells, including lung epithelial cells (A549), endothelial cells (HUVECs, Ea.hy926 cells, HBMECs), keratinocytes (HaCaT cells), fibroblasts (CCD32-SK) and osteoblasts (CRL-11372), by infecting them with different S. aureus strains (6850, USA300, LS1, SH1000, Cowan1) at an MOI of 50 for 3 h. After removing all extracellular staphylococci the number of intracellular bacteria was determined by plating serial dilutions of cell lysates, cell inflammation was measured by RT-PCR, and cell death was determined by propidium iodide staining. Results: While endothelial and epithelial cells showed a high capacity to take up bacteria, the other cell types (osteoblasts, fibroblasts and keratinocytes) contained much less bacteria. Inflammation and cytotoxicity following host cell invasion were strongly strain dependent and particularly enhanced in cells that ingest a high number of bacteria, namely endothelial and epithelial cells. To investigate bacterial long-term persistence, we infected Ea.hy923 (endothelial cell line), A549 (epithelial cell line) and CRL-11372 (osteoblast cell line), which take up high or low amounts of bacteria, respectively, and analyzed them for 21 days. In Ea.hy926 and A549 cells the number of intracellular bacteria was dramatically reduced (from approx. 106 to 102), but after 21 days we always recovered persisting bacteria. In CRL-11372 cells the ingested bacteria (strain 6850) were often cleared within 21 days. Discussion: Particularly, cellular barriers (endothelial and epithelial cells) take up high amounts of bacteria that can strain-dependently induce inflammatory and cytotoxic effects. Although the cells have the capacity to degrade ingested bacteria, they can still contain persisting bacteria after 21 days that is a possible reservoir for recurrent infections.