gms | German Medical Science

Background: S. aureus is able to invade and survive in non-professional phagocytes and can also survive engulfment by professional phagocytes. Here, bacteria can escape from the phagosomes and are released after the lysis of the host cells. S. aureus small colony variants (SCV) seem to be well adapted to the intracellular milieu and increasing evidence suggests that they can persist in non-professional phagocytes.

Objectives: We studied the potential of different S. aureus SCVs to persist within human macrophages and their ability to escape from these cells and the immune response and cytolytic effects that is triggered by different strains. Comparison of characteristics of clinical strains with site-directed mutants.

Methods: Human monocyte-derived macrophages were infected with different S. aureus SCV strains (clinical isolate 3878, menandione, hemin and thymidine dependent strains). Persistence and release from macrophages were monitored via protection assays and high resolution electron microscopy. Cytolytic effects of various strains were analyzed by FACS, apoptosis and necrosis were confirmed by independent assays. Inflammatory reaction was measured by IL-1 and TNF-α ELISAs.

Results: The isolate 3878 SCV is able to persist and survive in macrophages for more than 12 days and also displays low cytotoxic effects and a reduced pro-inflammatory effect when compared to wild type. Bacterial release started after 9–12 days. Defined site-directed mutants exhibit different patterns: Thymidine auxotrophs showed a decreased uptake by macrophages and were almost undetectable intracellular. Menadione auxotrophs were not able to persist for a longer duration of time. Hemin auxotrophs did not show a difference to its wildtype strain either. This could be attributed to high intracellular haemin content measured by hemin assays.

Conclusion: SCVs are able to persist intracellularly for several days without affecting the viability of macrophages which may have a potential role for dissemination of bacteria. Site-directed mutants cannot mimic the effects of clinical strains indicating that a complex phenotype is a pre-requisite for intracellular persistence.