Group A Streptococcus Persists in the Lysosomes of Phagocytic Cells

Abstract

Group A Streptococcus (GAS) is a gram‐positive bacterium that is responsible for invasive diseases such as necrotizing fasciitis and streptococcal toxic shock‐like syndrome. Normally, phagocytic cells of the innate immune system such as macrophages rid the body of pathogens by ingestion and digestion of pathogenic microbes. Pathogens are internalized into structures called phagosomes, which then fuse with lysosomes, facilitating the degradation of the pathogen. Acidification of the phagolysosome/lysosome compartment is an important part of the process to activate the proteolytic enzymes of the lysosome. To visualize GAS as it is trafficked through this pathway, we transformed bacteria with a pH‐sensitive green fluorescent protein, mWASABI. Continued mWASABI fluorescence and persistence of GAS within the lysosomal compartment suggests that GAS survives phagolysosomal degradation by preventing acidification of the compartment. We verified that the mWASABI protein expressed by GAS is appropriately quenched in low pH environments and that GAS is not acid‐tolerant. We hypothesize that streptolysin O (SLO), a GAS secreted protein capable of creating pores, contributes to GAS prevention of acidification. To study this, a mutant strain of GAS lacking the SLO gene (ΔSLO) was transformed to express mWASABI. Surprisingly, ΔSLO mutants also persisted in the lysosomal compartment and were equally fluorescent to wild type bacteria. We are currently exploring whether overexpression of mWASABI may be masking the quenching of the pH‐sensitive probe. We are also exploring whether other pore‐forming toxins contribute to preventing lysosomal acidification. Determining the mechanism for how GAS prevents lysosomal acidification will be important in the development of therapeutics aimed at enhancing the ability of phagocytic cells to destroy GAS.