Abstract
Mycobacterium smegmatis rarely causes disease in the immunocompetent, but reported cases of soft tissue infection describe abscess formation requiring surgical debridement for resolution. Neutrophils are the first innate immune cells to accumulate at sites of bacterial infection, where reactive oxygen species and proteolytic enzymes are used to kill microbial invaders. As these phagocytic cells play central roles in protection from most bacteria, we assessed human neutrophil phagocytosis and granule exocytosis in response to serum opsonized or non-opsonized M. smegmatis mc2. Although phagocytosis was enhanced by serum opsonization, M. smegmatis did not induce exocytosis of secretory vesicles or azurophilic granules at any time point tested, with or without serum opsonization. At early time points, opsonized M. smegmatis induced significant gelatinase granule exocytosis compared to non-opsonized bacteria. Differences in granule release between opsonized and non-opsonized M. smegmatis decreased in magnitude over the time course examined, with bacteria also evoking specific granule exocytosis by six hours after addition to cultured primary single-donor human neutrophils. Supernatants from neutrophils challenged with opsonized M. smegmatis were able to digest gelatin, suggesting that complement and gelatinase granule exocytosis can contribute to neutrophil-mediated tissue damage seen in these rare soft tissue infections.
Highlights
Mycobacterium smegmatis is a rod-shaped, Gram-positive, environmental mycobacterium that typically inhabits soils and natural and municipal waters, leading to frequent human exposure [1]
The percentage of neutrophils with no associated bacteria decreased from 69% in the non-opsonized condition to 19% when co-cultured with opsonized M. smegmatis
As a measure of gelatinase granule exocytosis, we measured the levels of MMP9 in supernatants of neutrophils cultured with media, P. stomatis, or M. smegmatis by enzyme-linked immunosorbent assays (ELISAs) (Figure 6B)
Summary
Mycobacterium smegmatis is a rod-shaped, Gram-positive, environmental mycobacterium that typically inhabits soils and natural and municipal waters, leading to frequent human exposure [1]. M. smegmatis typically causes a recurrent infection where antibiotic courses are ineffective. This results in a general histopathology of chronic skin ulceration with violaceous edges, rolled margins, and significant subcutaneous necrosis that requires aggressive debridement of all infected tissues and skin [12]. M. smegmatis and other saprophytic mycobacteria share a unique, lipid-rich, hydrophobic cell wall architecture that confers a high tolerance to physical and chemical damage and contributes to protection against host oxidative and other defense mechanisms [15,16,17]. M. smegmatis expresses a cell surface phosphoinositol-capped lipoarabinomannan (PILAM) that has Pathogens 2020, 9, 123; doi:10.3390/pathogens9020123 www.mdpi.com/journal/pathogens
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