Bacterial lipases and adaptation to the host lipid environment

About This Grant

Project Summary The host lipid environment is a well-described barrier to bacterial infection at several sites including the skin, lung, and upper airways. In addition, both bacterial and host lipids serve as signals that promote inflammation. Bacterial lipids activate Toll-like receptors which initiates cytokine production, immune cell recruitment, and induction of oxidative burst to control infection. Several pathogenic bacteria evade these lipid-mediated defenses by secreting lipolytic enzymes (lipases). Bacterial lipases can cause host membrane dissolution, facilitate escape from cellular compartments, disrupt physical barriers, detoxify antimicrobial lipids, and release nutrient fatty acids. Despite these established contributions of lipases to infection there exist critical gaps in knowledge on how lipases drive host immunity and physiology. This is especially true for Staphylococcus aureus which produces three lipases that have long been presumed to promote virulence, despite scant mechanistic studies. Recent work from our lab started to fill this gap by providing evidence for a direct role for lipases in immune evasion and nutrient acquisition during infection. We made the unexpected observation that the S. aureus secreted lipase, Geh, inhibited activation of innate immune cells in culture. Further, Geh blunted pro-inflammatory cytokine production during infection and was responsible for bacterial persistence in some tissues but not others, highlighting tissue-specific contributions of Geh to survival. The blunted cytokine response was not due to direct functions of Geh on mammalian cells, but rather a result of inactivation of S. aureus lipoproteins, a major pathogen-associated molecular pattern (PAMP) of Gram-positive bacteria, via ester hydrolysis. While our studies indicate S. aureus uses lipases to inactivate lipoprotein PAMPs containing saturated straight and branched chain fatty acids that it synthesizes de novo, it can also acquire host unsaturated fatty acids in a lipase-dependent manner and use them for lipoprotein biogenesis. We found that S. aureus uptake of host fatty acids disrupted innate immune responses and infection dynamics in tissues involved in lipid metabolism via attachment of host fatty acids to bacterial lipoproteins. This deleterious response correlated well with Toll-like receptor 2 signaling. Finally, infections of the skin uncovered a role for Geh that appeared to be independent of Toll-like receptor 2 and instead was linked to liberation of host fatty acid esters that promoted fitness. Altogether, our data support the hypothesis that S. aureus lipases modify both bacterial and host lipids to calibrate innate immunity and promote a fitness advantage that allows adaptation to a wide range of tissues during infection. Aim 1 will determine the cellular and molecular basis for Geh-mediated infection persistence. Aim 2 will test the roles of the three lipases secreted by S. aureus in virulence, lipid utilization, and immune defense. Aim 3 will determine how lipases disrupt immunity and allow S. aureus to adapt to increased circulating lipids in the host.