The role of staphyloxanthin in the regulation of membrane biophysical properties in Staphylococcus aureus

Staphylococcus aureus is an opportunistic pathogen that is considered a global health threat. This microorganism can adapt to hostile conditions by regulating membrane lipid composition in response to external stress factors such as changes in pH and ionic strength. S. aureus synthesizes and incorporates in its membrane staphyloxanthin, a carotenoid providing protection against oxidative damage and antimicrobial agents. Staphyloxanthin is known to modulate the physical properties of the bacterial membranes due to the rigid diaponeurosporenoic group it contains. In this work, preparative thin layer chromatography and liquid chromatography mass spectrometry were used to purify staphyloxanthin from S. aureus and characterize its structure, identifying C15, C17 and C19 as the main fatty acids in this carotenoid. Changes in the biophysical properties of models of S. aureus membranes containing phosphatidylglycerol, cardiolipin, and staphyloxanthin were evaluated. Infrared spectroscopy shows that staphyloxanthin reduces the liquid-crystalline to gel phase transition temperature in the evaluated model systems. Interestingly, these shifts are not accompanied by strong changes in trans/gauche isomerization, indicating that chain conformation in the liquid-crystalline phase is not altered by staphyloxanthin. In contrast, headgroup spacing, measured by Laurdan GP fluorescence spectroscopy, and lipid core dynamics, measured by DPH fluorescence anisotropy, show significant shifts in the presence of staphyloxanthin. The combined results show that staphyloxanthin reduces lipid core dynamics and headgroup spacing without altering acyl chain conformations, therefore decoupling these normally correlated effects. We propose that the rigid diaponeurosporenoic group in staphyloxanthin and its positioning in the membrane is likely responsible for the results observed.