Practice question

Gram-positive bacteria lack lipopolysaccharide and outer membrane, so their cytoplasmic membrane directly faces thick peptidoglycan. Primary lipid structure remains conventional phospholipid bilayer built from glycerol-3-phosphate esterified to straight-chain or branched fatty acids, typically phosphatidylglycerol, cardiolipin, and lysylphosphatidylglycerol that modulates surface charge to resist cationic antimicrobial peptides. These phospholipids provide fluid matrix housing respiratory complexes, transporters, and lipid II flipping machinery essential for wall synthesis. Glycerol diethers with phytanyl chains are hallmark of archaeal membranes, not Gram-positives. Lipopolysaccharides define Gram-negative outer leaflet, while hopanoids, pentacyclic triterpenoids similar to eukaryotic cholesterol, are produced by some bacteria for membrane ordering but are not primary bulk lipids in typical Gram-positives like Staphylococcus aureus or Bacillus subtilis, where hopanoid synthesis genes are often absent. Hence classic ester-linked phospholipid bilayer remains accurate description, forming permeability barrier and scaffold for peripheral wall polymers such as teichoic acids and capsular polysaccharides via undecaprenyl phosphate carriers. In some Gram-positives, membrane composition includes branched-chain fatty acids and small amounts of menaquinone electron carriers, influencing fluidity and susceptibility to membrane-targeting antimicrobials like daptomycin that inserts into phosphatidylglycerol-rich domains causing depolarization, highlighting importance of bilayer chemistry beyond simple barrier function.