7.3 Lipids

Chemical Structure and Composition of Lipids

Lipids are hydrophobic organic molecules built mainly from carbon and hydrogen. Their nonpolar nature makes them largely insoluble in water, which is exactly what makes them so useful for building membranes and storing energy in microbial cells.

Three main types of lipids show up in microbial cells:

• Fatty acids: Long hydrocarbon chains with a carboxyl group ($-COOH$) at one end. They can be saturated (all single bonds between carbons, making the chain straight and packable) or unsaturated (one or more double bonds that introduce kinks in the chain).
• Glycerolipids: Built on a glycerol backbone with fatty acids attached via ester bonds. This category includes triacylglycerides and phospholipids.
• Sterols: Defined by a four-fused-ring structure. Cholesterol is the classic example in eukaryotic cell membranes, though most bacteria lack sterols entirely and use other molecules (like hopanoids) to fill a similar role.

Triacylglycerides and Phospholipids

Triacylglycerides vs. phospholipids in microorganisms

These two glycerolipids share a glycerol backbone but differ in structure and function.

• Triacylglycerides (triglycerides) have three fatty acids attached to glycerol. They're entirely hydrophobic, which makes them efficient energy storage molecules. Some microorganisms accumulate them in cytoplasmic lipid droplets as a carbon and energy reserve.
• Phospholipids have two fatty acids and a phosphate group attached to glycerol. The phosphate group is linked to a polar head group such as choline, ethanolamine, or serine. This gives phospholipids their amphipathic nature: the fatty acid tails are hydrophobic while the phosphate head is hydrophilic. That dual character is what makes them the primary structural component of microbial cell membranes.

Phospholipids in microbial membranes

Because phospholipids are amphipathic, they spontaneously arrange into bilayers in water:

1. The hydrophobic fatty acid tails face inward, shielded from water.
2. The hydrophilic phosphate head groups face outward, interacting with the aqueous environment on both sides.

This phospholipid bilayer is the foundation of the cell membrane. It acts as a barrier between the cell's interior and the outside environment, maintaining cellular integrity and shape. Membrane proteins sit within or span this bilayer, handling selective transport of molecules, enzymatic reactions, and cell signaling.

Membrane fluidity depends on phospholipid composition, and microorganisms actively adjust it in response to environmental conditions:

• Shorter fatty acid chains and unsaturated fatty acids (with their kinked tails) increase fluidity because they prevent tight packing.
• Longer fatty acid chains and saturated fatty acids decrease fluidity because they pack together more tightly.

For example, a bacterium growing at low temperatures will incorporate more unsaturated fatty acids into its membrane to keep it fluid enough to function. This adaptation is called homeoviscous adaptation.

Lipid rafts are specialized microdomains within membranes that are enriched in cholesterol and sphingolipids. These regions help organize membrane proteins and play roles in cell signaling. They're more relevant in eukaryotic microbes than in most bacteria.

Diverse Structures and Functions of Lipids

Microbial lipids go well beyond membranes and energy storage. Several specialized lipids contribute to cell wall structure, virulence, and communication.

• Lipopolysaccharides (LPS): Found in the outer membrane of Gram-negative bacteria. LPS has three parts: lipid A (which anchors it in the membrane and acts as an endotoxin), a core polysaccharide, and the O-antigen (which varies between strains and helps bacteria evade the immune system). LPS is a major trigger of host immune responses.
• Lipoteichoic acids: Found in the cell wall of Gram-positive bacteria. These molecules are anchored in the cell membrane and extend through the peptidoglycan layer, helping maintain cell wall integrity and participating in host-pathogen interactions.
• Mycolic acids: Exceptionally long-chain fatty acids (up to 90 carbons) found in the cell wall of Mycobacterium species, including the agents of tuberculosis and leprosy. They create a waxy, highly impermeable barrier that makes these bacteria resistant to many antibiotics and harsh conditions.
• Hopanoids: Pentacyclic lipids found in some bacteria that function similarly to cholesterol in eukaryotes. They stabilize membranes and regulate permeability in organisms that lack sterols.
• Lipid-derived signaling molecules: Acyl-homoserine lactones (AHLs) are used in quorum sensing, a process where bacteria detect their own population density and coordinate gene expression accordingly. This can regulate biofilm formation, virulence factor production, and other group behaviors.
• Lipid-soluble pigments: Carotenoids in photosynthetic bacteria serve dual roles: they participate in light harvesting for photosynthesis and protect cells against oxidative damage from reactive oxygen species.

Lipid Metabolism in Microorganisms

Microorganisms both build and break down lipids through specific metabolic pathways.

• Esterification is the process of forming ester bonds between fatty acids and glycerol. This is how cells assemble triacylglycerides and phospholipids from their building blocks.
• Hydrolysis is the reverse: breaking ester bonds with water to release free fatty acids and glycerol. Enzymes called lipases catalyze this reaction, and many pathogenic bacteria secrete lipases to break down host lipids for nutrients.
• Beta-oxidation is the primary pathway for fatty acid catabolism. It works by sequentially removing two-carbon units from the fatty acid chain, generating acetyl-CoA with each cycle. That acetyl-CoA then feeds into the TCA cycle for further energy extraction. Each round of beta-oxidation also produces $FADH_2$ and $NADH$, which donate electrons to the electron transport chain for ATP generation.