Lipopolysaccharide

Lipopolysaccharides, often called LPS, are massive molecules found in the outer membranes of gram-negative bacteria. These bacteria include well-known species like E. coli and Salmonella.

LPS is essential for the survival of many of these microbes. It provides structural integrity and protects the cell from chemical attacks. In many gram-negative bacteria, LPS can make up as much as 80% of the outer membrane. Without the specific genes required to build these molecules, many bacteria simply cannot survive.

An LPS molecule is composed of three distinct chemical domains. The first is the O-antigen, which is a long chain of sugars called a glycan polymer. This part sits on the very outside of the cell and is highly variable. In fact, E. coli strains can produce over 160 different O-antigen structures. The presence of these long chains makes the bacteria appear "smooth" under a microscope. If the chains are missing or reduced, the bacteria are called "rough."

The second part is the core oligosaccharide. This middle section is much more stable and common among different bacterial groups. It often contains specific sugars like KDO, which stands for 3-Deoxy-D-manno-oct-2-ulosonic acid. The third part is lipid A. This is a phosphorylated glucosamine disaccharide decorated with fatty acids. These fatty acid chains act as anchors, embedding the molecule into the bacterial membrane. Lipid A is the most bioactive part and is responsible for most of the molecule's toxicity.

The process of building LPS is a complex biological assembly line. It begins on the inner membrane of the bacterial cell. First, a molecule called lipid A-Kdo2 is created. Other sugars are then added to this molecule on the inner membrane. A protein called MsbA then moves these core-lipid A molecules into the periplasmic space. Meanwhile, the O-antigen subunits are moved across the inner membrane by a protein called Wzx. These subunits are then linked together into long chains by the protein Wzy. Finally, a protein called WaaL attaches the completed O-antigen to the core-lipid A. To reach the final destination, the completed LPS molecules are transported to the outer membrane by a bridge of proteins known as the Lpt system.

Scientists have long studied how these molecules interact with living things. Richard Friedrich Johannes Pfeiffer first identified the toxic activity of LPS. He named them endotoxins to distinguish them from exotoxins. Exotoxins are substances that bacteria release into their environment. In contrast, endotoxins were thought to stay inside the cell until the membrane was destroyed. Later research revealed a more nuanced process. Bacteria can actually secrete LPS through small bubbles called outer membrane vesicles (OMVs) during normal activity. This means the cell does not always need to disintegrate to release these molecules.

LPS has a profound impact on human health because it interacts deeply with the immune system. It is a potent pyrogen, which is a substance that triggers a fever. When LPS enters the bloodstream, it can cause a massive immune response. In severe cases, this leads to acute organ failure and septic shock. Humans are remarkably sensitive to this effect. A dose of only 1 μg/kg can induce shock in a human. By comparison, mice can tolerate doses up to a thousand times higher. This extreme sensitivity is a major factor in how dangerous gram-negative infections can be.

Our bodies use specific biological tools to detect and manage these molecules. Many immune cells, such as macrophages and B cells, use a receptor complex to find LPS. This complex includes a protein called TLR4, which stands for Toll-like receptor 4. Bruce Beutler was awarded the Nobel Prize in 2011 for discovering that TLR4 is the specific receptor for LPS.

Once detected, the immune system responds by releasing pro-inflammatory cytokines. To prevent the immune response from becoming too violent, the body uses enzymes for detoxification. One such enzyme is acyloxyacyl hydrolase, or AOAH. This enzyme removes specific fatty acid chains from lipid A, making the molecule less active. Another enzyme, intestinal alkaline phosphatase, can also help by removing phosphate groups from lipid A. This helps reduce inflammation in the gut during certain infections.

Beyond causing disease, LPS plays many roles in bacterial ecology. It helps bacteria stick to surfaces and affects how they interact with predators like amoebae. It is also necessary for the function of omptins, which are a type of bacterial protease. Some bacteria, like Neisseria meningitidis, have a slightly different version called lipooligosaccharide (LOS). LOS lacks the long O-antigen chain but still helps maintain the membrane. These variations allow bacteria to adapt to different environments and even evade the host's immune defenses.