Lipid A is a lipid component of an endotoxin held responsible for toxicity of Gram-negative bacteria.It is the innermost of the three regions of the lipopolysaccharide (LPS, also called endotoxin) molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane.ipid A is a lipid component of an endotoxin held responsible for toxicity of Gram-negative bacteria.