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Lipid A is a component of the lipopolysaccharide (LPS) layer, which is a significant structural feature of the outer membrane of Gram-negative bacteria. This molecule plays a critical role in the physiology of these bacteria, serving as an endotoxin that can trigger a strong immune response in host organisms. The presence of Lipid A is a defining characteristic that distinguishes Gram-negative bacteria from Gram-positive bacteria, which lack this outer membrane structure.

While aerobic bacteria and facultative anaerobes can be found among both Gram-negative and Gram-positive classifications, the specific association of Lipid A is exclusive to the Gram-negative group. Thus, the answer highlighting Gram-negative bacteria accurately reflects the biological context surrounding Lipid A's role and significance in microbiology.