Which compound is unique to bacterial cell walls and not present in archaea?

Peptidoglycan is a polymer that forms a vital structural component of the cell wall in most bacteria. It provides rigidity and strength to the bacterial cell wall, helping to maintain cell shape and protect against osmotic pressure. This compound consists of a sugar backbone composed of N-acetylglucosamine and N-acetylmuramic acid, cross-linked by short peptide chains.

In contrast, archaea, although they may have cell walls, do not contain peptidoglycan. Instead, their cell walls are typically composed of different polymers, such as pseudopeptidoglycan, which has a different structure and composition. This distinct feature of bacterial cell walls, marked by the presence of peptidoglycan, is an essential characteristic that differentiates bacteria from archaea in microbiology and taxonomy.

The other options, such as cellulose, chitin, and collagen, are compounds found in other organisms but are not specific to bacterial cell walls. Cellulose is mainly found in plant cell walls, chitin is a component of fungal cell walls and some exoskeletons, and collagen is a structural protein found in animal tissues.