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can human degrade the D-amino acid present in bacterial cell wall, I'm confused about it i have read somewhere that human can do so.If yes than why we need antibiotic to kill bacteria???If it is not so than why??

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Humans have at least one enzyme which can break down D-amino acids, the D-amino acid oxidase (DAAO). They are able to break down these amoino acids, interestingly D-serine seems to play a role as a neurotransmitter. For more information have a look at these papers:

  1. Physiological functions of D-amino acid oxidases: from yeast to
    humans.
  2. Human D-amino acid oxidase: an update and review.
  3. d-Amino-Acid oxidase and its physiological function

We still need antibiotics, since only a small part of the bacterial cell wall is composed of D-amino acids. The cell wall is made of monomers from N-acetylglucosamine and N-acetylmuramic acid which are crosslinked by small peptides (these are short amino acids chains) to form the peptidoglycane polymer. A part of the amino acids in the crosslinking peptides are D-amino acids. I am not sure if the DAAO is able to attack these peptides in the polymer. Additionally most of the enzyme in the human body is present in liver and kidney, indicating that it breaks down D-amino acids that are transported there by the bloodstream.

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