Nitrate tests are positive since nitrate is generally used as the final electron acceptor rather than oxygen [4]. Citrate (positive test) is used by Serratia marcescens to produce pyruvic acid. It is positive for decarboxylase, which is the removal of a carboxyl group from an amino acid, producing an amine and carbon dioxide. The red pigmentation (prodigiosin) that Serratia marcescens is known for can be a distinguishing factor but is only present in some strains. It is not exactly known why this is, but it is hypothesized that it is a tightly regulated gene product. Prodigiosin can trigger a body’s immune system (antibodies and T cells), so it is possible that Serratia marcescens living in a human body will limit prodigiosin synthesis and hence escape detection by the host’s immune system. Many strains appear to have lost the ability to produce it at all