<prompt> Proteus antigen cross react with ?KlebsiellaRickettsiae KlebsiellaChlymydiaeE.coli  <response>: Ans. is b i.e., Rickettsiae . Proteus antigen cross reacts with rickettsial antigens. The application of this propey is done in the Weil Felix test. PROTEUS . They are gram negative, non capsulated, pleomorphic, motile bacilli. . Proteus bacilli are divided in to three genera. - Proteus -- Proteus vulgaris, Proteus mirabilis - Morganella -- Morganella morganii - Providencia -- Prov. alcalifaciens, Prov stuaii, Prov. rettgeri . Most of them except providencia strains, produce powerful urease. . A characterstic feature of proteus bacilli is PPA reaction . It is due to presence of enzyme phenylalanine deaminase which conves phenylalanine to phenyl pyruvic acid. . Culture of proteus bacilli have a characteristic putrefactive odour described as fishy or seminal. . They produce swarming growth over the surface of agar, except for morganella morganni which does not swarm. . Some non motile proteus strains OX-2, OX-19 and OX-K are used in weil-felix agglutination reaction for the diagnosis of rickettsiae infections. 