This invention relates to the protein, Mycobacterium paratuberculosis acylase (mpa) and the gene encoding mpa, which we have identified in the pathogen Mycobacterium paratuberculosis Mptb (also designated Mycobacterium avium subspecies paratuberculosis MAP), and to their use in the diagnosis of Mptb/MAP infections in animals and humans, as well as their use as components of vaccines for the prevention and treatment of diseases caused by Mptb/MAP. The importance of an intact uninterrupted mpa gene as a determinant of pathogenicity in Mptb/MAP is recognized and the invention also provides attenuated strains of normally pathogenic Mptb/MAP and other mycobacteria in which mpa has been inactivated, for use as vaccines.