Disclosed is an attenuated, avirulent Mycoplasma bovis bacterium strain, wherein the attenuated, avirulent Mycoplasma bovis strain is selected from the group consisting of the attenuated Mycoplasma bovis bacteria strains deposited with the ATCC under accession numbers PTA-8694, PTA 8695, or PTA 8696, and any attenuated, avirulent Mycoplasma bovis bacterium strain capable of inducing a humoral immune response in a calf within 14 days after administration of one dose of 2.1E9 CFU via the subcutaneous or intranasal route to the calf, and which does not cause clinical signs normally caused by an infection with a pathogenic Mycoplasma bovis wild-type strain. Further disclosed is a method of attenuating Mycoplasma bovis, comprising, a. passaging Mycoplasma bovis bacteria more than 10 times b. obtaining the cultured Mycoplasma bovis bacteria c. testing the cultured Mycoplasma bovis bacteria obtained under step b) for their pathogenicity and immunogenicity, wherein the immunogenic testing comprises: A. infecting cattle with the passaged Mycoplasma bovis bacteria B. monitoring the development of the humoral antibody response against Mycoplasma bovis in the infected cattle d. propagating the non-pathogenic, but immunogenic Mycoplasma bovis bacteria to obtain the attenuated Mycoplasma bovis bacteria. Further disclosed is an immunogenic composition comprising live bacteria of an attenuated, avirulent Mycoplasma bovis bacterium strain, wherein the bacteria have been passaged more than 10 times, adapted for the treatment or prophylaxis of infections caused by Mycoplasma bovis in cattle or for reducing the severity or incidence of clinical signs caused by Mycoplasma bovis in cattle.
