The present invention relates to mutant strains of Mycoplasma hyopneumoniae and to a method for preparing them. It also relates to carrier vectors which are used in said method, to vaccine compositions and to vaccination kits comprising the compositions against porcine enzootic pneumonia and other swine diseases. It also relates to the use of M . hyopneumoniae as a host for expressing recombinant proteins and other DNA sequences of interest.
