Updated project metadata. Mycoplasma hyopneumoniae pathogenic strains, like 7448, are the causative agents of porcine enzootic pneumonia. Non-pathogenic strains, like M. hyopneumoniae J, does not cause disease, although shares the entire repertoire of known virulence-related genes with M. hyopneumoniae 7448. In this context, the differential expression of ortholog genes is likely responsible, at least in part, for differences in pathogenicity or virulence level between these strains. Moreover, in the porcine lung, M. hyopneumoniae faces a hostile environment, with both oxidative and heat stresses. The performed comparative proteomics analyses provided evidence of differential stress responses between a pathogenic and a non-pathogenic M. hyopneumoniae strain, involving tens of proteins, including some known virulence factors. The results suggest that stress conditions trigger the expression of potential virulence factors in the pathogenic M. hyopneumoniae 7448, but not in the non-pathogenic M. hyopneumoniae J.