Mycoplasma hyopneumoniae. Mycoplasma hyopneumoniae is a species of bacteria known to cause the disease porcine enzootic pneumonia, a highly contagious and chronic disease affecting pigs. As with other mollicutes, M. hyopneumoniae is small in size (400–1200 nm), has a small genome (893–920 kilo-base pairs (kb)) and lacks a cell wall.