Chromosomal heterogeneity of various Mycoplasma hyopneumoniae field strains

Restriction enzyme digestion and field inversion gel electrophoresis were used to analyze the chromosomes of strains of Mycoplasma hyopneumoniae and the related organism Mycoplasma flocculare. The chromosome size for the M. hyopneumoniae type strain was calculated from individual fragments to be 1,011.3 +/- 32.9 kbp. The chromosomes of M. hyopneumoniae field strains were approximately the same size. The restriction patterns obtained for the chromosomes of phenotypically similar M. hyopneumoniae strains were quite different. Therefore, the species M. hyopneumoniae seems to be very heterogeneous. A field inversion gel electrophoresis analysis of the entire chromosomes allowed us to distinguish M. hyopneumoniae strains easily and hence to characterize further the species M. hyopneumoniae. The chromosome size for M. flocculare was calculated to be 988.3 +/- 39.5 kbp. Restriction enzyme XhoI, which statistically should cut the M. hyopneumoniae chromosome frequently, did not cut the DNA of any of the M. hyopneumoniae strains but did digest M. flocculare DNA, indicating that there is a site-specific modification at CTCGAG which probably belongs to a restriction modification system in M. hyopneumoniae and is absent in M. flocculare.