Microscopic appearance of mutant M. hyopneumoniae colonies. M. hyopneumoniae strain 232 was transformed with plasmid pMHC9-1 and within 14 days, tetracycline resistant colonies formed on Friis agar (A). No colonies were observed on “no DNA” control transformations (B). Individual colonies exhibited a similar appearance to non-transformed M. hyopneumoniae colonies. Tiny deposits on the surface of the agar are typically seen and are considered to arise from dead M. hyopneumoniae that have failed to grow in the presence of tetracycline or from proteinaceous material in the fresh yeast extract used (A and B).