Mycoplasma pneumoniae: a reduced-genome intracellular bacterial pathogen.

Mycoplasma pneumoniae has classically been considered an extracellular (or membrane-associated) organism. Nevertheless, the recently elucidated genomic structure of this pathogen strongly suggest that this organism may have been subjected to the process of reductive genetic evolution which is characteristic of intracellular bacteria. We studied the Mycoplasma pneumoniae RYC15989 strain, recovered from a pericardial biopsy sample from a patient with atypical pneumonia and acute pericarditis. The interaction of this strain with human hepatocytes Hep-G2 and mouse neuroblastoma N2-A cell lines was investigated. Confocal laser scanning microscopy and electronic microscopy evidence is presented of the intracellular location of fluorochrome-labelled Mycoplasma pneumoniae in cell lines infected with the organism in vitro. This finding provides preliminary evidence of cellular invasive capacity of Mycoplasma pneumoniae and casts some new light on the pathogenic potential of Mycoplasma pneumoniae in host infection.