Abstract
Mycoplasma hyorhinis (strain MCLD) was recently isolated from a melanoma cell culture. Growth of MCLD was considerably improved by 24 serial passages in a modified Hayflick's mycoplasma medium. Transmission electron microscopy showed that MCLD exhibits a polymorphic appearance, with ovoid or elongated cells frequently harboring an electron-dense core at one of the poles. Adherence of M. hyorhinis to melanoma cells followed saturation kinetics. Furthermore, although M. hyorhinis has been considered to remain attached to the surface of the host cells, we show for the first time, qualitatively by confocal laser scanning microscopy and quantitatively by a gentamicin resistance assay, that MCLD is able to invade melanoma cells. The ingested mycoplasmas were randomly distributed in the cytoplasm, tending to concentrate near the plasma membrane. Both adherence to and invasion of melanoma cells by M. hyorhinis strain MCLD were dramatically enhanced by mild proteolytic digestion with proteinase K (2.5 microg/mg cell protein for 2.5 min at 37 degrees C) that affected the surface-exposed proteins of this organism, mainly the major 47-kDa lipoprotein. We suggest that the intracellular location of M. hyorhinis strain MCLD is a privileged niche, which may explain the survival of M. hyorhinis in tissue cultures. The enhanced binding to and invasion of melanoma cells by protease treatment may be due to either the activation or the enhanced exposure of an adhesin(s) on the mycoplasmal cell surface.