Cytotoxicity of Mycoplasma penetrans and Mycoplasma genitalium upon extracellular infection of urethral tissue culture cells

Mycoplasma penetrans and Mycoplasma genitalium are two atypical bacteria associated with urethritis and other diseases in the urogenital tract, but their mechanisms of host damage remain poorly understood. M. genitalium has been more extensively studied in relation to interactions with urogenital cells, while limited research has been done and few interactions characterized with M. penetrans. We examined these organisms and their interactions with PURL cells, a tissue culture model derived from human penile epithelium. Scanning electron microscopy was used to evaluate how M. genitalium strain G-37 and three strains of M. penetrans bind to and grow on PURL cells. Cytotoxicity and intracellular invasiveness of these strains were measured. The effects of these bacteria on apoptosis of PURL cells were also evaluated. The results indicate that in contrast to other cell types, neither M. genitalium nor M. penetrans effectively invaded PURL cells but established extracellular infections. M. genitalium formed biofilm tower-like structures on PURL cells, but M. penetrans aggregates appeared to be two-dimensional and did not distinctly increase in size over the course of infection. M. penetrans strains isolated from immunocompetent patients with non-gonococcal urethritis induced significant cytotoxicity and triggered apoptosis in PURL cells, whereas a strain isolated from an immunocompromised patient exhibited minimal cytotoxic effects. These data demonstrate that M. genitalium and M. penetrans both establish extracellular infections on urethral epithelial cells and that they employ different mechanisms to achieve cytotoxicity. Moreover, M. penetrans strains from different sources vary in their impact on urethral cells.