Adherence and penetration of vascular endothelium by Candida yeasts

Metastatic infection after hematogenous dissemination of Candida species is presumably dependent on the fungus traversing the vascular endothelium. An in vitro model of the earliest events of metastatic Candida infection was developed with whole vascular strips. Freshly obtained porcine blood vessels were secured in a perforated Lucite template that allowed the application of yeasts directly to the endothelial surface. Multiple wells allowed experimental and control observations on the same vascular segments. Adherence to endothelium was greatest with Candida albicans and Candida tropicalis, less with Candida Krusei, and least with Candida parapsilosis, Candida pseudotropicalis, and Torulopsis glabrata. This hierarchy of adherence parallels that in other in vitro systems employing mucosal epithelial cells or fibrin-platelet matrixes and reflects the known virulence of the respective species and their potential for hematogenous dissemination. C. albicans and C. tropicalis yeasts that adhered were capable of directly traversing the endothelial surface before the production of germ tubes. Heat or Formalin-killed yeasts and viridans group streptococci, although adherent, were incapable of vascular penetration, a process presumably attributable to enzymatic digestion of host tissue. Loss of integrity of penetrated endothelial tissue was verified by loss of dye exclusion, lactic dehydrogenase release, and ultramicroscopic changes. These two steps, adherence and penetration, provide direct insight into the earliest events in hematogenous Candida species dissemination and suggest that C. albicans and C. tropicalis yeasts are capable of initiating tissue invasion before germ tubes have had the opportunity to form and participate in the invasive process.