Comparative effect of osmium tetroxide and ruthenium tetroxide on Lacazia loboi ultrastructure

Lobomycosis is a skin infection produced by the fungus Lacazia loboi, which mainly affects some indigenous and afro-descendant populations in Tropical America. We previously reported the comparative effect of osmium tetroxide (OsO₄ ) and ruthenium tetroxide (RuO₄ ) in the electron microscopy (EM) of other related microorganisms. The objective of this study is to compare the effect of postfixation with OsO₄ and RuO₄ in the ultrastructure of L. loboi yeasts. Skin biopsies on patients diagnosed with lobomycosis were fixed in glutaraldehyde at 3% and postfixed in the following solutions: (a) 1% OsO₄ , (b) 0.2% RuO₄ , and (c) OsO₄ at 1% followed by RuO₄ at 0.2%. They were then processed using the conventional method for EM. Unlike OsO_4, the treatment with RuO₄ revealed different shades of gray and electron dense bands in the cell wall and other cell components of L. loboi. The most notable finding was the presence of radial filamentous structures around the yeast, which made the image look like the sun. Postfixation with RuO₄ revealed ultrastructural details that had not been previously reported for L loboi. The combined use of OsO₄ and RuO₄ in EM of microorganisms with cell walls can be useful to evaluate the effect of microbicide substances.