Transmission electron microscopy (TEM) is a key tool for the ultrastructural analysis of biological samples; however, it requires optimized fixation and contrast enhancement methods to achieve accurate results. Here, we evaluated the use of tannic acid as a mordant during primary fixation for TEM processing of Giardia intestinalis and Trichomonas vaginalis. When combined with osmium tetroxide (OsO4) post-fixation, tannic acid significantly improved in-block contrast in plasma membranes, organelle boundaries, and cytoskeletal elements, while preserving structural integrity. It increased electron density without introducing artifacts and, in some cases, allowed the omission of lead citrate staining, simplifying the protocol and reducing exposure to toxic agents. Even in the absence of OsO4, samples processed with tannic acid retained sufficient contrast to visualize basal bodies, axonemes, and other cytoskeletal filaments. Moreover, tannic acid enhanced the visualization of poorly characterized structures in the transition zone. We also demonstrate its successful use as a post-staining agent, replacing uranyl acetate for ultrathin sections while maintaining high image quality. These findings support tannic acid as a safe, cost-effective, and efficient alternative to traditional contrast agents, particularly under biosafety constraints, and contribute to the improvement of TEM protocols for studying protozoan morphology and cell biology:
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