Beyond bacteria: large agents with analogies to Mimiviruses detected in canine cancers: reexamining Gram staining in cancer diagnostics

While Gram staining is traditionally used for classifying bacteria based on their cell wall properties, Mimiviruses and large mammalian agents can also retain the Gram stain, despite not being typical bacteria. In fact, Mimivirus-like agents that exhibit Gram-positive staining were first found in human tissues, particularly in malignant samples, suggesting that these agents may be involved in a unique carcinogenic process. In order to translate the findings published in human medi- cine to animal models, we evaluated for the first time the presence of analogous Gram-positive agents in canine malignancies and differentiate them from tradi- tional bacteria. Using Gram staining, we analyzed 35 canine tumors across vari- ous malignancy types, including 7 sarcomas, 15 carcinomas, and 13 round cell tumors such as mast cell tumors, transmissible venereal tumors and melanomas. Normal tissues and bacteria were used as controls. We were able to identify Gram-positive granulations, exhibiting intracytoplasmic, intra-nuclear and peri- nuclear patterns, measuring 1–2 μm that were distinct from traditional bacteria. This study, the first of its kind in the veterinary literature, supports comparable published findings in human research and advances our knowledge of the patho- physiology of cancer across species.