18/05/2026
🔬𝐂𝐘𝐓𝐎𝐈𝐍𝐒𝐈𝐆𝐇𝐓𝐒: 𝐇𝐢𝐝𝐝𝐞𝐧 𝐁𝐚𝐜𝐭𝐞𝐫𝐢𝐚, 𝐕𝐢𝐬𝐢𝐛𝐥𝐞 𝐂𝐥𝐮𝐞𝐬
by Mario Cabezas-Calvo DVM, CSAVP
Did you know that not all bacteria stain — and some prefer to hide? Mycobacterium spp. are classic examples, often appearing only as subtle “ghost-like” structures within macrophages on routine cytology. This was exactly the finding in a conjunctival mass aspirated from a tortoise.
Cytology revealed numerous large vacuolated macrophages, many of them multinucleated. This is a common finding in granulomatous inflammation often associated with long-standing infections or foreign body reactions. Within the cytoplasm of several macrophages, clear unstained linear structures were identified. Although subtle, these structures were suspicious for Mycobacterium spp.
Mycobacteria can be challenging to recognise on routine Romanowsky-type stains because of their lipid-rich cell wall, which prevents normal stain uptake. For this reason, organisms may appear as negative structures within inflammatory cells. To further investigate, a Ziehl–Neelsen acid-fast stain was performed. This special stain takes advantage of the same lipid-rich wall: once stained, the organisms retain their characteristic bright red-pink colour even after exposure to acid-alcohol, unlike surrounding cells.
The clinical and cytological appearance of mycobacterial infections are often similar across species. Clinical presentation, progression, affected species, and zoonotic potential vary depending on the Mycobacterium species involved. Definitive identification of the Mycobacterium species generally requires additional testing such as culture or molecular diagnostics (PCR).
Cytological evaluation of skin masses is a rapid and valuable tool for identifying inflammatory processes and, in some cases, the causative agents, thereby supporting timely clinical decision-making and therapeutic planning. This case also highlights the value of special stains, such as Ziehl–Neelsen stain, in the further investigation of these types of lesions.
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