Acid-fast and modified acid-fast stains These stains are used to identify the following: Acid-fast organisms (Mycobacterium species) Moderately acid-fast organisms (primarily Nocardia species) Rhodococcus and related genera Oocysts of some parasites (eg, Cryptosporidium, microsporidia, Cystoisospora [Isospora] belli, Cyclospora, Balantidium coli) Although detection of mycobacteria in sputum requires at least 10, 000 organisms / mL, mycobacteria are often present in lower levels, so sensitivity is limited.