acid-fast bacillus
An acid-fast bacillus is a rod-shaped bacterium that, once stained red, refuses to give up the colour even when rinsed with acid — a trick that sets it apart from almost all other bacteria. In the lab this stubbornness is a fast, cheap clue that mycobacteria, the family that includes the TB germ, may be present.
The property comes from the bacterium's thick, waxy cell wall, rich in mycolic acids. Ordinary bacteria release a stain the moment acid-alcohol is applied; the waxy coat of mycobacteria traps the dye inside, so they stay red against a contrasting background after the acid wash. Hence the name “acid-fast,” meaning the colour holds fast against acid.
Because the test reports a category rather than a species, “acid-fast bacilli seen” tells you mycobacteria are present but not which one — it could be TB or a non-tuberculous mycobacterium. Confirmation of the exact organism needs molecular testing or culture, but spotting AFB on a smear is often the first rapid step in diagnosing TB.
A microbiologist scans a stained sputum smear and reports “acid-fast bacilli seen,” prompting urgent isolation while molecular tests identify the species.
AFB seen — mycobacteria present, species still to be confirmed.
The classic stain is Ziehl–Neelsen (hot carbol fuchsin); fluorescent auramine staining is faster to read and now widely used.