Some mycobacterial species are unable to convert niacin ribonucleotide to nicotinamide adenine dinucleotide (NAD). Growth of these species will result in the accumulation of niacin in the culture media.
Culture for testing: heavy growth on Lowenstein-Jensen media > 3 weeks old
A small amount sterile water is placed on the culture and left to stand for 20-30 minutes. If growth is confluent, then the agar should be stabbed several times. The water is then collected and placed in a screw tap tube. A niacin filter paper strip is then placed in the water, with the tube sealed immediately.
Interpretation:
• A positive reaction (presence of niacin) is development of a yellow color.
Species that can give a positive niacin test:
(1) M. tuberculosis
(2) M. simiae
(3) some strains of M. chelonae
(4) some strains of M. bovis
(5) some strains of M. marinum
Reasons for a false negative test:
(1) use of a culture medium other than L-J media
(2) insufficient growth
(3) failure to stab the agar if growth in confluent
(4) bacterial contamination of the culture
(5) reading the color too soon (< 12 minutes) or too late (> 30 minutes) after adding the niacin test strip
