Acid-fast bacilli can frequently be found in the mouths

of water taps, but some at least seem to be affected by 6%

H2SO4 when an attempt is made to grow them, but survive

treatment with weaker acid.

Anonymous Strains

Many of the so-called anonymous strains are classified by

their reaction to light. Photochromogens are those which

produce orange colonies when they are exposed to light

(day-light/artificial light). Scotochromogens are those

which produce colored colonies even in the dark. It is

therefore, important in dealing with this classification not

to grow them in an incubator that is opened frequently,

not to let them stand on the laboratory bench unless the

observation is actually being made. The yellow-orange

pigment develops after exposure to light in 6–24 hours. The

scotochromogens produce a yellow growth but this turns

more orange on exposure to light. The Battey type either

does not change, or if it does, the change is very slow.

Animal Inoculation

Sometimes for final identification it is necessary to

inoculate an animal to see if the organism will produce

disease. The human strain of M. tuberculosis will produce

disease in both guinea pigs and rabbits, but the rabbit will

show little sign of tuberculosis. With the bovine strain,

however, the rabbit is also highly susceptible. In some

specimens, the material may be directly inoculated, in

other, only after a primary culture has been grown.

OVERVIEW OF M. TUBERCULOSIS: DIAGNOS- TIC APPROACH, AFB STAINING, CULTURE AND

SENSITIVITY