AFB Culture and Isolation

The modern bacteriology has many mycobacteriological

media available to it. An ideal medium should be able to

Microbiology and Bacteriology 855

produce rapid and abundant growth, enhance phenotype

characteristics, inhibit the growth of contaminants and

should be usable for antimicrobial techniques. However,

despite advances, the isolation of Mycobacterium tuberculosis is still a slow process ranging from 10 days to 8 weeks.

Solid media: LJ medium produces a slightly higher rate of

TB isolation however, it is prone to slant contamination.

A good LJ medium is non-selective, light green in color,

smooth slant without bubble formation so as to view

mycobacterial growth easily. The concentration of

Malachite green is critical for achieving a good color

contrast for visualization of mycobacterial colonies.

Suboptimal concentration of Malachite green in the

medium produces higher contamination rates whereas

excessive Malachite green can suppress and delay the

Mycobacterium growth itself.

Agar based medium such as Middlebrook are transparent, allow quicker examination of colony morphology.

Middlebrook is more resistant to contamination and

produces growth of Mycobacterium tuberculosis faster than

LJ medium. Some commercially available 7H11 medium

have been modified to increase the amount of Malachite

green. Laboratory workers should be careful to determine

this, for while the increase content of aniline dye retards

growth of contaminating bacteria, it can also inhibit the

growth of Mycobacterium.

When laboratories rely primarily on solid medium it

will take a minimum of 3 weeks to produce colonies of

Mycobacterium tuberculosis.

READY TO USE LJ SOLID MEDIUM FOR MYCO- BACTERIUM TUBERCULOSIS ISOLATION

Mycocult®

(Courtesy: Tulip Group of Companies)