Question
Lowenstein jensen medium
Answer
Lowenstein-Jensen (LJ) medium is the standard egg-based selective solid medium used for the primary isolation and culture of Mycobacterium tuberculosis and other mycobacteria.
Composition: contains coagulated whole egg (provides protein and lipid, essential for mycobacterial growth), potato flour, glycerol (promotes growth of M. tuberculosis specifically, though it inhibits M. bovis, which requires pyruvate-enriched LJ medium instead), mineral salts, and malachite green dye, which inhibits the growth of contaminating bacteria while permitting mycobacterial growth (mycobacteria are relatively resistant to the dye due to their waxy, mycolic-acid-rich cell wall).
Preparation: the liquid medium is dispensed into tubes/bottles and coagulated by inspissation (slow heating at ~85°C) to solidify the egg protein, forming a slanted solid surface.
Use and interpretation: specimens (decontaminated sputum, etc.) are inoculated onto the slant and incubated at 37°C for up to 6–8 weeks; M. tuberculosis produces characteristic rough, buff-coloured, breadcrumb-like (“cauliflower”) colonies, typically visible from 3–6 weeks — much slower than routine bacterial culture, reflecting the organism’s long generation time.
Limitations: slow (a major drawback for timely clinical decision-making, now largely supplemented/replaced for rapid diagnosis by liquid culture systems like BACTEC MGIT and molecular methods such as CBNAAT/GeneXpert); LJ culture remains important for drug-susceptibility testing and species confirmation.