Löwenstein-Jensen medium, better known as LJ medium, is a selective egg-based medium specifically used for the culture and isolation of Mycobacterium species, including Mycobacterium tuberculosis, from clinical specimens. LJ media was originally formulated by Lowenstein who incorporated congo red and malachite green to inhibit undesirable bacteria. The current formulation includes an egg-based glycerin medium and is based on Jensen's modification. Jensen modified the Lowenstein medium by modifying the citrate and phosphate levels, removing the congo red dye and increasing the concentration of malachite green. Gruft further modified the L. J. medium with the addition of penicillin, nalidixic acid and ribonucleic acid (RNA) to increase selectivity. This medium promotes the growth of a wide variety of mycobacteria and can also be used for the niacin test. Low levels of penicillin and nalidixic acid are present in the LJ medium to inhibit the growth of Gram-positive and Gram-negative bacteria, limiting growth to Mycobacterium species only. The presence of malachite green in the medium inhibits most other bacteria.