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Mycobactin is the name suggested for a growth factor, present in acid-fast bacteria, which is essential for the growth of Mycobacterium johnei. A preliminary account ofthe work reported here has already been published (Francis, Madinaveitia, Macturk & Snow, 1949). Myco. johnei is an organism first observed by Johne & Frothingham (1895) in cases of chronic enteritis of cattle. It could not be cultivated on ordinary laboratory media. Twort & Ingram (1913) supplemented such media with various extracts ofnormal cattle tissues, but none produced growth. They postulated that failure to cultivate the bacillus 'must be due to the absence of some necessary foodstuff', and that the missing factor would most likely be found in tubercle bacilli. They showed that when dried, killed, human tubercle bacilli were added to egg medium, a good growth of Myco. johnei developed. They were also able to obtain growth by addition of killed cells of other mycobacteria which had been grown on suitable media, and even by addition of cells of Myco. johnei which had been acclimatized to grow on beef broth without added growth factor. They made the original suggestion that the specific growth substance was essential to the metabolism of the whole group of acid-fast organisms, Myco. johnei differing from the other mycobacteria in lacking the power to initiate synthesis of this factor on an egg medium. This hypothesis has been reinforced by the subsequent discovery of many parallel cases in other families of organisms where some members can synthesize a particular factor while others require the same factor to be supplied from an external source. For example, among the Lactobacilli Lb. casei, Lb. delbruckii and Lb. Iactis have been shown to require pyridoxine for growth, whilst Lb. arabinosu8 and Lb. pentosus synthesize pyridoxine (Bohonos, Hutchings & Peterson, 1942). Twort & Ingram prepared extracts of mycobacteria with organic solvents and found that they promoted the growth ofMyco. johnei. This observation was one of the earliest indications of the existence of a specific organic growth factor for a micro-organism, and was made at a time when even * Present address: Department of Preventive Medicine, University of Queensland Medical School, Brisbane. the concept ofvitamins was novel. Although simple extracts ofMyco. phlei have been used regularly for the purpose of culturing Myco. johnei, the literature contains no record of any further attempt to concentrate and isolate the active substance. Woolley & McCarter (1940) claimed that growth of Myco. johnei on liquid medium could be promoted by phthiocol or 2-methylnaphthaquinone, but the amount ofgrowth observed was less than that given by addition of extracts ofMyco. phlei. They gave no evidence of the occurrence of quinones in Myco. phlei. Francis et al. (1949) found no phthiocol in Myco. tuberculos8i, though a substance resembling vitamin K2 was present in considerable amounts. Considering that the growth factor in Myco. phlei may well represent an important and specific component in the metabolism of mycobacteria, it is surprising that no further attempt has been made to investigate its constitution. The preparation of chemotherapeutic agents against this group of organisms might well be helped by a knowledge of the chemical structure of the growth factor. The extraction and purification of the factor was therefore undertaken, and the results are presented in this paper. The experiments have led to the isolation of the hitherto undescribed compound, mycobactin, which has the growth-promoting action of the original extracts; its main properties are outlined here. RESULTS

Mycobactin, a growth factor for Mycobacterium johnei. 1. Isolation from Mycobacterium phlei