In the cultivation of Bacillus lepræ the initial multiplication outside the body cannot be obtained unless amino-acids are present in the medium. The amino-acids are believed to be essential nutritives for the initial growth of the organisms.
It has been demonstrated that the primary growth of the leprosy bacilli occurs only in the presence of the products of tryptic digestion.
Hence, putrefactive and other bacteria which are capable of splitting nucleo-proteids into their end acid products are, in consequence, of value in the isolation and cultivation of the leprosy bacilli. Amebæ are not necessary for securing the primary multiplication of the leprosy bacilli upon artificial media and are detrimental since they feed with avidity upon the bacilli themselves.
Two methods may be employed for recovering in culture Bacillus lepræ from the tissues. In one (the direct), tryptophane or a mixture of albumen and trypsin are employed with a culture medium; in the other (indirect), bacterial species capable of digesting the albumen constituent of the culture medium are introduced into the medium. In both, the end result is identical, since they both provide for the presence of the amino-acids in the medium, without which the primary multiplication of the leprosy bacilli cannot be secured.