SUMMARY: The standard methods for assessing viability of micro-organisms are not applicable to rat and human leprosy bacilli since neither organism can be grown . McFadzean & Valentine (1958, 1959) suggested that the electron microscope might provide a quantitative guide to the viability of these organisms by allowing dead forms to be identified. This technique is further investigated and shown to be valid and reasonably accurate for provided the organisms are not morphologically fixed by the killing agent. It is concluded that the method can assess death of leprosy bacilli in the host when this occurs either naturally or aided by bactericidal drugs, and also loss of viability on storage, but not sudden killing by more violent chemical or physical means. The method has been found useful for following the survival of in tissue cultures. It is suggested that death in the host but not death occurring on storage can be measured by a simple classification of the bacilli seen with the light microscope after the conventional carbol fuchsin stain. It is indicated that many of the bacilli obtained from untreated human cases of leprosy are dead, while from rats the percentage which is degenerate is low. The significance of various features of the leprosy bacilli seen with the electron microscope is discussed. There is no evidence that the bacilli form spores or capsules.


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