ETIOLOGY OF YELLOW FEVER : VI. CULTIVATION, MORPHOLOGY, VIRULENCE, AND BIOLOGICAL PROPERTIES OF LEPTOSPIRA ICTEROIDES.

By the employment of methods designed to promote the growth both of aerobic and anaerobic organisms, particularly those belonging to the class of spirochetes, it was possible to obtain a pure culture of a delicate organism, the morphological features of which place it in the genus Leptospira. On three occasions, that is, from three out of eleven cases of yellow fever, the organism was directly cultivated. These three strains were found to induce the characteristic symptoms and lesions when tested on guinea pigs. The organism was designated Leptospira icteroides. Leptospira icteroides was also obtained in pure culture from the blood of guinea pigs which succumbed to infection after being inoculated with the blood or organ emulsions from patients suffering from yellow fever. These cultures also proved to be virulent when tested on susceptible animals. The morphological characteristics and certain biological properties of the organism were considered in detail. It is invisible under translucent illumination and is difficult to stain by most aniline dyes. It is highly sensitive to the presence of bacteria and is rapidly destroyed in a medium in which certain other organisms are present. The presence of blood serum (man, sheep, horse, rabbit, etc.) seems to be essential for its growth. It grows well at a temperature of about 25–26°C. and more quickly at 37°C., though at the latter temperature it dies out within a few weeks. At 25°C. under favorable conditions and in suitable culture media it remains viable for several months without losing its virulence. Leptospira icteroides multiplies by transverse division. The virulence attained by some strains was such that 0.00001 cc. of a culture could induce typical fatal infection in guinea pigs. There exists a considerable variation among guinea pigs in their susceptibility to Leptospira icteroides. The organism is killed within 10 minutes at a temperature of 55°C. and is also destroyed by complete desiccation or freezing and thawing. Bile and bile salts dissolve it in certain concentrations, but not saponin. Leptospira icteroides passes through the pores of Berkefeld filters V and N, and there is a possibility of its having a granular phase of life under certain conditions.