A Study of Rickettsiae Grown on Agar Tissue

The main objective of the recent work in this laboratory on typhus fever has been the development of a method by which it might be possible to obtain large amounts of virulent Rickettsia for purposes of vaccine production. I t has been relatively easy to obtain enormous yields of the murine type of Rickettsia by the x-ray rat method, a technique, previously discussed (1), which has been in weekly routine use in this laboratory for several years. This method has never, however, been successful with the classical or European strain of typhus virus, a fact which, more than any other, has convinced us that there is a deep seated biological difference between the two closely related strains. We have indicated elsewhere that there is reason to assume that the classical virus may have developed from the routine by centuries of man-louse-man transmission (2). Since the Weigl method of vaccinating with the phenolized intestinal contents of infected lice is, for technical reasons, inapplicable to large scale lmmumzation, the efforts of a number of investigators turned to tissue culture by the Maitland method as modified by Nigg and Landsteiner (3). This method has been found effective for vaccination purposes both by Kligler and Aschner (4) and by Macchiavello and one of us (5). After considerable modification of the original Maitland technique, however, we concluded that for practical, large scale vaccination this technique, while feasible, was unnecessarily time-consuming and difficult. In consequence we sought for better methods of cultivating Rick-