38 



In addition to these two classes of cases we have a third, where 

 smears show a few typically shaped bacilli, or where a considerable 

 number of typical-looking bacilli are found along with many other 

 bacterial forms. There is no safe rule for reaching a conclusion in 

 these cases, and one must resort to culture or to inoculation methods, 

 or both. In any such case it is always a good plan to let the macro- 

 scopical findings have more weight than the microscopical. 



The bipolar appearance of B. pestis is so largely dependent upon 

 the technique of staining, fixing, length of time the stain is allowed to 

 act, and the length of the washing, that it should never be given 

 great weight. Here, as elsewhere in bacteriology, many errors are to 

 be avoided by not depending too much upon the morphology of the 

 organism under investigation. 



BACTERIOLOGICAL DIAGNOSIS OF RAT PLAGUE. 



While the gross lesions of rat plague are often sufficiently character- 

 istic to justify a positive diagnosis, and the gross lesions in conjunc- 

 tion with the microscopical examination will in other cases enable 

 us to say definitely that a rat is plague infected, still a certain number 

 of cases occur in which it is necessary to resort to other methods, and 

 there are circumstances, such as the first case in a community, that 

 make a complete bacteriological confirmation of a diagnosis necessary. 



This is not the proper place in which to discuss fully the bacteri- 

 ology of plague. However, a brief outline of w T hat is necessary to 

 establish beyond question the existence of plague infection in an 

 ammal will be given. 



B. pestis may often be isolated in culture from the tissues (bubo, 

 liver, spleen, or heart's blood) of an infected rat. Unless the tissues 

 are badly contaminated with other organisms, plate or stroke culture 

 will yield a growth of B. pestis in pure culture, or isolated pest-like 

 colonies may be transferred to other media. 



It is unwise, however, to trust to cultural methods alone. In the 

 majority of doubtful cases it is advisable to inoculate guinea pigs or 

 white rats. The lesions of plague in these animals are quite charac- 

 teristic, and B. pestis may readily be recovered from their tissues if 

 cultures are made at once after death. 



A pure culture of the organism under suspicion is obtained from 

 the naturally infected animal or from a laboratory animal inoculated 

 from the one under suspicion. This culture is studied in regard to its 

 morphology; first, on agar, where it grows as a short rod, or often in 

 the shape of a coccus; second, in broth, where it often grows in 

 streptococcus-like chains; third, on agar containing 3 per cent 

 sodium chloride, where most extraordinary alterations in morphology 

 occur,. giving large balloon-shaped bodies, objects resembling gigantic 

 cocci and enormous trypanosome-shaped forms, the so-called "invo- 



