912 THE SPIROCHETES 



is now used extensively for study of the leptospirse, and antigenic tyj^es are deter- 

 mined by cross-agglutination, and sometimes cross-absorption, tests. Protective 

 bodies in serum are tested for in the usual way by animal inoculation. 



A valuable method of study is afforded by the test described by Rieckenberg 

 (1917), known sometimes as the thrombocytoharin reaction or the adhesion pheno- 

 menon. It depends on the fact that if a suspension of blood platelets or bacteria 

 is added to a mixture of a spirochaete and its specific antiserum, and the preparation 

 is observed under dark-ground illumination after preliminary incubation at 30'^ C. 

 for 20 minutes, the platelets or bacteria are seen to have become adherent to the 

 spirochsetes in the form of small clumps. In the presence of a non-specific serum 

 no such clumping or adhesion occurs. By this reaction it is possible to distinguish 

 rapidly between two such closely-allied forms as Lepto. icterohcBmorrhagice and 

 Lepto. hebdomadis (Brown and Davis 1927). Living motile spirochaetes are 

 required and fresh complement must be present. The specific antibody is 

 destroyed by heating to 72° C. for 30 minutes (Inoue 1930). Pfeiffer's test and 

 cross-immunity protection tests in animals are likewise of value in distinguishing 

 between closely allied species, and even between variants of the same species. 



Pathogenicity. — The virulence of spirochaetes appears to be subject to consider- 

 able variation. Many members are strictly parasitic and give rise to infections 

 in man or animals, while others are saprophytic and appear to be devoid of any 

 pathogenic effect. There is evidence, however, that the virulence of the parasitic 

 members may undergo change as the result of residence in the body of the host. 

 In relapsing fever, for example, the strains that appear in the blood at the second 

 or third relapses may differ antigenically from the strain responsible for the original 

 attack, and by virtue of this change are able to multiply in the tissues of a host that 

 has become immunized to the original parent strain. Again, strains of certain spiro- 

 chaetes, such as Trep. pallidum, may be brought by passage to grow readily in an 

 animal which at first resists their invasion. Trep. pertenue, the organism that is 

 responsible for yaws, is regarded by many observers as merely a variety of Trep. 

 pallidum which, by residence in the negro, has developed dermotropic affinities (Par- 

 ham 1922). Residence outside the body of certain parasitic strains may apparently 

 be accompanied by a fall in virulence, which renders them indistinguishable from 

 naturally saprophytic strains. Thus Lepto. icterohcBmorrhagice, if kept in water, may 

 become indistinguishable from Lepto. biflexa (Zuelzer 1925). Whether the virulence 

 of naturally saprophytic species ever becomes increased so as to render them patho- 

 genic for man is doubtful. Baermann and Zuelzer (1927, 1928) have brought 

 a considerable amount of evidence to show that Lepto. biflexa may be trans- 

 formed by repeated animal passage into Lepto. icterohcBmorrhagice. Their findings, 

 however, are not in harmony with the experience of most other workers nor 

 with the epidemiological picture of Weil's disease, and it seems probable that such 

 instances are due to the recovery in virulence of a real icterohcBmorrhagice strain. 



Though the spirochaetes may be classified into the free-living, the commensal, 

 and the pathogenic types, it must be realized that there is no sharp line of demarca- 

 tion between the three groups. An organism that is pathogenic in one animal 

 may be purely commensal in another, and an organism that is highly pathogenic 

 at one time to a particular host may at another give rise to no more than a latent 

 infection. 



We append a description of some of the members that are of most interest to 

 the student of medical and veterinary bacteriology. 



