126 BACTERIOLOGICAL CHEMISTRY 



Protozoal Infections.— Very little progress was 

 achieved until Ehrlich's researches gave a stimulus to 

 the study of the subject. Ehrlich had observed that 

 some tissues were selectively stained when certain dyes 

 were injected into animals, whilst other tissues were 

 almost unaffected. In 1891 he recorded that the malaria 

 parasite was stained by methylene blue and could be 

 differentiated from the tissues of the host in this way. 

 This suggested that dyestuffs might be found which 

 would be so easily adsorbed by pathogenic micro- 

 organisms as to kill them without harming the host. 

 As a result of these investigations Ehrlich and Shiga, in 

 1904, showed that trypanosomes were readily stained 

 by the dye trypan-red, 



SCNa 



and that the substance cured the ordinarily fatal infection 

 of mice with Trypanosoma equinum. Unfortunately the 

 dye was effective only against acute laboratory infections 

 and not against the natural disease. 



In 1905 atoxyl, ^^-amino -phenyl arsonic acid, 



NH2<^^ NAsOgNa, was shown by Thomas to be 



lethal to Trypanosoma gambiense in infected mice. This 

 discovery led to the production of a number of arsenical 

 drugs, many of which were dangerously near the toxic 

 limits for therapeutic use. Their action was rarely 

 apparent if administered in the later stages of the disease, 

 and frequently arsenic resistant strains of trypanosomes 

 were developed when cure was not effected. These drugs 

 were not lethal to T, rhodesiense. Further research, 

 however, led to the discovery in 1920 of Baeyer 205 or 

 germanin (among other names). 



