80 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



TECHNIC. 



The methods employed in studying the pathogenic protozoa are very similar to 

 those used in bacteriology. Microscopes, with the highest magnifications, are used in 

 order that the appearance of the protozoa may be observed. The observation, with 

 the microscope, of living specimens is a most important means of studying organisms 

 which undergo such wide changes of form as do many of the pathogenic protozoa. 

 Some parts of a protozoon cannot be seen in a fresh, living specimen; in order to make 

 these structures visible, preparations must often be fixed and stained by appropriate 

 methods; one of the methods most widely used is fixation in absolute alcohol and stain- 

 ing by some modification of Romanowsky's stain, such as Giemsa's method. 



Much has been learned concerning bacteria and fungi by cultivating them on 

 artificially-prepared culture media. Some of the protozoa can be cultivated during a 

 part, or all, of their life cycle on similar media; but culture methods have not been so 

 important in the study of the protozoa as they have been in bacteriology. One reason 

 for this is, probably, that many of the parasitic protozoa live, naturally, upon certain 

 very highly specialized cells; it is difficult to prepare a culture medium sufficiently 

 resembling the substance of those cells to enable it to be substituted for them. Another 

 reason is that many of the protozoa exist under widely differing conditions at different 

 stages of their life history. Even though a culture medium might be devised suitable 

 for one of the stages of a protozoon, it is scarcely probable that the same medium would 

 be suitable for its growth at all stages of its development. Nevertheless, much has been 

 learned concerning the life histories of protozoa through the employment of culture 

 methods. Culture methods are sometimes useful in diagnosing diseases, caused by 

 protozoa; for the multiplication of the parasites, in a suitable medium, may reveal the 

 presence of pathogenic protozoa, when they are too few to be detected by the micro- 

 scopical examination of the infected material from which the culture was made; for 

 example, the flagellated stage of Leishmania furunculosa may develop in cultures of 

 material taken from a Delhi boil in which the resting stage of the parasite cannot be 

 found (p. 675). 



