248 MANUAL OF MICROBIOLOGICAL METHODS 



the hands become contaminated aids in preventing one from contaminat- 

 ing himself or his environment further. A sink and liquid-soap container 

 operated by foot pedals have much to recommend them for this purpose. 

 Cloth face masks may contribute to negligence by inspiring a false sense 

 of security. 



Probably the greatest assets to protect one are patience and fore- 

 thought. The time is well spent if the procedures to be employed are 

 gone over mentally in an attempt to recognize danger areas and make 

 plans to minimize or obviate them. Artificial active immunization when 

 practical is strongly advised. It should not be used as a substitute for 

 vigilance, however. 



STAINS 



Many viruses are submicroscopic in size when the light microscope is 

 used. These viruses can be viewed by the electron microscope. Con- 

 siderable knowledge and training are required to make proper prepara- 

 tions for the electron microscope and for the operation of this instrument. 

 It is believed that such procedure is beyond the scope of this chapter and 

 should be learned under the experienced investigator. Large viruses 

 and rickettsiae as well as certain cellular inclusions caused by viruses can 

 be observed by the light microscope when properly stained. 



Preparations. In many instances success or failure of a staining pro- 

 cedure depends on the initial preparation of the material to be stained. 

 Generally speaking, best results are obtained by fixing the tissue, section- 

 ing and staining it. By the use of this method the position of the infec- 

 tious agent or cellular inclusion is not disturbed in relation to the host 

 cell. The position of the infected cells in relation to adjacent cells or 

 tissue is also maintained. Fixation, sectioning, and staining more 

 properly belong in the field of histology or pathology. Special equipment 

 and experience are usually requisite to success. 



Nevertheless acceptable preparations of infectious agents and cellular 

 inclusions can be demonstrated by smears. The smear technic is quick 

 and does not require special equipment. Tissue should be spread in a 

 thin film over the slide, or impression smears may be made. The latter 

 are made by sectioning an involved organ or tissue and pressing the cut 

 surface against a slide. Only scrupulously clean slides should be used for 

 any preparation to be stained. It is of paramount importance that the 

 tissue is harvested at a time when the agent or inclusions should be 

 present. 



Yolk-sac smears require special preparation. A small portion of the 

 membrane is detached, rinsed in saline, and placed on a paper towel. 

 With a pair of fine forceps the membrane is teased across the towel to 



