198 BACTERIOLOGY. 



misleading results. After death we have an added dif- 

 ficulty, in that even the blood and body tissues become 

 invaded by bacteria from the intestines and elsewhere, 

 so that bacteria actually present in the diseased tissues 

 may have had no connection with the disease under in- 

 vestigation. Whenever bacteria are found, therefore, 

 the methods carried out in the investigation should be 

 most carefully examined, to see if some error in tech- 

 nique has not been committed. The aim of the 

 bacteriological examination of any material is to de- 

 termine whether bacteria are present or not, and, if 

 present, to ascertain their number and distribution, and, 

 if possible, their species. This is accomplished chiefly 

 by means of two methods viz., the direct examina- 

 tion with the microscope of cover glass preparations 

 and the results of cultures made from the material. 

 Sometimes animal inoculations are also employed. 



The direct microscopical examination of suspected sub- 

 stances for bacteria can be made either with or without 

 staining. Unstained, the bacteria are examined, to note 

 their motility, their form, and their general arrangement: 

 but for more exact study, they can be so much better 

 observed when stained that this step is always advisable. 

 A cover-glass preparation is made as follows : A 

 very small amount of the blood, pus, discharges from 

 mucous membranes, culture fluids, or other material to 

 be examined is removed by means of a sterile swab or 

 platinum loop and smeared undiluted in a thin film 

 over a clean, thin cover-glass. 1 From cultures on solid 



1 To render new cover-slips clean and free from grease, place them in strong 

 nitric acid for a few hours, then rinse them olF in water, then in alcohol, 

 then in ether. Place them finally for keeping in alcohol, to which a little 

 ammonia has been added. When used wipe with soft clean handkerchief. 

 If old cover-slips are used boil first in soda solution. 



