730 BACTERIOLOGICAL, DIAGNOSIS. 



When the differences noted are not permanent in character, de- 

 scription under a distinct name only leads to confusion. Among the 

 bacteria, as among higher plants, such differences, constituting more 

 or less permanent varieties, have been developed by cultivation under 

 various conditions, and, without doubt, are constantly being devel- 

 oped under natural conditions as a result of changes in the environ- 

 ment of these minute plants. Thus we have artificial varieties of 

 certain common chromogenic species in which no pigment is pro- 

 duced, non-pathogenic varieties of pathogenic species, and asporoge- 

 nous varieties of bacilli which usually form spores. 



The attempt to classify the bacteria in a systematic manner is 

 attended with especial difficulties, owing to their simple structure, 

 the comparatively slight morphological differences which they pre- 

 sent, and also because of the tendency to variation in their biological 

 characters above referred to. But bacteriologists are generally 

 agreed upon the importance of the following characters for their 

 differentiation, viz. : form micrococci, bacilli, spirilla, polymor- 

 phous ; relation to oxygen aerobic, facultative anaerobic, strict 

 anaerobic ; growth in nutrient gelatin liquefy, do not liquefy, do 

 not grow in nutrient gelatin at the "room temperature"; growth on 

 potato ; groivth in milk coagulate milk, do not coagulate, etc. ; 

 color of growth chromogenic, non-chromogenic ; spore forma- 

 tion ; independent movements ; pathogenic power. 



It is upon these characters that we must rely chiefly in our bacte- 

 riological diagnosis of known species. But the student must remem- 

 ber that the lines are not sharply drawn between the groups formed 

 when we classify bacteria with reference to any one of these charac- 

 ters. Thus we have microorganisms of this class which we find it 

 difficult to classify as regards form, because they are not round, and 

 yet are so slightly elongated in one diameter that it is difficult to 

 consider them bacilli. If we follow Cohn and group these short- 

 oval bacteria under the generic name Bacterium, we have not re- 

 moved the difficulty, but have made two arbitrary and artificial lines 

 instead of one. Again, liquefaction of gelatin is sometimes so slight, 

 or occurs at so late a date, that it may be a question whether a mi- 

 croorganism of this class should be included among the liquefying 

 or non-liquefying bacteria. And our division with reference to the 

 formation of pigment must, to a certain extent, be arbitrary ; for 

 many species which are not decidedly chromogenic present under 

 certain circumstances a slight tint of yellow, gray, brown, or pink. 

 A slight yellow or a decided brown color is often developed in potato 

 cultures of bacteria which upon other culture media present a col- 

 orless growth, and which are generally included by bacteriologists 

 among the non-chromogenic species. With reference to independent 



