59 



Inoculate for Exercise XXIX., a set of media with B. coli 

 communis from a culture furnished. 



For suggestions in studying cultures and microscopic 

 preparations of bacteria, see Exercise VI. and XII. 



95. Making drawings of bacteria with a definite 

 magnification. In measuring the bacteria (64) we obtain 

 the dimensions in microns or in units of i/iooo of a milli- 

 meter. In making a drawing, therefore, showing them mag- 

 nified 1000 diameters it is simply necessary to represent each 

 micron by one millimeter. Thus, if the organism is 2.5 /x in 

 length and i ^ broad, the drawing should be 2.5 mm. long 

 and i mm. broad. If the drawing is to represent the or- 

 ganism magnified 500 diameters then each micron should be 

 represented by 0.5 mm. For this purpose a metric rule and 

 a pair of dividers are necessary. 



EXERCISE XXIX. 



BACILLUS COLI COMMUNIS. 



96. Explanatory note. Of the bacteria normally pres- 

 ent on the mucous membranes of the animal body the colon 

 group is, on account of its close morphological relationship 

 to the bacilli of typhoid fever and hog cholera, of more than 

 ordinary interest. There are varieties of this organism 

 which approximate very closely in their biochemic properties 

 as well as in their morphology the typhoid and also the hog- 

 cholera bacilli. It is important that this existing variation 

 is recognized and that the list of properties which characterize 

 B. coli communis should be clearly determined. The differ- 

 entiation of the colon and typhoid bacilli, as they exist in 

 nature, is one of the difficult problems in practical bacterio- 

 logical work. The culture assigned approaches very closely 

 to the typical species. 



