CHAPTER IX. 



BOUILLON, AGAR, MILK AND MODIFIED MEDIA— THE INCUBA- 

 TOH AND ACCESSORIES. 



The work, thus far, has enabled the student to acquire 

 familiarity with the fundamental methods of bacteriology. 

 The various non-pathogenic bacteria have been isolated by 

 means of gelatin plates; their characteristics of growth in 

 the colony, in stab culture, and on potato, have been stu- 

 died. A special bread medium has been employed in culti- 

 vating the several important moulds. The gelatin, potato 

 and bread media are, however, by no means the only ones 

 employed in growing micro-organisms. 



While the nutrient gelatin is invaluable for the isola- 

 tion of certain bacteria, especially those of the saprophytic 

 group, it is not so generally useful in connection with the 

 pathogenic bacteria. Many of these grow only, at or near 

 the temperature of the body (37.5°), and, inasmuch as the 

 ordinary gelatin melts at about 25°, it cannot obviously be 

 employed to obtain colonies, or stab cultures, of such or- 

 ganisms. 



It is, therefore, desirable to have a substance which, 

 to a certain extent, can take the place of gelatin; one which 

 will make a medium such that it will not melt, even at the 

 temperature of the body. The sea-weed agar-agar, which 

 is gathered off the coast of Asia, and about the islands of 

 the Pacific, answers this requirement. A more than two 

 per cent, solution of agar is not made, and is not desirable. 

 The prepared agar medium melts at the temperature of 

 boiling water (100°), and becomes solid again when the 

 temperature is reduced to about 40°. This medium, there- 



