256 LABORATORY EXERCISES L\ BACTERIOLOGY. 



at a later period, usually about three or four weeks, the evidences of tuber- 

 cular disease will be manifested by the appearances of enlarged and case- 

 ated lymphatic glands and by emaciation of the animals. Such an animal 

 is killed and scrapings planted with the usual precautions upon solidified 

 blood-serum (select fresh, undried medium), the substance being well 

 rubbed into the surface of the serum, and the inoculated tubes placed at 

 body temperature in the incubator. In ten or twelve days pure growths 

 of Mycobacterium tuberculosis should be obtained, and may be identified by 

 'their gross and minute characteristics as set forth in any of the various 

 descriptive works on bacteriology. 



2. Selection by Culture Medium. In a like manner the different nutrient 

 media, because of peculiarity of their organic matter or the presence of restrictive 

 or favoring inorganic constituents, may exhibit a special fitness for the nourishment 

 of some particular germ, to the more or less absolute exclusion of associated microbes 

 and thus lead to the early and preponderating development of the former, while the 

 latter are inhibited from growth in the culture either entirely or in part. This may 

 be noticed in the rapid development of the diphtheritic organism on blood-serum, 

 Loeffler's medium, or ascitic agar (agar made up with ascitic fluid instead of bouillon, 

 and with five per cent, of glycerine added), the colonies of this microorganism appearing 

 a number of hours earlier on such medium than those of the commonly associated pus 

 bacteria; while on ordinary agar or gelatine this is almost or quite reversed. 



For this reason successful search for some definite form of bacterium demands 

 thoughtful selection of the media to be employed for its culture. It may not be, however, 

 that the material selected will favor isolation by increase of the rate of development of 

 the organism sought for, or by exclusion of growth of its associates ; all may grow with 

 equal rate, but the medium may have caused the desired germ to take on some well- 

 marked gross cultural peculiarities, so that its colonies are easily recognized from the 

 rest and their selection for transfer to fresh medium made certain. 



In this connection may be mentioned the practice of adding special substances to 

 the nutrient medium for the purpose of inhibiting the growth of undesired bacteria 

 in the culture, the substances thus used having little or no influence upon the develop- 

 ment of the particular microorganism which it is desired to obtain in pure culture. 

 Carbolic acid, hydrochloric acid, iodide of potassium, and other substances are some- 

 times added, with this in view, to the nutrient upon which fecal matter is implanted 

 for the purpose of restraining the growth of organisms other than Bacillus coli or 

 Bacillus typhosus; the employment of such substances also bringing out differences 

 in the rate of development and of gross cultural appearance as will permit the recogni- 

 tion of these two organisms from each other in the culture, and consequently their 

 mechanical separation. 



Exercise 76. With the sterile needle make stroke or smear inoculation 

 of normal human feces upon a dish of Eisner's medium and grow for twenty- 

 four hours (if the medium be made up with agar instead of gelatine, one 

 may place the inoculated dish in the incubator) . At the end of this time dis- 

 tinct colonies of a pale brownish tint may be recognized as the growth of 



