EXAMINATION OF CULTURES 73 



(d) The medium must be suitable to the needs of the organism to be 

 grown. All organisms cannot be grown indifferently on any medium ; for 

 while some require a medium rich in albuminoid matter, others prefer sugars^ 

 glycerin, etc., and others again will not grow on serum, or potato, and so on. 



In a later part of the book, when discussing individual organisms, mention 

 will be made of the media most suitable for the growth of each species. 



SECTION IV. THE EXAMINATION OF CULTURES. 



Cultures should be examined daily or even two or three times a day, and 

 the character of the growth, which is of great importance in determining the 

 species to which an organism belongs, noted. 



Attention should be particularly directed to the following points : 



A. In the case of micro-organisms growing in artificial cultivation, no 

 matter what the medium, note : 



1. (a) The optimum temperature of growth, (b) the limits of temperature 

 within which growth takes place. 



2. The time when growth first makes its appearance. 



B. When cultures are growing in liquid media, note : 



1. The mode of growth. Growth may produce : 



(a) A distinct cloudiness of the medium, which may be either an uniform 

 cloudiness or a cloudiness with a watered silk appearance, or sometimes a 

 cloudiness with a surface pellicle. In these different cases flocculent deposits 

 may ultimately form, and if so their occurrence should be noted. 



(b) No distinct turbidity of the medium. Under these conditions the 

 growth may show : (a) a surface pellicle, which may be either thin or thick, 

 fatty or wrinkled ; (/^) a ring of growth round the wall of the tube at the 

 surface of the liquid ; (7) flocculent deposits in the liquid, which may sub- 

 sequently precipitate ; (8) fine granular deposits, which in some cases adhere 

 to the walls of the tube and in others fall to the bottom of the medium. 



2. The colour of the growth. 



3. The production of any smell during growth. 



4. The development of any new substances in the medium (toxins, indol, 

 acid, ammonia compounds, etc.). 



5. The presence or absence of clot when grown in milk. 



C. In the case of stroke cultures : 

 (i) On agar, potato or serum, note : 



1. The mode of growth. 



(a) The growth may remain limited to the line of sowing, and in this case 

 it should be further noted whether (a) the culture takes the form of a delicate 

 homogeneous and transparent streak, or occurs as discrete colonies ; or (/3) 

 whether the streak be thick, and if so if it be moist, greasy, viscous, dry or 

 wrinkled. 



(b) The growth on the other hand may spread widely over the surface of 

 the medium, and the nature of the growth, that is to say whether it be 

 moist, greasy, viscous, dry or wrinkled, is to be noted. 



2. The colour of the growth. Whether the line of growth or the surrounding 

 medium is pigmented. 



3. The production of any odour. 

 (ii) On gelatin, note : 



1, 2, 3. The/orm, colour and smell of the growth as in the preceding cases. 



4. Whether the organism liquefies the medium, and if so, the time at which 

 liquefaction begins. 



