60 Biology of Micro-organisms 



cultivation. The statement that certain forms of bac- 

 teria can flourish in clean distilled water seems to be 

 untrue, as in this medium the organisms soon die and 

 disintegrate. If, however, in making the transfer, a drop 

 of culture material is carried into the water with the bac- 

 teria, the distilled water ceases to be such, and becomes a 

 dilute bouillon fitted to support bacterial life for a time. 

 Sometimes a species with a preference for a particular 

 culture medium can gradually be accustomed to another, 

 though immediate transplantation causes the death of the 

 organism. Sometimes the addition of such substances 

 as glucose and glycerin has a peculiarly favorable influ- 

 ence, the latter, for example, enabling the tubercle bacillus 

 to grow upon agar-agar. 



The yeasts grow best upon media containing sugars, but 

 can also be cultivated upon media containing diffusible 

 protein and non-fermentable carbohydrates and glycerin. 



The molds flourish upon almost all kinds of organic matter, 

 but perhaps attain their most rapid development upon media 

 containing fermentable carbohydrates. 



The saprophytic and parasitic protozoa live by osmosis and 

 absorb through the ectosarc such substances as are capable 

 of assimilation and nutrition. These forms are cultivable 

 only upon media containing the same or approximately 

 the same proteins as those to which they have been accus- 

 tomed. Thus, to cultivate trypanosoma, blood-serum 

 must be added to the media. 



The larger protozoa live upon smaller animal and vegetable 

 organisms, which they ingest entire. Such can only be arti- 

 ficially cultivated provided the attempt be made under con- 

 ditions of symbiosis with some other and smaller organism 

 that may constitute the food. Amoeba coli can thus be cul- 

 tivated in symbiosis with Bacillus typhosus. 



Moisture. A certain amount of water is indispensable 

 to the growth of bacteria. The amount can be exceedingly 

 small, however, Bacillus prodigiosus being able to develop 

 successfully upon crackers and dried bread. Artificial 

 culture-media should not be too concentrated; at least 

 80 per cent, of water should be present. 



The molds and oidia grow well upon bread that contains 

 very little moisture. Protozoa usually require fluid media. 

 Pond-water protozoa can only grow in water, not in con- 

 centrated culture-media. 



