52 Biology of Micro-organisms 



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 con- 

 taining the same or approximately the same proteins as those to 

 which they have been accustomed. Thus, to cultivate trypanosoma, 

 blood-serum must be added to the media. 



The larger protozoa live upon smaller animal and vegetable organ- 

 isms, which they ingest entire. Such can only be artificially culti- 

 vated provided the attempt be made under conditions of symbiosis 

 with some other and smaller organism that may constitute the 

 food. 



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 con- 

 centrated; at least 80 per cent, of water should be present. 



The molds and o'idia grow well upon bread that contains very 

 little moisture. Protozoa usually require fluid media. Pond-water 

 protozoa can only grow in w r ater, not in concentrated culture-media. 



Reaction. Should the pabulum supplied contain an excess of 

 either alkali or acid, the growth of the micro-organisms is inhibited. 

 Most true bacteria grow best in a neutral or feebly alkaline medium. 

 There are exceptions to this rule, however, for Bacillus butyricus and 

 Sarcina ventriculi can grow well in strong acids, and Micrococcus 

 urea can tolerate excessive alkalinity. Acid media are excellent 

 for the cultivation of molds. Neutral or feebly alkaline media serve 

 best for the cultivable protozoa. 



Light. Most organisms are not influenced by the presence or 

 absence of ordinary diffused daylight. The direct rays of the sun, 

 and to a less degree the rays of the electric arc-light, retard and in 

 numerous instances kill bacteria. In a careful study of this 

 subject Weinzirl* found that when bacteria were placed upon glass 

 or paper, and exposed to the direct rays of the sun, without any 

 covering, most non-spore-bearing bacteria, including Bacillus tubercu- 

 losis, B. diphtherias, B. typhosus, S. choleras asiaticae, B. coli, B. 

 prodigiosus, and others are killed in from two to ten minutes. 

 Certain colors are distinctly inhibitory to the growth, blue being 

 especially prejudicial. 



Treskinskajaf found that sunlight had a marked destructive effect 

 upon the tubercle bacillus, and varied according to altitude. By 

 direct sunlight at the sea-level they were destroyed in five hours: at 

 an altitude of 1560 meters, in three hours. In winter the time of 

 destruction was about two hours longer than in summer. In diffused 

 daylight the time required for destruction was about twice as long 



*"Centralbl. f. Bakt. u. Parasitenk. Ref.," XLVII, Nos. 22-24, P- 681. 

 t"Jour~ Infectious Diseases," 1907, vol. iv, Supplement, No. 3, p. 128. 



