620 



SCIENCE 



[N. S. Vol. XLI. No. 1060 



The author suggests that steps be undertaken 

 for mating out some uniform method of eonduet- 

 ing filtration experiments of testing the efficiency 

 of the candle and expressing or recording the re- 

 sults. 



Influence of the Concentration of the Nutrient 



Substrate upon Microorganisms: Zae Noethrup. 



1. Determination of the Influence of the Con- 

 centration of the Gelatin in Nutrient Gelatin, upon 



Liquefying and Non-liquefying Organisms. — 

 Gelatin media, having the same amount of other 

 nutrient substances than gelatin, per unit volume 

 were prepared, using 15 per cent., 25 per cent., 35 

 per cent., 50 per cent, and 75 per cent, gelatin. 



Difficulties were met in the preparation of the 

 highest percentage of gelatin on account of the 

 thick sticky nature of the mass, but an excess of 

 water was added to make the mixture homogene- 

 ous, this water being then driven off by evapora- 

 tion on a water bath. 



Pure cultures of both liquefying and non- 

 liquefying organisms were plated on the different 

 concentrations of gelatin. 



On account of the extreme viscidity of the 75 

 per cent, gelatin it could not be plated in the 

 usual manner ; a thin film of the gelatin was spread 

 over a sterile glass slide in a sterile petri dish and 

 inoculated by spreading a small drop of a 24-hour 

 culture of the organism on the surface of the gela- 

 tin. 



The number, size and appearance of colonies 

 were to be noted on the media of the respective 

 concentrations. 



In counting, the low power of the compound 

 microscope ocular No. 1 and objective No. 7 was 

 found to give counts S-i times as high as the 

 ordinary counting lens. 



The numbers of organisms developing on the 

 plates are influenced to some but not to any 

 marked extent, if the mechanical difficulties of 

 inoculating the gelatin and pouring the plates are 

 taken into consideration. The decrease or varia- 

 tions noted may be due only to experimental 

 error. 



The size of the colonies was found to be in- 

 versely proportional to the concentration of the 

 gelatin. This was especially marked in the case 

 of the organisms which are the most active in 

 liquefying gelatin. 



The type and appearance of the colonies were 

 also found to be worthy of note. The subsurface 

 colonies of both liquefying and nonliquefying or- 

 ganisms appeared like very fine gas biibbles dis- 

 tributed throughout the medium. The active 



liquefying organisms began to show a rectangu- 

 lar instead of a concave depression in surface col- 

 onies on 35 per cent, and 50 per cent, gelatin, 

 while with the slow liquefier a new type of growth, 

 a stalagmite-like or apieulate growth, appeared 

 on the 50 per cent, gelatin. This type of growth 

 was noted in the 25 per cent, gelatin of colonies of 

 the non-liquefying organisms. 



B. typhosus was the only organism among the 

 eight types used in the experiment which refused 

 to grow on the 50 per cent, gelatin. However 

 another trial might prove successful. 



The different phenomena observed in the course 

 of this experiment will most probably call upon 

 the sciences of physics and of physical chemistry 

 for their interpretation. 



Several questions have been called forth by the 

 results of this experiment and most of them re- 

 main as yet unanswered. 



What part does the medium or substratum and 

 what part does the organism play in the formation 

 of the so-called characteristic growths which are 

 obtained in solid media? What force or forces 

 cause the variation in types of liquefaction pro- 

 duced by various proteolytic enzyme-forming or- 

 ganisms? Does the inherent nature of the organ- 

 ism or its secretions play the greater part or are 

 physical or physico-chemical forces the greater 

 factor? 



Why is the size and the structure of the colony 

 so markedly influenced by the media of increas- 

 ing concentration? It is not due to osmotic pres- 

 sure, as gelatin is a colloid and consequently vrill 

 exert no osmotic pressure. 



Is it due to the lack of water or is it due to 

 some physical property of the gelatin, aa surface 

 tension, which is more evident in greater concen- 

 trations? 



What force causes the colony in a nutrient gela- 

 tin of high concentration to show a rectangular 

 depression when in ordinary nutrient gelatin the 

 depression is concave? 



In the liquefaction of ordinary nutrient gelatin 

 what part does the force of gravity play? 



An interesting occurrence was noted in the 

 "plates" made with the 75 per cent, gelatin. 

 Upon examining these plates, several days (exact 

 period of time not noted) after they were made, 

 the glass slides were found in very fine pieces aa 

 if crushed by a powerful force. This occurred in 

 every case. The crushing of the slide was evi- 

 dently due to the contraction of the highly con- 

 centrated gelatin upon cooling and solidification. 

 Just how much energy it will take to crush slides 

 by mechanical force is yet to be determined. 



