152 C. J. T. DORYLAND 



above table demonstrates clearly the possibilities of the pro- 

 posed synthetic media. It is apparent at once that the pure 

 species of bacteria included in these experiments may be grown 

 on some of the above synthetic media and controlled, to a cer- 

 tain extent, at least, by changing the energy or nitrogen source. 



SUMMARY 



The above scheme differs radically from the attempts to 

 secure a universal medium, i.e., one upon which most bacteria 

 of the saprophytic or parasitic groups will grow. It is on the 

 contrary an attempt, by the use of definite sources of energy 

 and definite sources of nitrogen, to exclude all species but those 

 which can use the particular source of energy and nitrogen in- 

 cluded in each case. If we are thus able to devise a series of 

 mutually inclusive and exclusive media, we shall, after having 

 tested the known species upon each, be able to state positively 

 that any growth which appears upon any given medium is a 

 member or members of a hmited number of types belonging 

 to that group (designated by media). Furthermore, we shall 

 be able to plate out a soil, a milk, a water, etc., on a series of 

 media and know, not only the number of bacteria present but 

 also the number of the different groups present. 



The schematic arrangement presented is tentative and is 

 limited to water soluble compounds. Many of the groups sug- 

 gested may have no energy value or the chemicals used may be 

 too toxic or too expensive for practicable purposes. Other 

 groups and compounds will no doubt be suggested by further 

 study. Likewise, it is possible that subdivisions of the above 

 groups may be made by using different sources of ammonia, 

 nitrate or cyanide, etc., or by modifying other nutrient con- 

 stituents or by changing the reaction. Up to the present time 

 16 of the above media have been tested. One using oxalate as 

 a source of energy was negative throughout, the others have 

 given promising results. It is hoped, therefore, that these 

 tentative statements will stimulate needed investigation along 

 this line, and criticism is heartily invited. 



