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For routine water work the selection of a differential test must be 

 governed to a great extent by the following considerations: 



1. The medium should be simple, easy to prepare, and permit of a 

 considerable degree of variation. 



2. The reaction should be distinct and constant. 



3. The test must be one which can be completed in a short time, 

 preferably not more than 24 hours. 



In the light of these considerations it becomes evident that the gas 

 ratio is out of the question. That it is permanent and constant has been 

 well demonstrated hy the investigations of Rogers and his associates, Hut 

 the skill and apparatus necessary together with the time required removes 

 it from the possibility of a routine water test. 



The uric acid reaction is a convenient and reliable test but the scrup- 

 ulous care required in preparation of the medium can not be obtained at 

 the present time at least, in laboratories which are concerned with routine 

 water analysis. The period of incubation, three to five days, is also a 

 disadvantage. 



The choice thus becomes limited to the Voges-Proskauer and methyl 

 red tests. Opinions are quite at variance as to which is preferable. The 

 methyl red reaction has been urged by Clark and Lubs and by Winslow. 

 The necessity for employing Witte's peptone together with the fact that the 

 reaction is based on a delicate adjustment of the source of carbon and 

 buffer substance and the insatiable desire among bacterialogists to deviate 

 from the media recommended and to make individual substitutions has 

 lead to numerous difficulties in the application of this test in practice. 

 The synthetic medium of Clark would of course eliminate some of these 

 difficulties. The methyl red reaction is simple, reliable, and when care- 

 fully performed, constant, but the period of incubation for accurate dif- 

 ferentiation is quite long (3 to 5 days), too long, in fact, to be conven- 

 iently employed as a routine test. 



The tendency recently has been toward the V. P. reaction. Thus Chen 

 and Rettger conclude from a study of 640 strains that the V. P. reaction 

 is even more satisfactory than the methyl red test in that it is simple in 

 operation, and when correctly carried out, is thoroughly constant in its 

 results. 



Clark and Lubs, on the other hand, point out that the production of 

 acetyl methyl carbonol, being the result of secondary reaction, and pos- 

 sibly synthetic to some extent, may not be intimately connected with the 

 main course of the fermentation and the quantity produced may be very 

 slight, thus giving a faint V. P. test even though the fermentation may be 

 very vigorous. Nevertheless, the reaction has been found to be very con- 

 stant and it has proven very satisfactory in the hands of practically all 

 who have tried it. The advantages of this reaction are: 



1. Any peptone medium in which the organisms will grow and 

 which contains glucose (in a wide range of concentration) is suitable. 

 It is preferable, however, to have the medium as free from color as pos- 

 sible. 



