1 88 



C.-E. A. WiNSLow AND Anne F. Rogers 



To our conception of the non-acid-forming cocci as typically 

 saprophytic organisms frequently occurring in packets and usually 

 Gram-negative, we may add the property of moderate, but not 

 very active, liquefaction of gelatin. The very high acid-producers 

 are generally parasitic, do not show sarcinae, stain by Gram, grow 

 faintly on agar, and fail to reduce nitrates or liquefy gelatin. Between 

 these two groups is a third type which forms a moderate amount 

 of acid and produces the most active liquefaction of gelatin. 



6. Reduction of Nitrates. 



As described above, the tests for nitrate reduction were made in 

 parallel in lo tubes, and a marked variation was found in the indi- 

 vidual tubes, as shown in Table 30. This is perhaps to be expected, 

 since the development of bacteria in such a nutrient medium as nitrate 

 solution must be subject to many chance variations in the number 

 and vigor of the organisms inoculated. 



TABLE 30. 

 Reduction of Nitrates. 



Table 30 shows a considerable number of cultures yielding posi- 

 tive results in one or two out of the 10 tubes tested, less in from four 

 to seven of the tubes, and more again giving check results in all 

 10 tubes. In order to compare this property with others, it was 

 necessary to distinguish between positive and negative cultures, and 

 we have therefore considered the test to be positive when five or 

 more of the tubes showed some reduction. The cultures then grouped 

 themselves into three classes — one a large one, of those organisms 

 which did not have the property of nitrate reduction, and the two 

 smaller classes, in which were those which formed nitrites and those 

 which formed ammonia. 



Table 31 shows a somewhat surprising lack of correlation between 

 the formation of the two reduction products for which we have made 

 tests. Only 17 cultures showed both nitrites and ammonia in five 



