32 BULLETIN 782, U. S. DEPARTMENT OF AGRICULTURE. 



of nitrate to nitrite in peptone broth was probably due to the fact, 

 which has been previously explained, that the organisms of the 

 group did not develop well in peptone solution. While they could 

 use it as a source of nitrogen with a suitable source of carbon, it 

 was a poor source of nitrogen when used alone. With beef extract 

 a source of carbon was probably supplied for the alkali-forming 

 bacteria from the salts of the organic acid present in the ex- 

 tract; consequently these bacteria could grow better, and as a 

 result there was a larger number of cultures which coidd reduce the 

 nitrate to nitrite. 



It has been seen, however, that these bacteria could utilize nitrogen 

 from nitrate ; consequently it should be expected that all the cultures 

 should show a reduction and a positive nitrite test. Tests were 

 made, therefore, in a medium containing 0.2 per cent of sodium 

 nitrate, 0.25 per cent sodium lactate, and 0.1 per cent sodium- 

 dibasic phosphate in distilled water. This medium was inoculated 

 and incubated at 30° C. for 14 days and then tested for the presence 

 of nitrite. Table 14 shows that 57 cultures gave a positive nitrite 

 test. Seven did not grow in the experiment, although at other 

 times they grew in the same medium. The remaining four cultures 

 did not show a positive nitrite test. In order to show that there 

 was a growth in all cases except the seven mentioned, titrations 

 were made to determine the production of alkaline carbonates due 

 to the oxidation of the sodium lactate. It will be seen from the 

 table that all the cultures except those which did not show any 

 growth produced an alkaline reaction. Even the five cultures 

 which showed negative nitrite tests produced an alkaline reaction, 

 showing that the nitrogen for growth miust have been obtained 

 from sodium nitrate; consequently there must have been a reduction. 

 The probable reason why there was growth without a nitrite reaction 

 is because nitrites may be utilized by this group of alkali-forming 

 bacteria as well as nitrates. This is shown in Table 14 through the 

 alkaline change in reaction in the medium composed of 0.2 per cent 

 sodium nitrite, 0.25 per cent sodium lactate, and 0.1 per cent dibasic- 

 sodium phosphate and distilled water. 



It will be seen from the results that aU but 5 cultures produced an 

 alkaline change, showing the ability of the organisms to utihze sodiiun 

 nitrite as a source of nitrogen and consequently reduce it. The five 

 cultures that failed to show the nitrite test in the sodium-nitrate 

 medium all produced an alkaline change in the sodium-nitrite-lactate 

 medium. It is probable that these organisms failed to show a 

 positive nitrite test because the nitrite was used up as soon as formed 

 from the nitrate. Since these alkah-forming bacteria could reduce 

 nitrite, it was natural to expect that it might be possible to determine 

 the reaction by means of the determination of ammonia as shown by 



