CHARACTERISTICS OF THE NITRATE-BACTERIA 193 



§6. MORPHOLOGICAL CHARACTERISTICS OF THE 

 NITRATE-BACTERIA. 



The colonies on agar plates take a long time to grow, as they are 

 not visible, even as small specks, till about two 



weeks after inoculation. Then round, homogeneous ^ ^ft • 



colonies, which may attain a diameter of from ^ C5^^^ 



100 ytt to 180 fi., gradually develop. The individuals ^^^^ if^» 



composing the colonies are rod-shaped, and very ^ 



small, being about 1 fx. long and 0-3-0-4 /x broad ^'°- il^J^i^i?''^'''- 

 (Fig. 117). They have not been observed to exhibit 

 motility, neither do they unite to form the Zoogloeae which are so 

 characteristic of the nitrite-bacteria. 



PHYSIOLOGICAL CHARACTERISTICS OF THE 

 NITRATE-BACTERIA. 



The statements that have been made above with regard to the 

 physiological characteristics of the nitrite-bacteria apply equally to the 

 nitrate-bacteria, for they also can assimilate the atmospheric carbon 

 dioxide without the aid either of chlorophyll or of light ; and further, 

 they thrive only in media from which organic substances have been 

 carefully removed. In the same way also, the presence of organic 

 substances is detrimental to the growth of the nitrate bacteria, 

 though they are not so sensitive in this respect as are the nitrite- 

 bacteria. 



They differ from the nitrite-bacteria in that they obtain their 

 nitrogen supply from nitrite- instead of from ammonium-compounds, 

 hence they take up a substance which has been excreted by the 

 nitrite-bacteria. 



As explained above, the nitrites are oxidised into nitrates as a result 

 of passing through the bodies of the nitrate-bacteria. We have seen 

 that ammonium compounds are indispensable to the nitrite-bacteria. 

 To the nitrate-bacteria, however, the presence of an ammonium com- 

 pound is unfavourable to growth. In a mixed culture containing 

 ammonia the development of the nitrate-bacteria, if present, is arrested 

 until all the ammonia has been used up. Among the different nitrite 

 salts, that of sodium is best suited for the needs of these bacteria, 

 and it is in this form that the nitrite is usually supplied. 



