164 MICRO-ORGANISMS AND FERMENTATION. 



Sulphur bacteria occurring in water, many of which pro- 

 duce a red-colouring matter, have been described by Cohn, 

 Warming, Engler, and especially Winogradsky. Under the 

 microscope they are distinguished by the roundish bodies they 

 contain, strongly refractive to light, and consisting of pure 

 sulphur. They are aerobic, and occur especially in waters 

 containing sulphuretted hydrogen. This substance is oxidised 

 by the bacteria, and the sulphur split off is stored in the cells. 

 Among the thread-like species the Beggiatoa alba may be 

 mentioned. It occurs in cylindrical filaments without sheaths, 

 which have a crawling motion, rotate round their longer axis, 

 and swing from either end. They may expand to a great 

 length. If the liquid contains sulphuretted hydrogen the 

 filaments will be found to contain rounded grains of sulphur, 

 strongly refractive. The threads divide by means of cross 

 sections, and if no sulphuretted hydrogen is present they 

 break up into single pieces, and gradually die off. 



An important part is played in nature by bacteria which 

 convert ammoniacal salts into nitrates ; they are highly 

 important for the nutrition of many plants. Schloesing and 

 Miintz first described them ; their observations were con- 

 firmed by Winogradsky, who made use of pure cultures. 

 Among these nitrifying bacteria, as they are termed, there 

 are some which oxidise ammonia into nitrous acid (nitrite 

 bacteria), which is converted by other species into nitric acid 

 (nitrate bacteria). As already mentioned, in an earlier section, 

 these bacteria possess the power of living entirely without 

 organic food, and in culture experiments in the laboratory 

 they are grown in solutions of inorganic compounds. They 

 form cocci and short rods, aerobic, motile, or motionless \. 

 they may grow in the dark, although they assimilate carbon 

 dioxide from the air. Saltpetre may again be attacked by 

 the denitrifying bacteria, which are capable of decomposing 

 saltpetre in presence of organic substances with evolution of 

 free nitrogen. The nitrifying bacteria also cause the efflor- 

 escence of nitre from walls, which often brings about the 

 decay of brickwork, snow-like masses of calcium nitrate 

 becoming detached. This evil can be remedied by means 

 of antiseptics. 



