AUTOTROPHIC BACTERIA 79 



the ammonium sulfate is to the Nitrosomonas and Nitrosococcus, the 

 nitrous acid and nitrite to Nitrobacter and the carbon compounds to the 

 heterotrophic bacteria. 



The sulfur bacteria, or those bacteria which are capable of obtaining 

 the energy necessary for their growth from the oxidation of sulfur or its 

 compounds, should be distinguished from other bacteria taking part in 

 the sulfur cycle, such as those liberating H 2 S in the hydrolysis of pro- 

 teins or in the reduction of sulfates. 



Classification of sulfur bacteria. The sulfur oxidizing bacteria can 

 be divided into five groups. 



1 . Thread-forming, colorless bacteria, accumulating sulfur within their 

 cells; Beggiatoa and Thiothrix are representatives of this group. 



2. Non-thread forming, colorless bacteria, accumulating sulfur within 

 their cells; a number of forms (Thiospirillum, Thiovulum, Achromatium, 

 etc.) of various sizes and shapes, the distinguishing characteristic of 

 which is the fact that they oxidize H 2 S and accumulate sulfur within 

 their cells, are included in this group. Some of these have been isolated 

 in pure culture. 



3. Purple bacteria. Some of these seem to play a part in the sulfur 

 cycle, although none of the sulfur forms have yet been isolated in pure 

 culture. 



4. Colorless, non- thread forming sulfur oxidizing bacteria, which do 

 not accumulate sulfur within their cells, but which produce an abun- 

 dance of sulfur (from H 2 S and thiosulfates) outside of their cells. The 

 two characteristic and most important forms belonging to this group are 

 Thiobacillus denitrificans Beijerinck, an anaerobic form, deriving its 

 oxygen from the decomposition of nitrates, and Thiobacillus thioparus 

 Beijerinck, which oxidizes thiosulfates, H?S, and S and allows an exten- 

 sive accumulation of sulfur from the first two. 



5. The fifth group is similar to group 4 in morphology (but is less than 

 1/jl in length) and is distinctly different physiologically. The organisms 

 belonging to this group can act upon thiosulfates and H 2 S, but 

 they oxidize elementary sulfur very rapidly, allowing the medium to 

 become acid up to a reaction of pH 0.6 to 1.2. The only known 

 representative of this group is Thiobacillus thiooxidans Waksman and 

 Joffe. 



The sulfur bacteria can also be divided into, a, sulfide bacteria, or those 

 organisms which act primarily upon H 2 S and sulfides and which would 

 include the first three groups in the previous classification; b, thiosulfate 

 or "thionic acid" bacteria, equivalent to group 4; c, sulfur bacteria, 



