PRODUCTS OF MICROBIAL ACTIVITIES 235 



characters of the organisms involved and the specific type of fermenta- 

 tion. Practically all the alcoholic organisms are yeasts, and the lactic 

 acid-producing organisms are streptococci or closely related bacteria. 

 The lactic bacteria, as they are briefly named, such as are responsible 

 for lactic fermentation, are readily recognized by their scanty growth 

 on agar, and their excellent growth in milk, bringing about a solid 

 curdling in one to three days. They change sugar to lactic acid only. 



C 6 H ]2 6 = 2C 3 H 6 O 3 



No gas and no volatile acids are formed by these bacteria. The best- 

 known representative of this group is the organism which causes the 

 normal souring of milk. It was originally called Bacterium lactis acidi, 

 but on account of its very close relation to the streptococci, it is more 

 commonly now named Streptococcus lacticus. Many streptococci will 

 produce the true lactic fermentation. 



The last two groups of organisms, alcoholic and lactic, represent 

 complex fermentations. There are several products formed, and as has 

 already been pointed out in the paragraph on the equation of fermen- 

 tations, the entire fermentation cannot be described accurately by one 

 equation, for different fermentations operate independently and simul- 

 taneously in the same cell. Under slightly different experimental 

 conditions the one or other of these simultaneous fermentations may be 

 favored, accordingly a varying proportion of the products is formed. 



The typical representatives of the acid-gas forming group of micro- 

 organisms which cause acid-gas fermentation are B. coli, and its near 

 relative, Bact. aerogenes. Many of the gas-formers in nature belong in 

 this group; the bacteria of the fermentations of pickles, sauerkraut, 

 salt-rising bread, the gassy fermentation of milk are some of the many 

 representatives. They are distinct rods, with good surface growth, 

 and do not liquefy gelatin. They are commonly spoken of as the coli- 

 aerogenes group. Some of them have peritrichate flagella, while 

 others are not motile. 



The fermentation of dextrose brought about by these organisms 

 has been described originally by Harden in the equation: 



2C 3 H 12 6 + H 2 = 2 C 3 H 6 3 + CH 3 C0 2 H + C 2 H 6 OH + 2 CO 2 + 2 H 2 



Dextrose Lactic acid Acetic acid Alcohol 



Harden himself stated later that this equation holds only for one 

 strain, and that we have several different strains distinguished by a 



