36 BACTERIOLOGY. 



bacteria, possess the property of producing light or 

 of illuminating the medium on which they grow by 

 a peculiar phosphorescence. These are found in sea- 

 water and in decomposing phosphorescent fish and 

 meat. 



Still others, the so-called eymogenie bacteria, are con- 

 cerned in the various fermentations, such, for instance, 

 as acetic-, lactic-, and butyric-acid fermentations ; and 

 many of the industries, such, for example, as those 

 concerned in the making of wine, beer, cheese, butter, 

 and indigo, are more or less directly dependent upon 

 the fermentation that accompanies the growth of pecu- 

 liar species of bacteria in those materials. 



Other species of bacteria liberate in the course of 

 their growth soluble proteolytic ferments or enzymes, 

 more or less analogous to pepsin and trypsin in their 

 action upon proteids ; while others generate ferments 

 analogous to diastase and to invertin : the former con- 

 verting starch into sugar, and the latter having the 

 power of converting cane- into grape-sugar. 



The saprogenic bacteria are those that produce the 

 particular fermentation that we know as putrefac- 

 tion. 



Another very important saprophytic group comprises 

 the so-called nitrifying and denitrifying bacteria, whose 

 activities are concerned in specific forms of fermentation : 

 the former oxidizing ammonia to nitrous and nitric acids ; 

 the latter reducing nitric acid to nitrous acid and am- 

 monia. It is through their association (symbiosis) with 

 the nitrifying bacteria that certain plants, the legumi- 

 nous, are enabled to make up their nitrogen deficit in 

 part from the free nitrogen of the air. The discovery 

 of this phenomenon gave to free atmospheric nitro- 



