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 zymogenic 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. 



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- 

 gen a biological significance that had hitherto been 

 denied it. 



The so-called thiogenic bacteria convert sulphuretted 

 hydrogen into higher sulphur compounds. 



The bacteria concerned in the foregoing changes per- 

 form the functions, as said, by virtue of special fer^ 

 ments or enzymes that are elaborated by them. 



