128 Dr. J. M. Hobson on 



of alcohol is exposed to the air at a temperature of about 80'^ to 

 100° F. The Micrococcus aceti lights upon this mixture, and under 

 the circumstances grows, fermenting the alcohol into acetic acid. 

 This is a partly oxidising process. Other fermentations produced 

 by bacteria are instanced as follows : — The tuniinf/ of milk, the 

 sugar of milk being converted into lactic acid, and the acid pre- 

 cipitating the casein ; the conversion of urea into carbonate of 

 ammonia ; the complicated fermentations of albuminous com- 

 pounds through stages into carbonic acid and ammonia, the 

 nitrifying of the resulting ammonia forming nitrates. Other 

 products of bacterial growth are pigments, which have also been 

 spoken of as ferment products. Again, there is the production 

 of certain bodies of the diastase type. Germinating seeds, as is 

 well known, produce an albuminous body called diastase, which 

 has the power of converting starch into grape sugar. The yeast 

 plant also inverts cane sugar into grape sugar by means of a 

 similar body. The term enzyme has been applied to these sub- 

 stances. In the animal economy these bodies are represented by 

 pepsine, pancreatin, and so forth. They have this in common, 

 that under suitable conditions they have the power of converting 

 organic compounds into other forms without themselves entering 

 into chemical combination. Lastly, bacteria have the power of 

 forming out of their pabulum various alkaloids. The term 

 ptomaines has been applied to these, from Trrw/^a, a dead body, 

 because they have been found in decaying animal matter. Some 

 of these alkaloids are highly poisonous, and have been obtained 

 from cases of cholera, hydrophobia, tetanus, &c., and when given 

 to animals have produced symptoms resembling some of those 

 obtaining in these several specific diseases. There is reason to 

 think that the production of specific alkaloids sometimes requires 

 the collaboration of two different organisms. It is certainly the 

 case that bacterial products are dependent very largely upon the 

 character of the nutrient medium, and upon the presence or 

 absence of oxygen. The fermentative phase of Saccharomyces is 

 really anaerobic, although the presence of oxygen is necessary for 

 a ;j)-e- fermentative stage in which the production, simply, of fresh 

 cells takes place. I shall give an instance of this again when I 

 come to speak of cholera. 



Parasitic Bacteria. — By this are meant bacteria which ac- 

 tually live and multiply within the living fluids and tissues. 

 Bacteria which merely find a home within the alimentary canal, 

 where they regularly exist without producing any harmful 

 results, are not regarded as parasitic. A more strictly scientific 

 term would be infective bacteria, for it is their power of pro- 

 ducing more or less profound effects upon the living tissues, of 

 causing local or general infective diseases, which has called so 

 much attention to them, and has in fact created the science of 



