CH.VI.] MORPHOLOGY OF BACTERIA. 35 



is an essential element for them, and a certain temperature is 

 in many instances a stimulant of their growth. Most pathogenic 

 bacteria require for their propagation a temperature varying in 

 the different cases between 18" and 40 C. The bacteria obtain 

 their nitrogen from organic compounds ; some are capable of 

 obtaining it from compounds as simple as ammonium tartrate ; 

 others, especially pathogenic organisms, require much more 

 complex combinations, such as occur in the animal body. 

 Carbon they obtain likewise from organic compounds, such as 

 carbohydrates, amongst which sugar is the chief, and vegetable 

 acids combined as salts are also to be mentioned. It is essential 

 for all that certain inorganic salts, phosphates, potassium and 

 sodium salts, should be present, since their own substance 

 contains a large percentage of it 4 to 6 per cent. 



While all are capable of disintegrating organic combinations 

 containing nitrogen, they in their turn help to produce certain 

 chemical products, which in some cases are definite for a de- 

 finite species (see below). Such is the case with the various 

 bacteria connected with the fermentations producing lactic 

 acid, butyric acid, and acids belonging to the aromatic series. 

 On many bacteria connected with putrefaction, and also on 

 some pathogenic organisms, these chemical products have a 

 deleterious effect. Small quantities impede their growth, and 

 sufficiently large quantities kill them altogether. 



Most bacteria are killed by heat below the temperature of 

 boiling water, many of them when exposed for several hours 

 to a temperature above 50 60 C. Exceptions are the spores 

 of bacilli, which in some instances (spores of hay bacillus, 

 Cohn) require exposure to the heat of boiling water for as much 

 as half an hour. By raising the boiling point above 100, 

 it does not require more than a few minutes to kill them 

 (Sanderson). 



Drying destroys most bacteria, except the spores of bacilli. 

 Freezing destroys likewise most bacteria, except the spores of 

 bacilli, which survive exposure to as low a temperature as 

 15 C., even when exposed for an hour or more. No spores 

 survive exposure to a temperature of 120 C. 



Amongst those substances which inhibit the growth of, or 

 altogether destroy the bacteiia, are carbolic acid, salicylic acid, 

 thymol, &c. ; corrosive sublimate is the most powerful (Koch) 

 since even solutions as weak as 1 : 300,000 are said to inhibit 

 the growth of bacillus anthracis. 



The best classification of bacteria is that given by Cohn, 1 

 and this we shall adopt : (1) spherobacteria micrococci ; 



1 Lt lr. z. S. d. Pjl. Ld. i. 



D 2 



