64 THE CHEMISTRY OF BACTERIA 
thrive in media in which nitrogen is presented to them as peptones, 
albumoses, or even certain amino-acids; in other words, upon the pro- 
ducts of protein digestion.^ The more strictly pathogenic organisms, 
as the gonococcus, may require nitrogen in the form of highly specific 
tissue proteins. Generally speaking, animal protein or its derivatives 
is more easily utilized by bacteria than protein of vegetable origin. It 
will be recalled, however, that Bacillus botulinus produces its potent, 
soluble poison readily from vegetable protein. 
{h) Carbon.— The simplest carbon compound which occurs naturally, 
CO 2, cannot be used by bacteria, except possibly certain nitrifying 
bacteria, as a source of energy, for it is already fully oxidized. The 
carbon of proteins and their derivatives, of carbohydrates, and of 
fats, on the contrary, is readily utilizable by most bacteria. As a 
rule, hydrocarbons of the aliphatic series are not attacked by the 
microorganisms, but compounds containing oxygen as well as carbon 
and hydrogen are better adapted for microbial food. Organic acids, 
as acetic acid, aspartic, tartaric and many oxyacids are utilizable by 
some organisms. The simpler alcohols can be used, but by very few 
bacteria. The complex alcohols, like glycerol and mannitol, on the 
other hand, are available food materials for many. 
The best nitrogen-free food compounds for microorganisms are the 
carbohydrates, particularly those containing six and twelve carbon 
atoms, the hexoses and bioses respectively. Carbohydrates containing 
four, five or any number of carbon atoms not a multiple of three are 
usually not readily attacked by bacteria. Starches and cellulose are 
not generally utilizable, although certain types of organisms, notably 
those found in the intestinal tracts of herbivora, appear to decompose 
them very readily. 
(c) Hydrogen.— Hydrogen is readily obtained by microorganisms 
from organic compounds containing available carbon, nitrogen and 
hydrogen, but not apparently from water. 
{d) Oxygen.— Oxygen is indispensable to the life of all living organ- 
isms as a source of energy and for structural purposes. A few bac- 
teria, the obligately aerobic bacteria, can live only in the presence of 
free oxygen; another small group, the obligately anaerobic bacteria, 
live either in the absence of free oxygen or at best in the presence of 
minimal amounts of it; more than minimal amounts of free oxygen 
act as specific poisons to them. The majority of bacteria are faculta- 
tive with respect to their oxygen requirements; that is, they can either 
live in the presence of free oxygen or derive their oxygen needs from 
organic compounds, usually the carbohydrates or proteins. 
{e) Inorganic Salts.-— Inorganic salts are used by bacteria almost 
wholly for structural purposes. The requirement for mineral com- 
pounds is very little, for these substances do not on the average make 
> See p. 52 for effects of specific amino-acids upon bacterial growth. 
2 See Falk: The Role of Certain Ions in Bacterial Physiology, Abst. BacterioL, 
1923, 7, .33, 87, 137 for good discussion. 
