32 
principles of bacteriology 
A large number of bacteria may thrive without any pro¬ 
tein at all. 
Hydrogen.— Hydrogen is obtained largely in combina¬ 
tion with water. 
Salts. —Salts are absolutely necessary and are chiefly 
phosphates and chlorides of magnesium, calcium, potas¬ 
sium, and sodium, and iron (in some of the higher bac¬ 
teria) . 
VII. Aerobes and Anaerobes 
Those bacteria that must have oxygen and can not live 
without it are called obligatory aerobes. 
Those that can not live in the presence of oxygen (e. g., 
tetanus bacillus) are called obligatory anaerobes. Inter¬ 
mediate between these two extreme groups are those that 
prefer oxygen but can grow without it—faculative anae¬ 
robes—and those that prefer to grow without oxygen but 
can live in its presence—facultative aerobes. 
VIII. Longevity of Bacteria 
The duration of bacterial life is unknown, but is, com¬ 
paratively speaking, brief. Dried spores may not only 
live, but retain their virulence (invasiveness), for years. 
IX. Parasites and Saprophytes 
Parasites are those bacteria which live and multiply in 
the human or animal body, while saprophytes are those 
which can not hold their own in animal tissues, but are 
found everywhere in the air, soil, water, and manure. Par¬ 
asitic bacteria are fastidious as regards their food, temper¬ 
ature, etc., while the saprophytes are easily satisfied and 
live on simplest media. The main distinction, however, lies 
in our conception that the parasitic bacteria are those 
