52 GENERAL PHYSIOLOGY OF BACTERIA 
The association of various kinds of bacteria in this process, where 
each succeeding kind profits by the activities of the preceding kind, 
is a symbiotic one; that is, the several types of organisms mutually 
profit by their combined activities. 
It frequently happens that the products of symbiotic activity may 
be greater than the sum of the products of the separate activities 
of the organism. 1 On the contrary, many instances are known in 
which one kind of organism by its activity actually crowds out a 
preexisting organism, as for example, the lactic acid bacteria which 
sour milk. They produce sufficient lactic acid from the fermenta- 
tion of the lactose to kill the proteolytic forms. This substitution of 
one type of organism by another is known as antibiosis: the latter 
organism profits wholly at the expense of the first organism. 
It not infrequently happens that one type of bacterium profits by 
the activity of another type of organism without benefiting the former 
in return. If two types of bacteria are concerned, the process is known 
as metabiosis; if the bacterium is living on a host, the relationship is 
spoken of as parasitism. 
N. MEDIA-COMPOSITION AND REACTION. 
Most bacteria grow best in a medium containing a large percentage 
of moisture in which dift\isible proteins or protein derivatives are 
present as sources of nitrogen: these substances are better adapted to 
the dietary needs of the majority of bacteria than are ammonium 
salts or even simple amino-acids. Gordon and McLeod- studied the 
effects of fourteen amino-acids, separately, and with respect to their 
effects upon bacterial growth. They found no effects upon organisms, 
like B, coli and staphylococci, which grow freely. More delicate 
organisms, however, were aft'ected: 
(a) Indifferent amino-acids: arginine, glutamic acid, 1-histidine, 
1-leucine, 1-lysine, tyrosin and valine. 
(h) Favoring amino-acids: proline, aspartic acid, and alanine. 
(c) Inhibitory amino-acids: cystine, glycocoll, phenyl alanine and 
tryptophan. 
(d) Tyrosin, the most toxic of amino-acids. 
A very few bacteria (nitrifying bacteria) cannot grow in media con- 
taining organic nitrogen compounds. Tamura^ has shown that bac- 
teria may synthesize typical purines, phosphatids and proteins from 
non-protein media. A few strictly pathogenic bacteria appear to 
require nitrogen as it exists in the highly complex tissues of man or 
animals for their growth. Many bacteria can utilize carbohydrates 
for their carbon, hydrogen and oxygen requirements. Some bacteria 
appear to be able to utilize fats for their carbon requirement. 
1 Kendall: Jour. Am. Med. Assn., 1911, 56, 1084. 
2 Jour. Pathol, and Bacteriol., 1926. 29, 13. 
3 Ztschr. f. phys. Chem., 1913, 88, 190. 
