Accurate determination of limits have been made on but few organisms. The reaction is a most important factor in growing bacteria on artificial media (see Making of Media, [Chapter XVI]).
INJURIOUS CHEMICAL SUBSTANCES.
(See Disinfection and Disinfectants, [Chapter XIII].)
CHEMICAL COMPOSITION.
The chemical composition is subject to wide variation chiefly for two reasons: First, the cell wall in most instances seems to exert only a slight selective action in the absorption of mineral salts so that their concentration within the cell is very nearly that of the surrounding medium. Second, the chief organic constitutents vary remarkably with the kind and amount of food material available—a rich protein pabulum increases the protein, a plentiful supply of carbohydrates or of fat results in the storing of more fat, especially and vice versa. These facts must be borne in mind in considering the chemistry of bacteria.
Of the chemical elements known, only the following seem to be essential in the structure of bacteria: carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, potassium, calcium, magnesium, iron, manganese. Other elements, as sodium, iodine, silicon, aluminum, lithium, copper, etc., have been reported by different analysts, but none of them can be regarded as essential, except possibly in isolated instances.
These elements exist in the bacterial cell in a great variety of combinations of which the most abundant is water. The amount of water varies in different species from 75 to 90 per cent. of the total weight in growing cells, and is less in spores. The amount of ash has been shown by different observers to vary from less than 2 per cent. to as much as 30 per cent. of the dry weight. The following table compiled from various sources will give an idea of the relative abundance of the different elements in the ash.
| S as SO3 | 7.64 per cent. (much more in sulphur bacteria) |
| P as P2O5 | 18.14 per cent. to 73.94 per cent. |
| Cl | 2.29 per cent. to 73.94 per cent. |
| K as K2O | 11.1 per cent. to 25.59 per cent. |
| Ca as CaO | 12.64 per cent. to 14.0 per cent. |
| Mg as MgO | 0.7 per cent. to 11.55 per cent. |
| Fe as Fe2O3 | 1.0 per cent. to 8.15 per cent. (iron bacteria) |
| Mn | traces |
As to the form in which the last six elements in the table exist in the cell, little is known. The sulphur and phosphorus are essential constituents of various proteins. The high percentage of phosphorus points to nuclein compounds as its probable source.
The carbon and nitrogen, together with most of the hydrogen and oxygen not united as water, make up the great variety of organic compounds which compose the main substances in the bacterial cell.