CHEMICAL ENVIRONMENT.
REACTION OF MEDIUM.
Most bacteria are very susceptible to changes in the degree of acidity
or alkalinity of the medium in which they grow. Some kinds prefer a
slightly acid reaction, some a slightly alkaline, and some a neutral
(with reference to litmus as indicator). The organism which is the
commonest cause of the souring of milk thrives so well in the acid
medium it produces that it crowds out practically all other kinds,
though its own growth is eventually stopped by too much acid. Acid
soils are usually low in numbers of bacteria and as a consequence
produce poor crops. The disease-producing bacteria as a class grow best
in a medium which is slightly alkaline.
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 „ to 73.94 per cent.
Cl 2.29 „
K as K₂O 11.1 „ to 25.59 „
Ca as CaO 12.64 „ to 14.0 „
Mg as MgO 0.7 „ to 11.55 „
Fe as Fe₂O₃ 1.0 „ to 8.15 „ (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.
It has already been stated that the essential structures in the
bacterial cell are cell wall and protoplasm, including the nuclein.
These differ markedly in chemical composition. It is well known that
the cell walls of green plants consist largely of cellulose and closely
related substances.[7] _True cellulose_ has been recognized in but
very few bacteria. (_Sarcina ventriculi_, Migula; _Mycobacterium
tuberculosis_, Hammerschlag, Dreyfuss, Nishimura; _Bacillus
subtilis_, Dreyfuss; _Acetobacter xylinum_, Brown; _Acetobacter
acidi oxalici_, Banning; and a few others.) It is certainly not an
important constituent of the cell wall in many. On the other hand,
_hemicellulose_ and _gum-like_ substances have been identified in
numerous organisms of this class as important constituents of the cell
wall and of the capsule which is probably an outgrowth from the latter.
Practically always associated with these substances are compounds
containing nitrogen. One of these has been certainly identified as
_chitin_ or a closely similar substance. Chitin is the nitrogenous
substance which enters largely into the composition of the hard parts
of insects, spiders and crustaceans. It is an interesting fact to find
this substance characteristic of these animals in bacteria, as well as
other fungi.
Though it is extremely difficult to separate the cell wall of bacteria
from the cell contents, in the light of our present knowledge it can be
stated that the cell walls are composed of a carbohydrate body closely
related to cellulose, though not true cellulose, probably in close
combination with chitin.
Of the organic constituents of the cell contents the most abundant are
various proteins which ordinarily make up about one-half of the dry
weight of the entire cell. The “Mycoproteid” of Nencki, 1879, and other
earlier workers is deserving of little more than historical interest,
since these substances were certainly very impure and probably
consisted of mixtures of several “proteins” in the more recent sense.
From later studies it seems probable that substances resembling
the albumin of higher forms do not occur in bacteria, at least in
appreciable quantities. Globulin has been reported by Hellmich in an
undetermined bacterium, but is certainly not commonly found. The larger
portion of the protein is of a comparatively simple type, in fact,
consists of protamins most of which are in combination with nucleic
acid as nucleoprotamins. Practically all recent workers find a high
percentage of nuclein, both actually isolated and as indicated by the
amounts of purin bases--xanthin, guanin, adenin--obtained, as well as
by the abundance of phosphorus in the ash, already mentioned. Some of
these nucleins have been shown to have poisonous properties.
Closely related to but not identical with the proteins are the enzymes
and toxins which are formed in the cell and exist there as endo-enzymes
or endo-toxins respectively. These substances will be discussed later
under the heading “Physiological Activities of Bacteria” (Chapter XII).
Carbohydrates are not commonly present in the cell contents, though
glycogen has been observed in a few and a substance staining blue
with iodine in one or two others. This latter substance was at first
considered to be starch “granulose,” but is probably more closely
related to glycogen.
Fats seem to be very generally present. The commoner fats--tri-olein,
tri-palmitin, tri-stearin have been found by many analysts. The
“acid-fast bacteria” are particularly rich in fatty substances,
especially the higher wax-like fats. Lecithins (phosphorized fats) and
cholesterins (not fats but alcohols) have been repeatedly observed and
probably occur in all bacteria as products of katabolism.
Organic acids and esters occur as cell constituents but will be
discussed in connection with their more characteristic occurrences
as products of bacterial activity, as will also pigments which may
likewise be intracellular in some instances.
The following analysis of tubercle bacilli, from de Schweinitz and
Dorset, while not intended as typical for all bacteria, still
illustrates the high percentage of protein compounds which undoubtedly
occurs in most, as well as showing the large amount of fatty substance
in a typical “acid-fast” organism:
{ 8.5 per cent. tuberculinic acid
{ 24.5 „ nucleoprotamin }
In the dried { 23.0 „ neucleoprotein } 55.8 per cent.
organisms { 8.3 „ proteinoid } protein.
{ 26.5 „ fat and wax
{ 9.2 „ ash