PHYSIOLOGICAL ACTIVITIES (CONTINUED).
PRODUCTION OF ACIDS.
The production of organic acids has been sufficiently discussed in
preceding chapters. It should be noted that not only these in great
variety are produced by bacteria but that under certain conditions
mineral acids, such as nitric, sulphuric and phosphoric may be formed
(see Oxidation, p. 114). Acid production is of great value in the
identification of bacteria in dairy and soil work and in connection
with certain types of pathogenic bacteria.
GAS PRODUCTION.
It will be sufficient merely to enumerate collectively the various
gases mentioned in preceding paragraphs and to state that those
commonly observed in the study of pathogenic bacteria are the first
six mentioned. Most of them come in in dairy work either in the study
of bacteria causing milk and cheese “failures” or as affecting the
flavors of butter or cheese. In the study of soil organisms, any or all
of them are liable to be of importance. The gases are: CO₂, H, CH₄, N,
NH₃, H₂S, gaseous mercaptans, gaseous ptomaines, volatile fatty acids,
ethereal salts or esters and others, both of pleasant and of foul odor,
but of unknown composition.
PRODUCTION OF ESTERS.
The production of esters, as mentioned in Chapters IX and X, of various
alcohols and aldehydes are activities which are sometimes of value in
the study of bacteria, but need not be further discussed.
PRODUCTION OF “AROMATIC” COMPOUNDS.
These have been mentioned in discussing the putrefaction of proteins,
as indol, skatol, phenol and various cresols. Of these only the first
is ordinarily tested for in the study of bacteria, though others of the
group become of value in certain special cases.
[Illustration: FIG. 72.--Culture of phosphorescent bacteria in an
Ehrlenmeyer flask photographed by their own light. Time of exposure
twelve hours. (Molisch, from Lafar.)]
PHOSPHORESCENCE OR PHOTOGENESIS.
This is a most interesting phenomenon associated with the growth of
some bacteria. The “fox fire” frequently seen on decaying wood which
is covered with a slimy deposit is most commonly due to bacteria,
though also to other fungi. Phosphorescent bacteria are very common
in sea water, hence they are frequently found on various sea foods,
especially when these are allowed to decompose, such as fish, oysters,
clams, etc. The light is due to the conversion of the energy of
unknown easily oxidizable compounds directly into _visible_ radiant
energy through oxidation without appreciable quantities of heat.
The light produced may be sufficient to tell the time on a watch in
absolute darkness, and also to photograph the growths with their own
light, but only after several hours’ exposure (Fig. 72). None of the
phosphorescent bacteria so far discovered produce disease in the higher
animals or man.
PRODUCTION OF PIGMENT OR CHROMOGENESIS.
One of the most striking results of bacterial activity is this
phenomenon. The particular color which results may be almost any one
throughout the range of the spectrum, though shades of yellow and of
red are of more frequent occurrence.
In the red sulphur bacteria the “bacteriopurpurin” which they contain
appears to serve as a true respiratory pigment in a manner similar to
the chlorophyl in green plants, except that these bacteria oxidize
H₂S in the light as a source of energy instead of splitting up CO₂.
The red pigment produced by certain bacteria has been shown to have
a capacity for combining with O resembling that of hemoglobin, and
some investigators have believed that such bacteria do store O in this
way for use when the supply is diminished. With these few exceptions
the pigments seem to be merely by-products of cell activity which are
colored and have no known function.
The red sulphur bacteria above mentioned and one or two other kinds
retain the pigments formed within the cell. Such bacteria are called
_chromophoric_ as distinguished from the _chromoparic_ bacteria whose
pigment lies outside the cell.
The chemical composition of no bacterial pigment has been determined
up to the present. Some are soluble in water, as shown by the
discoloration of the substances on which they grow. Others are not
soluble in water but are in alcohol, or in some of the fat solvents
as ether, chloroform, benzol, etc. These latter are probably closely
related to the _lipochromes_ or “fat colors” of higher plants and
animals. Attempts have been made to render the production of pigments
a still more reliable means of identification of species of bacteria
through a careful examination of the spectra of their solutions, but
such study has not as yet led to any valuable practical results.
The production of pigment depends on the same general factors which
determine the growth of the organism but does _not necessarily run
parallel_ with these. It is especially influenced by the oxygen
supply (only a very few organisms are known which produce pigment
anaërobically--_Spirillum rubrum_ is one); by the presence of
certain food substances (starch, as in potato, for many bacteria
producing yellow and red colors; certain mineral salts, as
phosphates and sulphates, for others); by the temperature (many
bacteria cease to produce color at all if grown at body temperature,
37°--_Erythrobacillus prodigiosus_--or if grown for a longer time at
temperatures a few degrees higher).
REDUCING ACTIONS.
Reduction of nitrates to nitrites or to ammonia or even to free
nitrogen is brought about by a great many different kinds of bacteria.
In many instances this phenomenon is due to a lack of free oxygen,
which is obtained by the bacteria from these easily reducible salts.
In other cases a portion of the nitrogen is removed to be used as food
material in the building up of new protein in the bacterial cell. This
latter use of the nitrogen of nitrates by bacteria might theoretically
result in considerable loss of “available nitrogen” in the soil as has
actually been shown in a few experiments. The reduction of nitrates as
above mentioned would also diminish this supply, but probably neither
of these results has any very great practical effect on soil fertility.
The building up of protein from these mineral salts by bacteria in the
intestines of herbivorous animals has been suggested by Armsby as a
considerable source of nitrogenous food, and this suggestion appears
possible.
The liberation of nitrogen from nitrates or nitrites, either as free
nitrogen or as ammonia, is spoken of as “dentrification,” though this
term was formerly applied to such liberations, from compounds of
nitrogen generally even from proteins.
Certain bacteria may also reduce sulphates and other sulphur compounds
to H₂S, a phenomenon frequently observed in sewage and likewise of
importance in the soil. It is possible that phosphates may be similarly
reduced.[14] Further and more careful study of the reducing actions of
bacteria is needed.
OXIDATION.
As has been stated in discussing the respiration of bacteria (Chapter
VIII) most of these organisms gain their energy through the oxidation
of carbon in various forms, chiefly organic, so that CO₂ is a product
of the activity of nearly all bacteria. Some few oxidize CO to CO₂,
others CH₄ and other paraffins to CO₂ for this purpose. One class of
bacteria even oxidizes H in small amounts for its energy and uses the
carbon dioxide of the air or traces of organic carbon in the air as a
source of carbon for “building” purposes.
One of the familiar oxidations of organic carbon is that of the acetic
acid bacteria in the making of vinegar. These oxidize the alcohol
which results from the action of yeast to acetic acid according to the
formula CH₃CH₂OH + O₂ = CH₃COOH + H₂O (see Fig. 67).
Of the various phenomena of oxidation due to bacteria, the formation of
nitrites and nitrates has the greatest practical importance, since it
is by this means that the ammonia which results from the decomposition
of animal and vegetable tissue and waste products is again rendered
available to green plants as food in the form of nitrates. Practically
all the nitrates found in nature, sometimes in large quantities, are
formed in this way. There are two distinct kinds of bacteria involved.
One, the nitrous bacteria, oxidizes the ammonia to nitrous acid
which forms nitrites with bases, and the other, the nitric bacteria,
oxidizes the nitrous to nitric acid, giving nitrates with bases. A
striking peculiarity of these two classes of organisms is that they
may live entirely on inorganic food materials, are proto-autotrophic,
prototrophic for oxygen (aërobic) and autotrophic for the other
elements. Their carbon is derived from CO₂ or carbonates. The
importance of such organisms in keeping up the supply of nitrates in
the soil can scarcely be overestimated.
[Illustration: FIG. 73.--Sprinkling filters of the Columbus
sewage-disposal plant--devices which provide a good supply of oxygen
for the bacteria that oxidize the organic matter in the sewage.]
The oxidation of the H₂S, which is formed in the putrefaction
of proteins, to free S by the sulphur bacteria and the further
oxidation of this free S to sulphuric acid, and of the phosphorus, so
characteristic of the nucleins, to phosphoric acid have been referred
to. These activities of bacteria are of great value in the soil.
Doubtless the commercial “phosphate rock” owes its origin to similar
bacterial action in ages past.
The oxidation of H₂S to free S may be an explanation of the origin of
the great deposits of sulphur which are found in Louisiana and along
the Gulf coast. These deposits occur in the same general regions as
natural gas and oil. The sulphur might have been derived from the same
organic material carried down by the Mississippi which yielded the oil
and gas.[15]
A purposeful utilization of the oxidizing power of bacteria is in
“contact beds,” “sprinkling filters” and “aërated sludge tanks” in
sewage disposal works. In these instances the sewage is thoroughly
mixed with air and brought in contact with large amounts of porous
material so as to expose an extensive surface for oxidation (Fig. 73).
[Illustration: FIG. 74.--One of the University hot beds.]
PRODUCTION OF HEAT.
A direct result of the oxidizing action of bacteria is the production
of heat. Under most conditions of bacterial growth this heat is not
appreciable. It may become well marked. The “heating” of manure is one
of the commonest illustrations. The temperature in such cases may reach
70°. The heating of hay and other green materials is due chiefly to
bacterial action. This heating may lead to “spontaneous combustion.”
The high temperatures (60° to 70°) favor the growth of thermophil
bacteria which cause a still further rise. The heat dries out the
material, portions of which are in a state of very fine division due
to the disintegrating action of the organisms. The hot, dry, finely
divided material oxidizes so rapidly on contact with the air that it
ignites.
A practical use of heat production by bacteria is in the making of “hot
beds” for forcing vegetables (Fig. 74).
ABSORPTION OF FREE NITROGEN.
[Illustration: FIG. 75.--Root tubercles on soy bean. × 3/7.]
This is likewise one of the most important practical activities of
certain types of bacteria present in the soil. The ability of plants
of the legume family to enrich the soil has been known and taken
advantage of for centuries, but it is only about thirty years since
it was demonstrated that this property is due to bacteria. These
plants, and several other kinds as well, have on their roots larger or
smaller nodules (Fig. 75) spoken of as “root tubercles” which are at
certain stages filled with bacteria. When conditions are favorable,
these bacteria live in symbiotic relationship with the plant tissues,
receiving carbonaceous and other food material from them and in return
furnishing nitrogenous compounds to the plant. This nitrogenous
material is built up from free nitrogen absorbed from the air by the
bacteria. The utilization of this peculiar property through the proper
cultivation of clover, alfalfa, soy beans and other legumes is one of
the best ways of building up and maintaining soil fertility in so far
as the nitrogen is concerned. The technical name of these bacteria is
_Rhizobium leguminosarum_.
[Illustration: FIG. 76.--Free-living nitrogen absorbing bacteria
“Azotobacter.” Note their large size as compared with other bacteria
shown in this book.]
There are also types of “free-living,” as distinguished from these
symbiotic, bacteria which absorb the free nitrogen of the air and aid
materially in keeping up this supply under natural conditions. One
of the most important of these types is the aërobic “Azotobacter”
(Fig. 76), while another is the anaërobic _Clostridium pasteurianum_.
The nitrogen which is absorbed is built up into the protein material
of the cell body and this latter must in all probability be “worked
over” by various types of decomposition bacteria and by the nitrous
and nitric organisms and be converted into utilizable nitrates just
as other protein material is, as has been discussed in Chapter X. At
any rate there is as yet no definite knowledge of any other method of
transformation. Up to the present no intentional practical utilization
of this valuable property of these free-living forms has been made.
=Nitrogen Nutrition of Green Plants.=--It is the belief of botanists
that green plants obtain their nitrogen chiefly in the form of
nitrates, though ammonium salts may be utilized to some extent by
certain plants at least. Exceptions to this general rule are those
plants provided with root tubercles (and the bog plants and others
which have mycorrhiza?). These plants obtain their nitrogen in the
form of organic compounds made for them by the bacteria growing in
the tubercles. That nitrogen circulates throughout the structure of
plants in organic combination is certain. There does not appear to
be any reason why similar compounds which are soluble and diffusible
(amino-acids?) should not be taken up through the roots of plants and
utilized as such. _It seems to the author that this is very probably
the case._ Arguments in favor of this view are: (1) The nitrogen
nutrition of leguminous and other plants with root nodules. (2) The
close symbiosis between “Azotobacter” and similar nitrogen-absorbing
bacteria and many species of algæ in sea water at least. (3) The
vigorous growth of plants in soils very rich in organic matter, which
inhibits the production of nitrates by the nitrous-nitric bacteria
when grown in culture, and possibly (?) in the soil, so that nitrates
may not account for the vigorous growth. (4) The effect of nitrate
fertilizers is to add an amount of nitrogen to the crop much in excess
of the amount added as nitrate. (5) The most fertile soils contain
the largest numbers of bacteria. The doctrine that nitrates furnish
the only nitrogen to plants was established before the activities of
bacteria in the soil were suspected, and, so far as the author is
aware, has not been supported by experiments under conditions rigidly
controlled as to sterility.
It would seem that one of the chief functions of soil bacteria is to
prepare soluble organic compounds of nitrogen for the use of green
plants and thus to make a “short cut” in the nitrogen cycle (p. 107),
as now believed in, direct from the “decomposition bacteria” to green
plants.
Experiments have been made by different observers in growing seedling
plants of various kinds in water culture with one or in some cases
several of the amino-acids as sources of nitrogen. Most of these
experiments were disappointing. Plant proteins are not so different
from animal proteins, or plant protoplasm (apart from the chlorophyl
portions of plants) from animal protoplasm as to lead one to suppose
that it could be built up from one or two amino-acids any more than
animal protoplasm can. The author is strongly convinced that this
subject should be thoroughly investigated. It will require careful
experimentation and perhaps rather large funds to provide the amounts
of amino-acids that would probably be needed, but might result in a
decided change in our ideas of soil fertility, and especially in the
use of nitrogen fertilizers.