PHYSIOLOGICAL ACTIVITIES (CONTINUED).
PUTREFACTION OF PROTEINS.
The word “_putrefaction_” is now restricted to the action of bacteria
on the _complex nitrogen-containing substances_, proteins, and their
immediate derivatives. The process is usually accompanied by the
development of foul odors.
Bacteria make use of proteins chiefly as a source of nitrogen, but also
as a source of carbon and other elements. Proteins contain nitrogen,
carbon, hydrogen, oxygen, sulphur and frequently phosphorus. Some of
the metals--potassium, sodium, calcium, magnesium, iron and manganese
and the non-metal chlorine--are nearly always associated with them more
or less intimately. Since these bodies are the most complex of natural
chemical substances it follows that the breaking up of the molecule to
secure a part of the nitrogen gives rise to a great variety of products.
There are marked differences among bacteria in their ability to
attack this class of compounds. Some can break up the most complex
natural proteins such as albumins, globulins, glyco-, chromo-, and
nucleoproteins, nucleins and albuminoid derivatives like gelatin. The
term _saprogenic_ (σαπρος = rotten) is sometimes applied to bacteria
which have this power. These proteins are large-moleculed and not
diffusible, so that the first splitting up that they undergo must occur
outside the bacterial cell. The products of this first splitting may
diffuse into the cell and be utilized there. The bacteria of this class
attack not only these proteins in the natural state or in solution,
but also in the coagulated state. The coagulum becomes softened and
finally changed into a liquid condition. The process when applied to
the casein of milk is usually called “digestion,” also when coagulated
blood serum is acted on. In the latter case the serum is more commonly
said to be “liquefied” as is the case when gelatin is the substance
changed. Most of these bacteria have also the property of coagulating
or curdling milk in an alkaline medium, and then digesting the curd.
A second class of bacteria has no effect on the complex proteins just
mentioned but readily attacks the products of their first splitting,
_i.e._, the proteoses, peptones, polypeptids and amino-acids. They are
sometimes called _saprophilic_ bacteria.
Other bacteria derive their nitrogen from some of the products of the
first two groups, and still further break down the complex protein
molecule. Under normal conditions these various kinds of bacteria
all occur together and thus mutually assist one another in what is
equivalent to a symbiosis or rather a metabiosis, a “successive
existence,” one set living on the products of the other. The result
is the complete splitting up of the complete protein molecule. A part
of the nitrogen is built up into the bodies of the bacteria which are
using it as food. A part is finally liberated as _free nitrogen_ or as
_ammonia_ after having undergone a series of transformations many of
which are still undetermined.
One class of compounds formed received at one time much attention
because they were supposed to be responsible for a great deal of
illness. These are the “ptomaines,” basic nitrogen compounds of
definite composition--amines--some few of which are poisonous, most of
them not. The basic character of ptomaines may be understood if they be
regarded as made up of one or more molecules of ammonia in which the
hydrogen has been replaced by alkyl or other radicals. Thus ammonia
(NH₃) may be represented as
/H
/
N--H
\
\H.
The simplest ptomaine is
/CH₃
/
N--H
\
\H,
in which one H is replaced by methyl, methylamine, a gaseous ptomaine.
With two hydrogens replaced by methyl,
/CH₃
/
N--CH₃
\
\H,
dimethylamine, also a gas at ordinary temperature, is formed.
Trimethylamine,
/CH₃
/
N--CH₃
\
\CH₃,
a liquid, results when three hydrogens are similarly replaced.
All three of these occur in herring brine and are responsible
for the characteristic odor of this material. Putrescin and
cadaverin--tetramethylene--diamine, and pentamethylenediamine
respectively--occur generally in decomposing flesh, hence the names.
They are only slightly poisonous. One of the highly poisonous ptomaines
is neurin C₅H₁₃NO or C₂H₃N(CH₃)₃OH = trimethyl-vinyl ammonium
hydroxide. This is a stronger base than ammonia, liberating it from
its salts. Numerous other ptomaines have been isolated and described.
These bodies were considered for a long time to be the cause of various
kinds of “meat poisoning,” “ice cream poisoning,” “cheese poisoning,”
etc. It is true that they may sometimes cause these conditions, but
they are very much rarer than the laity generally believe. Most of
the “meat poisonings” in America are due, not to ptomaines, but to
infections with certain bacilli of the _Bacterium enteritidis_ group.
Occasionally a case of poisoning by the true toxin (see Chapter XII) of
_Clostridium botulinum_ occurs, and in recent years has become entirely
too common due to insufficient heating of canned goods. _The boiling of
such material will destroy this toxin. The safest rule to follow is not
to eat any canned material that shows any departure from the normal in
flavor, taste or consistency._
As ptomaines result from the putrefaction of proteins, so they are
still further decomposed by bacteria and eventually the nitrogen is
liberated either as free nitrogen or as ammonia.
Another series of products are the so-called aromatic compounds--phenol
(carbolic acid), various cresols, also indol and skatol or methyl indol
(these two are largely responsible for the characteristic odor of human
feces). All of these nitrogen compounds are attacked by bacteria and
the nitrogen is eventually liberated, so far as it is not locked up in
the bodies of the bacteria, as free nitrogen or as ammonia.
The carbon which occurs in proteins accompanies the nitrogen in many of
the above products, but also appears in nitrogen-free organic acids,
aldehydes and alcohols which are all eventually split up, so that the
carbon is changed to carbon dioxide or in the absence of oxygen partly
to marsh gas.
The intermediate changes which the sulphur in proteins undergoes are
not known, but it is liberated as sulphuretted hydrogen (H₂S) or as
various mercaptans (all foul-smelling), or is partially oxidized to
sulphuric acid. Some of the H₂S and the sulphur of the mercaptans
are oxidized by the sulphur bacteria to free sulphur and finally to
sulphuric acid.
Phosphorus is present especially in the nucleoproteins and nucleins.
Just what the intermediate stages are, on whether there are any, so
far as the phosphorus is concerned, in the splitting up of nucleic
acid by bacterial action is not determined. The phosphorus may occur
as phosphoric acid in such decompositions, or when the conditions are
anaërobic, as phosphine (PH₃), which burns spontaneously in the air to
phosphorus pentoxide (P₂O₅), and water.[13]
The hydrogen in proteins appears in the forms above indicated: H₄C,
H₃N, H₃P, H₂S, H₂O and as free H. The oxygen as CO₂ and H₂O.
In the breaking down of the complex protein molecule even by a single
kind of bacterium there is not a perfect descending scale of complexity
as might be supposed from the statement that there result proteoses,
peptones, polypeptids, amino-acids. These substances do result, but at
the time of their formation simpler ones are formed also, even CO₂,
NH₃ and H₂S. It appears that the entire molecule is shattered in such
a way that less complex proteins are formed from the major part, while
a minor portion breaks up completely to the simplest combinations
possible. A more complete knowledge of these decompositions will aid
in the further unravelling of the structure of proteins. The presence
or absence of free oxygen makes a difference in the end-products, as
has been indicated. There are bacteria which oxidize the ammonia to
nitric acid and the H₂S to sulphuric acid. (See Oxidation, Chapter XI.)
Bacteria which directly oxidize phosphorus compounds to phosphoric
acid have not been described. It does not seem that such are necessary
since this is either split off from nucleic acid or results from the
spontaneous oxidation of phosphine when this is formed under anaërobic
conditions.
Not only are proteins decomposed as above outlined, but also their
waste products, that is, the form in which their nitrogen leaves the
animal body. This is largely urea in mammals, with much hippuric acid
in herbivorous animals and uric acid in birds and reptiles. These
substances yield NH₃, CO₂ and H₂O with a variety of organic acids
as intermediate products in some cases. The strong odor of ammonia
in stables and about manure piles is the everyday evidence of this
decomposition.
Where the putrefaction of proteins occurs in the soil with moderate
amounts of moisture and free access of air a large part of the products
is retained in the soil. Thus the ammonia and carbon dioxide in the
presence of water form ammonium carbonate; the nitric, sulphuric and
phosphoric acids unite with some of the metals which are always present
to form salts. Some of the gases do escape and most where the oxygen
supply is least, since they are not oxidized.
The protein-splitting reactions afford valuable tests in aiding in
the recognition of bacteria. In the study of pathogenic bacteria the
coagulation and digestion of milk, the digestion or liquefaction
of blood serum, the liquefaction of gelatin and the production of
indol and H₂S are those usually tested for. In dairy bacteriology
the coagulation of milk and the digestion of the casein are common
phenomena. Most bacteria which liquefy gelatin also digest blood serum
and coagulate and digest milk, though there are exceptions. In soil
bacteriology the whole range of protein changes is of the greatest
importance.
[Illustration:
+----<----------------------+
| |
| |
v |
_Nuclein of |
animal cells_ |
| |
| |
v |
| |
_Decomposition |
bacteria_ |
| \ |
| \ ^
v \ |
_Unknown _PH₃ |
P oxidizes |
compounds_ spontaneously |
| to_ |
| / |
v / |
_Phosphoric |
acid_ |
| |
| |
v |
_Phosphates |
in the |
soil_ |
| ^
| |
v |
_Green |
plants_ |
| |
| |
v |
_Nuclein of |
plant |
cells_ |
| |
| |
v |
_Animals_----->----------------+
FIG. 68.--Diagram to illustrate the circulation of phosphorus through
the agency of bacteria.]
[Illustration:
_Fats and
various C <---------------------------+
compounds_ |
| |
| |
v |
_Decomposition |
bacteria_ |
| |
| |
v |
_Various |
C |
compounds |
eventually ^
to_ |
| |
| |
v |
_CO₂ <---------------+ |
in | |
the air_<-+ | |
| | | |
| | _Plant respiration_ |
v | | |
_Green | | |
plants_---+ | |
| | |
| | |
v | |
_Carbohydrates, | |
fats and | |
other C compounds_ | |
| | |
| | |
v | |
_Animals_---->_Animal respiration_ |
| |
| |
+---->----------------------------+
FIG. 69.--Diagram to illustrate the circulation of carbon through the
agency of bacteria.]
[Illustration:
+---------------<-----------------+
| |
_Dead animal |
protein_ |
| ^
| |
v |
_Free N taken <-----_Decomposition <-------------+ |
up by free living bacteria_ <----------+ | |
N absorbers and | | | |
root tubercle | | | |
bacteria_ v | | |
| _NH₃ compounds | | |
| in soil_ | | |
| | | | |
| | | | |
| v | | |
| _Nitrite bacteria_ | | |
| | | | |
| | | | |
| v | | |
| _Nitrites_ | | |
| | | | |
| | | | |
| v | | |
| _Nitrate bacteria_ | | |
| | ^ ^ ^
| | | | |
| v | | |
| _Nitrates in soil_ | | |
| | | | |
| | | | |
| v | | |
| _Green plants_ | | |
| | | | |
| | | | |
| v _Dead plant | |
+------->_Plant protein_----> protein_ | |
| | ^
| | |
v _Animal waste, |
_Animals_---------->urea, etc._ |
| |
| |
+-------->------------------------+
FIG. 70.--Diagram to illustrate the circulation of nitrogen through the
agency of bacteria.]
[Illustration:
+--------------<---------------+
| |
| |
v |
_Dead animal |
protein_ |
| |
| |
v |
_Decomposition bacteria_ |
| | |
| | |
v ^ |
_H₂S_ | |
| | |
| | |
v | |
_Sulphur bacteria_ | |
| | |
| | |
v | |
_Free S_ | ^
| | |
| | |
v | |
_Sulphur bacteria_ | |
| | |
| | |
v | |
_Sulphates | |
in the | |
soil_ | |
| | |
| | |
v | |
_Green plants_ ^ |
| | |
| | |
v | |
_Plant protein_----->_Dead plant |
| protein_ |
| |
v |
_Animals_ |
| |
| |
+-------------->---------------+
FIG. 71.--Diagram to illustrate the circulation of sulphur through the
agency of bacteria.]
The three physiological activities already discussed explain how
bacteria break down the chief complex, energy-rich substances
--carbohydrates, fats and proteins which constitute the bulk of the
organic material in the bodies of plants and animals, as well as
the waste products of the latter--into energy-free compounds like
carbon dioxide, water, ammonia, nitric, sulphuric and phosphoric
acids--mineralize them, as is frequently said. By so doing the bacteria
act as the great scavengers of nature removing the dead animal
and vegetable matter of all kinds which but for this action would
accumulate to such an extent that all life, both on land and in the
water, must cease. It is further to be noted that not only is all this
dead organic matter removed; but it is converted into forms which are
again available for plant growth. Carbon dioxide forms the source of
the carbon in all green plants, hence in all animals; the sulphates and
phosphates are likewise taken up by green plants and built up again
into protein compounds; the ammonia is not directly available to green
plants to any large extent but is converted by the nitrifying bacteria
(Chapter XI) into nitrates which is the form in which nitrogen is
assimilated by these higher types. Even the free nitrogen of the air
is taken up by several kinds of bacteria, the symbiotic “root-tubercle
bacteria” of leguminous and other plants, and some free-living forms,
and made available. Hence bacteria are indispensable in nature,
especially in keeping up the circulation of nitrogen. They are also of
great service in the circulation of carbon, sulphur and phosphorus.
Though some few kinds cause disease in man and animals, if it were not
for the saprophytic bacteria above outlined, there could be no animals
and higher plants to acquire these diseases.