W. T. (William Thompson) Sedgwick.

An introduction to general biology online

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plus some such compound of nitrogen as annnonium tartrate
(C^H/NIIJ^Oe). Upon a much less complex organic compound
of nitrogen such as a nitrate it cannot thri\e, thus showing its
inferiority in constructive power to Protococcus and all green
plants, on the one hand, and its sui^eriority to Avxvha and all
animals, on the other.

Pasteur's fluid, composed of water and salts, amoii"; wliich is ammonium
tartrate (above), will suffice to support yeast. It will sui)port a much more
vigorous growth if sugar be added to it. But if ammonium nitrate is sub-
stituted for ammonium tartrate yeast will refuse to grow in the fluid.


Yeast is a Plant. The superior constructive faculty of yeast,
just described, separates it fiindamentallv from all animals in
respect to its physiology, and allies it closely to all plants. Its
inferiority to the chlorophyll-bearing plants or parts of plants, on
the otlier hand, in no wise separates it fundamentally from
plants ; for it must not be forgotten that the power, even of
plant- cells to utilize mineral matters as raw materials and from
them to manufacture foods like starch, ordinarily resides exclu-
sively in the chlorophyll bodies, and is operative only in the
presence of light. It follows, therefore, that most of the cells,
even of the so-called green plants, and a considerable portion of
the contents of the so-called green cells, must be destitute of
this synthetic power. Considerations of this kind show how
exceedingly localized and special the starch-making function is,
even in the ' ' green ' ' plants ; and yeast probably compares very
favorably in its synthetic powers with many of the colorless cells
of such plants, or even with the colorless protoplasmic portions
of chromatophore-bearing cells.

But yeast is vegetal rather than animal, morphologically as
well as physiologically. Its structure more nearly resembles
that of some undoubted plants (fungi) than any animal. Its
wall is composed of a variety of cellulose, called fungus-cellulose ;
and cellulose, though occasionally occurring in animal structures,
is, broadly speaking, a vegetal compound. Finally, in its
methods of reproduction by budding, and by spores, yeast is
allied rather to plants than animals.

Top Yeast. Bottom Yeast. In the process of brewing two well-
marked varieties of yeast occur, known as " top " and " bottom " yeast.
The former is used in the making of English ale, stout, and porter ; the
latter in the making of German or " lager " beer. The top yeast is culti-
vated at the ordinary summer temperature of a room, without special at-
tention to temperature ; the latter in rooms artificially cooled so that even
in summer, icicles often hang from the walls. The two yeasts also show
obvious differences in form, size, and structure ; and how much they must
differ in their function is plain from the very different products to which
they give rise.

Wild Yeasts, Besides the commercial or cultivated yeasts there are
also wild yeasts, and to them are due in the main the fermentations of
apple-juice, of grape-juice, and other fruit juices. A drop of sweet cider
shows under the microscope a good example of one of these species ; and
Pasteur long ago proved that the outer skins of ripe grapes and other fruits


are apt to harbor yeast-cells in the dust which lodges upon them. More
recently it has been shown that wild yeasts often live under apple-trees-
upon the surface of the earth. In a dry time the wind easily lifts the dust
containing them and conveys them over great distances (cf. Amwha,
Infusoria, etc.). The domesticated yeasts of to-day are probably the de-
scendants of similar wild yeasts.

Red Yeast. One of the finest of the wild yeasts is the so-called "red
yeast," which is furthermore very easy to study. Red yeast, and many
others not red, grow luxuriantly upon a jelly, made by thickening beer-
wort with common gelatine. In this way "pure" cultures — that is, cul-
tures free from other species of yeasts, or bacteria, and consisting of one
kind only — can be easily made and studied. The microscope shows that
the cells of red yeast, which form red dots upon such jelly, are not them-
selves colored, but the pigment appears to lie between the cells, as in the
case of the " miracle germ " {Bacillus prodigiosus).

Fermentation. To the processes where yeast is employed to-
produce chemical changes in various domestic, agricultural, and
industrial operations the term fermentation.^ or more often
alcoholic fermentation.^ is applied. In the '' raising" of bread
or cake, in brewing, cider-making, etc., yeast acting upon
sugar produces from it an abundance of alcohol and carbon
dioxide. Both products are sought for in brewing, and carbon
dioxide is especially desired in bread-making.

But alcoholic fermentation is only one example of a large
class, and yeast is only one of many ferments. We may, there-
fore, postpone further consideration of fermentation to the next

Related Forms. It has been shown by the researches of Hansen that
ordinary commercial yeast is seldom one single species, as was formerly
supposed, but rather a mixture of several species. It is therefore no
longer safe to speak of commercial yeast as Saccliaromyces cerevisiop, unless
careful examination by the modern methods has shown it to be such ; and
to determine what species exist in any particular specimen is often a labori-
ous and difficult matter.

Inasmuch as the natural position of yeast in the vegetal kingdc^m is
not established beyond all doubt, it is impossible to state precisely what
are its near relatives. There are numerous unicellular colorless plants, but
they are not necessarily closely related to yeast ; and the student must not
conclude for plants any more than for animals that because an organism
unicellular it is necessarily at the very bottom of the scale of life.



C. Bacteria.


The smallest, and the most numerous, of all living things are
the bacteria. Bacteria occur almost everywhere : they are lifted
into the atmosphere as dust particles, in it they float and with its
currents they are driven about; water — both fresh and salt —
often contains large numbers of them ; and the upj^er layers of
the soil teem with them. But they are most abundant in liquids
containing dissolved organic matters, especially such as have stood
for a time — for example, stale milk and sewage, these fluids
often containing millions of individual bacteria in a single cubic

In respect to their abundance in the surface layers of the
earth (one gram of fertile soil often containing a million or more),
and the work which they do there in producing the oxidation of
organic matters and changes in the composition of the soil, bac-
teria may well be compared with earthworms (cf . p. 42). They
are also of much general interest because some are what are
known as ' ' disease-germs. ' ' Most bacteria, however, are not
jparasitie^ hvX saprophytic^ i.e., live upon dead organic matters,
and therefore are not merely harmless, but positive]y usef-ul in
rendering back to the inorganic world useless organic matters.
Some species such as the vinegar bacteria are commercially

In systematic botany bacteria constitute a well-defined group,
the Schizomycetes {fissio7i-fnngi)^ their near allies being the
Cyanoj)hyce(B or " blue -green algae."

Morphology. Under the microscope bacteria appear as
minute rods {Bacilli) (Fig. 98), balls {Cocci) (Fig. 100), or spirals
{Spirilla) (Fig. 104), sometimes at rest, but often, at least in
the case of the rods and spirals, in active motion. Little or no




structure can be made out in them by the beginner, to whom
they usually appear at first sight like pale, translucent or watery
bits of protoplasm. Investigation has shown, however, that they
possess a cell-wall (probably composed of cellulose) and a ikju-
homogeneous protoplasm. Unlike Protococcus^ but like yeast-
cells, the cells of bacteria contain no chlorophyll. Nuclear mat-

Fia. 98. — Bacillus Megaterium=
Rods (unstained) in various
aggregations as commonly seen
with a high powor after their
cultivation in bouillon and
while rapidly Rowing and mul-
tiplying by transverse divi-

Fig. 9^. — Bacilli from.
Ha y I ifimo n (t/ nsta in-
ed). The filaments at
the left in a condition
of active vegetation.
The middle filament
forming spores. The
filament to the right
contains five spores
enclosed in otherwise
empty cells, the walls
of which bulge, proba-
bly from the absorp-
tion of water.

ter is present, either scattered about, or, if the views of Biitschli
be accepted, composing most of the protoplasmic body itself.
Many bacteria bear appendages in the shape of tiagella or
cilia; but these can only be demonstrated in special cases, and
by special methods. They are believed to be locomotor organs,
and in some cases have been seen in active motion (Fig. 103).


The minuteness of bacteria is extraordinary. Many bacilli are
not more than .005 mm. (-g-yVo inch) in length or more than .001
mm. {yt^wq iiich) in breadth. Some are very much smaller.

Most bacteria are at some time free forms ; but like other
unicellular organisms many of them have the power to pass
from a free-swimming {swariniiig) into a quiescent {resting)
condition. In the latter some undergo a peculiar change, in
which the cell -wall becomes mucilaginous, and by the aggrega-
tion of numerous indiyiduals or by repeated division lumjDs of
jelly-like consistency {zoogloea) arise. If the jelly mass takes
the shape of a sheet or membranous skin (as haj)23ens in the
mother- of -vinegar), it is sometimes described as Mycoderma
{fungus-skin) (Fig. 102).

Reproduction. The bacteria increase in numbers solely by
transverse division. Growth takes place and is followed by trans-
verse division of the original cell, usually into halves. Each half
then likewise grows and divides in its turn. In this way multi-
phcation may go on in geometrical progression, and with almost
incredible rapidity. It has been stated that such repeated divi-
sions may follow only an hour apart, and on this basis it is easy
to compute the enormous numbers to which a single cell may
give rise in a single day.

If separation after division is complete, strictly unicellular
forms arise. If actual separation is postponed, long rods, chains,

or plates (in the case of cocci)


'••f •••"y



may appear. Different names
are given to the resulting forms.
Streptococcus is a moniliform
or necklace-like arrangement;
Stajyhylococcus^ single cocci ;
, ^ Dijjloccoccus, cocci in jDairs;
(^ ^v-^ Lejytothrix^ a filament of
/ ] ^\" — ' bacilli; Sarcina, a plate of
O (3 <20cci resembling a card of bis-
cuit, or two or more cards

Fig. 100— Micrococci Fig. 101.— Short
(unstained) from hay Bacilli (un- SUperpOSed ; CtC. , CtC.

infusion. stained) from Spores. Soiiie bacteria pro-

hay infusion. ^

duce so-called sjpores [endo-
sjpo^^es) in the following way : The contents of the cell






Fig. 102.— The Mother-of-
Vinegar. The edge of a
film of zoogloea of mother-
of-vin?gar as it appears
under a high power. The
bajteria are seen imbedded
in the jelly which they
have secreted.

withdraw from the wall and condense into a (usually oval)
mass at one end of the cell, leavino^ the rest of it empty
It is at this time that the cell-wall
is best seen. The condensed mass
now becomes dark and opaque, appa-
rently from the deposit upon itself of a
greatly thickened and peculiar wall; it
refuses to absorb stains which the origi-
nal cell would have taken, and becomes
exceedingly resistant to extremes of
heat, cold, and dryness (Fig. 105). To
these spores the Germans give the
excellent term Dauersporen, i.e.,

often called

resting spores. When brought under
favorable conditions, these sprout
and, the ordinary bacterium cell
having been produced, growth and
fission proceed as before. Obviously
these spores are very different in
function from those of Pteris (p.
130), since they are protective
merely, and not rej^roductive. They
correspond, doubtless, to that phase
of animal life which is known as the
''encysted" state. Another mode
of spore-formation in bacteria is that
known as the production of arthro-
spores^ in which a long slender cell
mav become constricted and detach
daughter-cells from one or both ends.
This is obviously a sjiccial case of
Fig. ms.-Ciiiated Bacteria. The unequal ccll-division, but if it exists

c1?r;"^:o:''fspecfren'p"I at all (which has been a„uhted) it
by s. c. Keith, Jr. Drawn by J. H. clearly approaches agamogenesis


in such forms as Pteris.

Physiology. Income;, Metabolism, and Outgo. The bacteria


show a surprising diversity in the precise conditions of their
nutrition, and it is tlierefore difficult to make for them a
satisfactory general statement. As a group, however, and dis-
regarding for the moment certain important exceptions, they are
to be regarded as colorless plants living for the most part upon
complex organic compounds from which they derive their in-
come of matter and energy and wliich they decompose into

simpler compounds poorer in poten-
:^: tial energy. In so doing they
i^l bring about certain chemical
:/: changes in the substances upon
>.V: which they act which are of the
vy. highest theoretical interest, and
■^ sometimes of great practical im-
% portance. Perhaps the most pecul-
■i: iar feature of the physiology of
% bacteria is the fact that while tliey
;i:' are themselves individuallv invisi-

• ..■ «/

^;v;:^:•.:.^>:•.V■:^^•:••v.•:vv/-•;:•::v;::^■^^ ble, they collectively produce very
Firiw!-spirnium'nnduia^ couspicuous and important changes

bacteria deeply stained. Drawn yh their environment. For CXaui-
from the first photographic repre- . i , • .

sentation of bacteria ever pub- ple, Vinegar bacteria act upon
lished, viz., that of Robert Koch, alcohol (in cidcr, etc.) and by a

in Cohn's Beitmye, 1876.) \ ... i i

process oi oxidation slowly convert
it into acetic acid and water, thus : —

C,H.O + O, = C,H.O, + H,0.

Here it is not the bacteria that are most conspicuous, but the
effect which they produce. It is clear that the alcohol can be
only one factor in the nutriment of the organism, because it
contains no nitrogen, and the above reaction cannot represent
more than a phase in the nutrition of the bacterium. That this
is indeed the case is proved by the fact that if the conditions be
somewhat changed the same bacteria may go further and convert
the acetic acid itself into carbonic acid and water : —

C,H,0, + O, = 2C0, + 2H,0.

Chemical changes of this kind in which the effect upon the en-



vironment is more conspicuous than, and out of all proportion to,

the change in the agent are in some cases known as ferinen-

tations^ and the agent effecting the change is described as a

ferment. Some ferments are organized or living^ and some are

Fig. 105.— Bacillus megaterium (x 600). Spore formation and germination. .4,
a pair of rods forming spores, about 3 o'clock p.m. B, the same about an hour
later. C, one hour later still. The spores in C were mature by evening ; the one
apparently begun in the third upper cell of A and B disappeared ; the cells in
which did not contain spores were dead by 9 p.m. D, a five-celled rod with three
ripe spores, placed in a nutrient solution, after drying for several days, at 12.30,
P.M. E, the same specimen about 1.30 p.m. F, the same about 4 p.m. G, a pair of
ordinary rods in active vegetation and motion. (After De Bary.)

unorganized or lifeless. Of the former the vinegar bacterium
and yeast are good examples. Of the latter the digestive fer-
ments, like pepsin^ jptyalin., and trypsin^ and certain vegetal
ferments, like diastase of malt are familiar instances.

As a rule the bacteria seem to prefer neutral or slightly
alkaline nitrogenous foods. They therefore decompose more
readily meats, milk, and substances (such as beef-tea) made of
animal matters; less readily acid fruits, timber, etc. If in the
course of their activity they decompose meats, or fish, eggs, etc.,
with the production of evil-smelling gases or putrid odors, the
process is known 2i^ piotref action. Rarely, bacteria invade the
animal (or plant) body and act upon the organic matters which
they find there. In such cases disease may result, and the
bacteria concerned are then known as disease germs.

But while bacteria appear to prefer highly organized nitrog-
enous (proteid) food, they are by no means dependent upon it.
Experiments have shown that many species can thrive upon
Pasteur's fluid, a liquid containing only ammonium tartrate and
certain purely inorganic substances; and one bacterium, at least
(the "nitrous"), according to Winogradsky, can thrive upon
ammonium carbonate. If this proves to be true for other spe-
cies, it will show that bacteria can not only obtain their nitrogen
from the inorganic world, but their carbon also. Enough has


been said already to prove tliat the bacteria are plants, for only
plants can live upon inorganic food. But if the ex23eriments
just referred to are correct, bacteria are not only plants, but, in
spite of their lack of chlorophyll, some at least appear to be
able, like green plants to manvfacture their own food out of
the raw materials of the morganic world. The importance of
this fact in studies of the genealogy of organisms is very great,
for we are no longer obliged to suppose all chloroj^hylless plants
to be degenerate forms. They may have been the primitive
forms of life.

As was the case with yeast and Protococcus^ it is extremely
difficult to make any precise statement concerning the income or
outgo of bacteria. It is believed, however, that the income
always includes salts and water, and the outgo C02,H20 and
some nitrogenous compound or, possibly, free (dissolved) nitro-
gen. In more favorable cases the income appears to include
proteids, fats, and carbohydrates or their equivalents. Sugar is
freely used under some circumstances ; and fats (when saj^onified)
and proteids peptonized, or otherwise altered, might readily be
absorbed. It is probable that soluble ferments are excreted by
the bacteria, which dissolve, and make absorbable, solid matters,
such as meat or white of ^^^ ; and if this is true, bacteria exhibit
a kind of external digestion. However this may be, it is certain
that bacteria can live and multiply upon an amount of food ma-
terials so small as almost or quite to elude chemical analysis ; and
it is fair to say that they are among the most delicate of all

It must not be inferred from what has been said above that bacteria are
always oxidizing agents. Broadly speaking and in the long run they are
such, and in this respect they resemble animals. Like the latter they are
unable (because of want of chlorophyll) to utilize solar energy, and there-
fore must obtain their energy by oxidizing their food. Yet under certain
circumstances bacteria act as reducing agents, as, for example, when they
reduce nitrates to ammonia. This action only takes place, however, in
the presence of organic matter, and appears to be merely an incidental
effect, the oxygen of the nitrate being needed for the oxidation of carbon.
What at first sight appears to be an exception, therefore, proves in the end
to be a part of a general law that bacteria, like animals, are oxidizing
agents, are dependent for their energy upon the potential energy of their
foods, and are unable to utilize solar energy (p. 104).


It has recently been shown that many bacteria under circumstances
otherwise favorable are killed by exposure to sunlight.

Related Forms. According to our present ideas of classification the
bacteria form a somewhat isolated group, their nearest relatives being the
slitne-moulds {Myxomycetes) and especially the Mijxobacteria of Thaxter, on
the one hand, and the CyanopJiycece the "blue-green" or "fission" algae
on the other. Neither of these, however, need be considered here.

Why Bacteria are Considered to be Plants. The bacteria were
formerlj regarded as infusorial aninialcules (because they abound
in infusions, and many have the power of active m(jveiiient).
They are still regarded by some as animals. Most ])iologists,
however, regard them as plants, because they can live without
proteid food (which no animal, so far as known, can do), and
because in their method of reproduction and in their growtli-
forrns they more nearly resemble the Cydnojjhycece than they do
any animal. There is also reason to think that their cell- wall is
composed of cellulose.

Bacteria and their Environment. The relations of organisms to tem-
perature and moisture have been more thoroughly studied for the bacteria
than for any other unicellular organisms on account of tlieir bearing upon
modern theories of infectious disease. In general, temperatures above
70° C. are fatal to ordinary bacteria. In general, as is shown by common
experience with the "keeping" of foods in cold storage, bacteria are be-
numbed but not killed by moderate cold. But in special cases, particu-
larly when they are dried slowly, bacteria may withstand even prolonged
boiling or freezing or the action of poisons, so that the removal or destruc-
tion of the last traces of bacterial life is often very difficult.

Sterilization and Pasteurizing. The removal of all traces of living
matter from any substance, and in particular the destruction of all bac-
terial life, is known as sterilization. To free organic substances from the
larger forms of life is a comparatively easy matter; but bacteria are so
minute and so ubiquitous that scarcely anything is normally free from
them, and they are so hardy that it is exceedingly difficult to destroy them
without at the same time destroying the substances which it is desired to
sterilize. They are not normally present in the living tissues of plants or
animals which are sealed against their entrance by skins or epitiielia ; but
after these are broken or cut open (as in wounds) bacteria speedily invade
the tissues. Ordinary earth, as has been said above, teems with bacteria,
which are easily dried and disseminated in dust driven by the wind. What-
ever is in contact, therefore, with the air or exposed to dust or dirt is never
free from bacteria, and meat or milk which in the living animal are nor-
mally sterile, if exposed to the air soon become contaminated with bacteria.
Sterilization (such as is required to preserve canned goods, for example)


may be effected by heat and continued, after cooling, by exclusion of
germ -laden air. Disinfection, which is the destruction of bacterial life by
powerful poisons, is another form of sterilization. Still another is filtra-
tion through media impervious to germs, such as occurs in the well-
known clay, or porcelain, water-filters. In the last case the pores of the
filter are large enough to allow the water very slowly to pass, but too small
for the bacteria.

In some cases, especially those in which disease-producing (j9(7^^05re???!c)
germs may be present and yet it is impossible to use poisons and undesira-
ble to use a high temperature. Pasteurization is resorted to. This con-
sists in heating to a temperature (usually 75^ C.) high enough to destroy
the particular pathogenic germs supposed to be present, but not high
enough to alter the digestibility or other valuable properties of the liquid

in quostion.

For the medical, economic, and sanitary aspects of problems relating
to the Imcteria, reference must be had to the numerous treatises upon
Bacteriology, perhaps the youngest, and certainly one of the most impor-
tant, of the biological sciences.

J i



If a wisp of liaj is put into a beaker of water and the mix-
ture allowed to stand in a warm place there is soon formed what
is known as a hay infusion. Microscopical examination of a
drop of the liquid at the end of the first hour or two reveals
little or nothing, and if the beaker be held up to the liglit the
liquid appears clear and bright. But after some hours a marked

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Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 17 of 20)