When bacteria develop in a liquid in a sufficient quantity, they become visible to the naked eye. They appear either as a slight cloud, or gathered in little masses in the liquid, or forming a pellicle upon its surface, or as a deposit upon the walls of the vessel and upon the objects contained in the liquid. However, we must hasten to say with M. Cohn, that the fact of the absence of all turbidity in a liquid does not exclude the possibility of the presence of bacteria. In liquids more dense than water (serum, lymph, etc.), when the refractive power of these corpuscles is the same as that of the liquid, their presence may not be revealed to the naked eye. We will add that sometimes their color serves to indicate their presence in a liquid, although this color is often very feeble, and can only be perceived when a considerable thickness of the liquid is examined.
If we examine these clouds, these accumulations, these deposits, with the microscope, we see that they are formed of a myriad of little bodies isolated or grouped, globular or linear, gifted or not with motion, sometimes colored. These variations constitute so many characters which require to be studied with some detail.
Form. The bacteria, as understood to-day by most botanists, when considered in their separate state, are of two principal forms, globular bodies, or monads, and bodies more or less filiform, or bacteria properly so called.
The globular bacteria comprise organisms round ed, ovoid, sometimes elongating themselves into a tube (Warming). The Monas crepusculum of Ehrenberg may be taken as a type. This form includes also the Micrococcus of Hallier, the Mi crosporon of Klebs, the round forms of the Amy lobacter of M. Trecul, and perhaps the Microzyma of M. Bechamp. We will see farther on that these are very probably phases of development of the spores of bacteria, properly so called.
The bacteria, not globular, present a greater diversity of form ; they may be straight, undulating, or twisted in a spiral.
The rectilinear bacteria are usually exactly cylindrical throughout their whole extent ; and in this case they form very short cylinders, as in the Bacterium,Cohn, or cylinders of which the length is several times as great as the thickness, as in the Bacteridies (Bacillus ulna Cohn) ; others are swollen in the middle, with their extremities rounded, such as certain forms of Vibrio serpens (Warming) ; others again are fusiform, swollen in the middle and attenuated at the extremities, Bacterium fusiforme (Warming) ; rectilinear bacteria swollen at the two extremities are met during the life of certain species, B. lineola and
B. termo, for example, above all when they are transported to a more favorable medium : this modification usually precedes segmentation ; final ly, one meets sometimes bacteria swollen at one extremity only; the swollen part presents often a clear point and sometimes an evident spore : we shall see later the signification of this peculiarity.
With these claviform bacteria we may include the Bacterium capitalum Dav., the Helobacteria of Billroth, and certain Amylobacter, with heads of the Ficus carica, etc. (Ch. Robin).
The undulating bacteria constitute the Vibrios properly so called (V. Rugula, serpens, etc.). The spiral bacteria of which the turns are more or less elongated are named Spirillum, Spiro chceta, etc. Dimensions. The dimensions of the bacteria oscillate between the most variable limits, but in a general way it may be said that they are the smallest of all microscopic beings. Some of them are situated at the extreme limit of our highest magnifying powers; and their proportions, as to length and thickness, are comprised within the limits of errors of observation.
The globular bacteria are the smallest, and the dimensions of some species are so minute that they cannot be measured directly.
The largest are the Spirillum, which attain a length of 2/15th of a millimetre. Between these two extremes, there are all intermediary sizes possible.
Several authors, considering exclusively this character of dimensions, have divided the monera and the bacteria according to their size. Thus Hoffmann recognizes in addition to the monera, only the microbacteria, the mesobacteria, and the macrobacteria. In the same way Billroth classifies the monads according to their dimensions into micro, meso, megacoccos, and the bacteria into micro, meso, mega bacteria. Finally, Klebs separates the Micrococcos from the Microsporines, which do not differ from them except by their smaller dimensions, both forms being able to pass to the state of bacteria (rods).
Color of Bacteria
The phenomena relating to the color of bacteria have only recently been pointed out.
" But little attention has been given to the color of the bacteria, regarded generally as colorless," said M. de Seynes in 1874 ; and recently M. de Lanessan, " The bacteria are ordinarily quite color less." However, M. Cohn had already insisted upon the globular bacteria chromogenes, or of pig mentary fermentation, and upon the colors pro duced by different monads, which have long since been studied by microscopists.
Upon this subject, let us observe that the bac teria which are colored belong to two very dif ferent groups. First, colored organisms always known as such, but which were not formerly in cluded with the bacteria, as the different monads, which have become the Micrococcus prodigiosus, cyaneus, aurantiacus,Cohn, etc. ; the second group includes the bacteria properly so called, which absorb the coloring matter of vegetables upon which they are fixed as parasites, or of the media in which they live. This is the case with the bac teria observed by M. de Seynes upon the Penicil lium glaucum, and perhaps with the Vibrio syn xanthus and sy?icyam^s,Ehrenb., which give to milk a yellow or blue color according to the species.
We will return to this subject when we speak of the nutrition of the bacteria.
As to the purple-colored monads, they have been especially studied as early as 1838 by Dunal, then by Morren and Ehrenberg, and in our own day by Kay-Lankester, Cohn, Klein, and finally by Warming and Giard. They are found in various media in seawater, in hot sulphur springs, in fresh water containing animal or vegetable mat ter in a state of putrefaction. They appear some times upon bread, meats, and in general upon cooked food placed in a humid atmosphere. The different colors which they present are red, yel low, orange, and blue. It is probably to anal ogous organisms that we must attribute the blue color presented by pus under certain circum stances, the green and blue color studied by
M. Chalvet, and the orange-yellow, bright red, and blue colors observed by C. Eberth in perspi ration.
In Norway, red bacteria appear in summer in such masses that the borders of the sea are some times colored of an intense red (Warming).
Movement of Bacteria
The bacteria are met in two different states. They are active or motionless ; but it is now well settled for the greater number that the same species may present itself sometimes in a state of repose, sometimes in a state of move ment.
The movements of the bacteria are of two kinds, a movement of the corpuscle upon itself and a movement of translation. The first is sometimes nothing more than a molecular or brownien move ment, which occurs in the smallest forms. But at other times it is more extended, and consists in a movement of rotation round the axis, or a bend ing of the body. This flexibility is, above all, observed in the long forms, the Bacillus, the Vibrions, etc. As to the movement of translation, it is very variable ; at one time slow, at another rapid, it is in relation with the length and form of the bacterium. M. Cohn has well described all the modifications of movement in the following lines : " Almost all the bacteria possess two different modes of life, characterized by repose and by movement.
" In certain conditions, they are excessively mobile ; and when they swarm in a drop of water, they present an attractive spectacle, similar to that of a swarm of gnats, or an ant-hill.
The bacteria advance, swimming, then retreat without turning about, or even describe circular lines. At one time they advance with the ra pidity of an arrow, at another, they turn upon themselves like a top ; sometimes they remain motionless for a long time, and then dart off like a flash. The long rod-bacteria twist their bodies in swimming, sometimes slowly, sometimes with address and agility, as if they tried to force for themselves a passage through obstacles. It is thus that the fish seeks its way through aquatic plants. They remain sometimes quiet, as if to re pose an instant: suddenly the little rod commences to oscillate, and then to swim briskly backwards, to again throw itself forward some instants after.
All of these movements are accompanied by a second movement analogous to that of a screw which moves in a nut. When the vibrios" in the shape of a gimlet turn rapidly round their axis, they produce a singular illusion : one would be lieve that they twisted like an eel, although they are extremely rigid."
The causes of these movements have been sought, at first, in the supposed animal nature of the bac teria, and the movements assimilated, consequently, to voluntary movements ; but this opinion can no longer be sustained, as similar movements are to be seen in a great number of vegetable organisms, such as the diatoms, the oscillatorise, the spores of algae and some fungi, etc. They have also been attributed to the existence of locomotor appen dices (Ehrenberg) ; but, although the cilia, denied at first by most microscopists, have been seen since in nearly all the bacteria, the botanists who have best studied them, M. Warming, for example, rec ognize that it is scarcely probable that these or gans are the cause of their movements, for " one meets some examples in which the body remains motionless while the cilia are in violent agitation, and others in which the body moves while the cilia remain inert, or dragging behind."
The movements appear to depend rather upon the nutrition, or respiration, and especially upon the presence of oxygen (Cohn); indeed when this gas is wanting the bacteria become motionless.
Immobility may also be produced by want of nutriment, poisoning by different toxic substances, (chloroform, iodine, etc.), dessication, etc. The attempt has been made to use the characters derived from the existence or absence of motion, and the form of the bacteria, in order to classify them ; but what has just been said shows clearly that these transitory phenomena cannot be taken for generic or specific characters.
Structure. It was for a long time believed that the bacteria were constituted of amorphous masses of protoplasm, or of solid rods. The researches of Hoffmann have shown that they have a true cellu lar structure. We shall describe, then, succes sively, their membrane, the contents, and the cilia, which may be considered as belonging to the pro toplasm.
==Bacterial Cell-membrane== The extreme minuteness of the bacteria usually prevents a direct demonstra tion of the cell-membrane, and the existence of this envelope has not, heretofore, been clearly demonstrated except by indirect proofs ; chemical reactions, for example.
Thus Hoffmann verifies the existence of a cellular envelope when " the contents, which is a trans parent plasma, are partly coagulated, as sometimes happens, or disappear, and are then replaced by air which shows precisely the form of the normal bacterian cell." Warming, also, has not been able to see the membrane, " which only appears dis tinctly when a vacuole has formed just against the periphery."
On the other hand, the action of chemical agents upon bacteria proves that they have an envelope of cellulose, which is colored by tincture of iodine ;
Is not destroyed by caustic potash, ammonia, or even acids; and resists putrefaction for an ex ceedingly long time. In this respect, it resembles the membrane of cellulose of vegetable cells (Cohn).
We should add that Cohn claims to have succeeded with high powers in seeing directly the cell-membrane. On the other hand, Yarming has never succeeded in so doing. The last observer remarks also that the resistance of bacteria to acids, to alkalis, etc., does not seem to prove the existence of a membrane, " inasmuch as this may be the result of a particular condition of the plasma, which in all the bacteria is of a more con sistent nature than in other plants."
Finally, the membrane may be, in certain bacteria, tender, flexible and susceptible of move ments of torsion. In others, it is rigid and incapable of bending. Cohn thinks also that it may swell and dissolve into mucilage, a fact which would explain the origin of this substance in the Zooglcea.
Protoplasm. The contents of the cell is a nitrogenous substance, generally colorless, more highly refractive than water.
In the smallest species, this protoplasm appears homogeneous ; but in the bacteria of medium size, and above all in the large species, the contents of the cell encloses portions more highly refractive, vacuoles, special granules, and sometimes diverse coloring matters.
Cohn first pointed out the movements of the protoplasm, in which currents occur, above all in the central portion, the peripheral portion remain ing homogeneous and motionless. The vacuoles are also found in the central portion ; Warming, however, who has observed them in Monas Okenii, Vibrio rftgula, V. serpens and Spirillum undula var. Littoreum, has sometimes seen them in the mid dle of the plasma, at another near the exterior wall.
The granules which are seen in the protoplasm were considered by Ehrenberg as stomachal vesi cles or ovules. They are of two sorts ; the one, highly refractive and not bordered by a dark circle, are considered by Warming as nothing more than mere compact masses of protoplasm ; the second, also highly refractive, but surrounded by a dark circle, resemble drops of oil, and have been taken for fat granules ; but the recent researches of Cramer, Cohn, and Warming have proved that some of them, at least, are formed of crystalline sulphur. They are not soluble either in hydro chloric acid or in water, but they are dissolved in absolute alcohol, in hot caustic potash and sulphite of soda, in nitric acid and chlorate of potash at ordinary temperatures, and in bisulphide of carbon, when the membrane, which is permeable with dif ficulty, has been previously destroyed by sulphuric acid. Although their small dimensions and great refractive power prevent them from being dis tinguished with certainty as crystals of sulphur, as they are doubly refractive to polarized light their crystalline nature cannot be doubted.
These globules of sulphur have been observed in Monas Okenii, Bacterium sulphuratum, Ophi domonas, and the different species of Beggiatoa, both in fresh water, in putrid seawater, and in thermal sulphur waters. It will be seen when we speak of the physiology of these organisms what their role is in the elimination of sulphui* and the formation of sulphuretted hydrogen.
We have said, in speaking of the colored bac teria, that some borrow their color from the sur rounding medium, and that others, on the contrary, have a color of their own. The protoplasm of the latter contains a granular coloring matter, which is ordinarily yellow, blue, or red. The red color ing matter is most common, and this has been best studied, and appears to be the best known.
One of these colors which gives a pink tint (peach color) to Bacterium rubescens, Ray-Lank.
( Clathrocystis roseopersicina, Cohn); Monas tnnosa, Ehrb., M. Ojcenii, Cohn ; M. gracilis, Warming ; Rhabdomonas rosea, Cohn; M. Warmingii, Cohn; Ophldomonas sanguinea, Ehrb. ; Merismopedia littoralis, Rabenh. ; etc., has been studied by Ray Lankaster, who has given to it the name of bac terio-purpurine. It is insoluble in water, soluble in alcohol, ether, carbolic acid, glycerine, and fatty oils, characteristics which make it resemble chlorophyll. It has also a characteristic spectrum.
Other red coloring matters which appear to be different have been found in Monas prodigiosa 9 Ehrb.; Bacillus ruber, Cohn; and Micrococcus ful vus, Cohn. These should not be confounded with the purple coloring matter of other algae, as that of the Porphyridium cruentum, which comes from a mixture of chlorophyll and of phycoerythrine.
The bacteria never contain chlorophyll.
In this connection, it is interesting to recall the protoplasmic constitution of the Amylobacter of Trecul. These organisms are, according to Yan Tieghern, bacteria, to which he has given the name of Bacillus Amylobacter, and which does not differ from B. subtilis, except by a specific character, extremely transitory, the presence of amorphous starch, formed and stored in reserve during the period of growth, to be again used later, and con sumed during the process of reproduction.