The Fundamentals of Bacteriology. Charles Bradfield Morrey
chemical substance in nuclei, which when aggregated forms the nucleus, is scattered throughout the cell contents and thus intimately mingled with the protoplasm, and cannot be differentiated by staining as in most cells.
The close association of nuclein and protoplasm may explain the rapid rate of division of bacteria (Chapter VIII, p. 91).
The chemical composition of the bacterial cell is discussed in Chapter VII.
In addition to the essential parts just described the bacterial cell may show some of the following accidental structures: vacuoles, capsules, metachromatic granules, flagella, spores.
Vacuoles.—Vacuoles appear as clear spaces in the protoplasm when the organism is examined in the living condition or when stained very slightly (Fig. 18). During life these are filled with liquid or gaseous material which is sometimes waste, sometimes reserve food, sometimes digestive fluids. Students are apt to confuse vacuoles with spores (p. 47). Staining is the surest way to differentiate (Chapter XIX, p. 209). If vacuoles have any special function, it is an unimportant one.
Fig. 19.—Bacteria seen within capsules.
Fig. 20.—Metachromatic granules in bacteria. The dark round spots are the granules. The cells of the bacteria are scarcely visible.
Capsule.—The capsule is a second covering outside the cell wall and probably developed from it (Fig. 19). It is usually gelatinous, so that bacteria which form capsules frequently stick together when growing in a fluid, so that the whole mass has a jelly-like consistency. The term zoöglœa was formerly applied to such masses, but it is a poor term and misleading (zoön = an animal) and should be dropped. The masses of jelly-like material frequently found on decaying wood, especially in rainy weather, are in some cases masses of capsule-forming bacteria, though a part of the jelly is a product of bacterial activity, a gum-like substance which lies among the capsulated organisms. When these masses dry out, they become tough and leathery, but it is not to be presumed that capsules are of this consistency. On the contrary, they are soft and delicate, though they certainly serve as an additional protection to the organism, doubtless more by selective absorption than mechanically. Certain bacteria which cause disease form capsules in the blood of those animals which they kill and not in the blood of those in which they have no effect (Bacterium anthracis in guinea pig’s blood and in rat’s blood). The presence of capsules around an organism can be proved only by staining the capsule. Many bacteria when stained in albuminous fluids show a clear space around them which appears like a capsule. It is due to the contraction of the fluid away from the organism during drying.
Metachromatic Granules.—The term “metachromatic” is applied to granules which in stained preparations take a color different from the protoplasm as a whole (Fig. 20). They vary widely in chemical composition. Some of them are glycogen, some fat droplets. Others are so-called “granulose” closely related to starch but probably not true starch. Others are probably nuclein. Of many the chemical composition is unknown. They are called “Babes-Ernst corpuscles” in certain bacteria (typhoid bacillus). Since they frequently occur in the ends of cells the term “polar granules” is also applied. Their presence is of value in the recognition of but few bacteria (“Neisser granules” in diphtheria).
Flagellum.—A flagellum is a very minute thread-like process growing out from the cell wall, probably filled with a strand of protoplasm. The vibrations of the flagella move the organism through the liquid medium. Bacteria which are thus capable of independent movement are spoken of as “motile bacteria.” The actual rate of movement is very slight, though in proportion to the size of the organism it may be considered rapid. Thus Alfred Fischer determined that some organisms have a speed for short periods of about 40 cm. per hour. This is equivalent to a man moving more than 200 miles in the same time.
It is obvious that bacteria which can move about in a liquid have an advantage in obtaining food, since they do not need to wait for it to be brought to them. This advantage is probably slight.
Fig. 21.—A bacterium showing a single flagellum at the end—monotrichic.
Fig. 22.—A bacterium showing a bundle of four flagella at the end—lophotrichic.
An organism may have only one flagellum at the end. It is then said to be monotrichic (Fig. 21) (μόνος = alone, single; τριχος = hair). This is most commonly at the front end, so that the bacterium is drawn through the liquid by its motion. Rarely it is at the rear end. Other bacteria may possess a bundle of flagella at one end and are called lophotrichic (Fig. 22) (λοφος = tuft). Sometimes at approaching division the flagella may be at both ends and are then amphitrichic (Fig. 23) (αμφι = both). It is probable that this condition does not persist long, but represents the development of flagella at one end of each of a pair resulting from division of an organism which has flagella at one end only. In many bacteria the flagella arise from all parts of the surface of the cell. Such bacteria are peritrichic (Fig. 24) (περι = around). The position and even the number of the flagella are very constant for each kind and are of decided value in identification.
Fig. 23.—A bacterium showing flagella at each end—amphitrichic.
Fig. 24.—A bacterium showing flagella all around—peritrichic.
Flagella are too fine and delicate to be seen on the living organism, or even on bacteria which have been colored by the ordinary stains. They are rendered visible only by certain methods which cause a precipitate on both bacteria and flagella which are thereby made thick enough to be seen (Chapter XIX, p. 210). The movement of liquid around a bacterium caused by vibrations of flagella can sometimes be observed with large forms and the use of “dark-field” illumination.
Flagella are very delicate and easily broken off from the cell body. Slight changes in the density or reaction of the medium frequently cause this breaking off, so that preparations made from actively motile bacteria frequently show no flagella. For this reason and also on account of their fineness the demonstration of flagella is not easy, and it is not safe to say that a non-motile bacterium has no flagella except after very careful study.
The motion of bacteria is characteristic and a little practice in observing will enable the student to recognize it and distinguish between motility and “Brownian” or molecular motion. Dead and non-motile bacteria show the latter. In fact, any finely divided particles suspended in a liquid which is not too viscous and in which the particles are not soluble show Brownian motion or “pedesis.” This latter is a dancing motion of the particle within a very small area and without change of place, while motile bacteria move from place to place or even out of the field of the microscope with greater or less speed. There is a marked difference in the character of the motion of different kinds of bacteria. Some rotate around the long axis when moving, others vibrate from side to side.
Among the higher thread bacteria there are some which show motility without possessing flagella. Just how they move is little understood.
Spores.—Under certain conditions some bacterial cells undergo transformations which result in the formation of so-called spores. If the process is followed under the microscope,