Читать книгу The Fundamentals of Bacteriology - Charles Bradfield Morrey - Страница 5

Speculation as to the first origin of life is as old as history and doubtless older. Every people of antiquity had its own legends, as for example, the account in Genesis. This question never can be definitely settled, even though living matter should be made in the laboratory.

The doctrine of the “spontaneous origin” of particular animals or plants from dead material under man’s own observation is a somewhat different proposition and may be subjected to experimental test. The old Greek philosophers believed it. Anaximander (B.C. 610–547) taught that some animals are derived from moisture. Even Aristotle (B.C. 384–322) said that “animals sometimes arise in soil, in plants, or in other animals,” i.e., spontaneously. It can be stated that this belief was general from his day down through the Dark and Middle Ages and later. Cardano (A.D. 1501–1576) wrote that water gives rise to fish and animals and is also the cause of fermentation. Van Helmont (1578–1644) gives directions for making artificial mice. Kircher (1602–1680) describes and figures animals produced under his own eyes by water on plant stems.

However, many thinkers of the seventeenth century doubted the truth of this long-established belief. Francesco Redi (1626–1698) made a number of experiments which tended to prove that maggots did not arise spontaneously in meat, as was generally believed, but developed only when flies had an opportunity to deposit their eggs on the meat. It seems that by the latter part of this century the idea that organisms large enough to be seen with the naked eye could originate spontaneously was generally abandoned by learned men.

The work of Leeuwenhoek served to suspend for a time the subject of spontaneous generation, only to have it revived more vigorously later on. He is usually called “The Father of the Microscope,” though the compound microscope was invented probably by Hans Zansz or his son Zacharias, of Holland, about 1590. Leeuwenhoek used a simple lens, but his instruments were so much more powerful that they opened up an entirely new and unknown world. (Fig. 1.)

Anthony van Leeuwenhoek (1632–1723) was apprenticed to a linen draper and accumulated a comfortable fortune in this business. He became interested in the grinding of spectacle lenses, then an important industry in Delft, Holland, where he lived, and did a great deal of experimental work in this line, mainly for his own enjoyment. Finally he succeeded in making a lens so powerful that he could see in water and various infusions very minute living bodies never before observed. Leeuwenhoek contributed 112 papers to the Royal Society of Great Britain, the first in 1673, many of them accompanied by such accurate descriptions and drawings, for example a paper submitted September 12, 1683, that there is no doubt that he really saw bacteria and was the first to do so (Fig. 2). Rightly may he be styled “The Father of Bacteriology,” if not of the microscope. He says in one paper: “With the greatest astonishment I observed that everywhere through the material I was examining were distributed animalcules of the most microscopic dimness which moved themselves about in a remarkably energetic way.” Thus he considered these living objects to be animals, from their motion, and this belief held sway for nearly two hundred years.

Fig. 1.—Leeuwenhoek’s Microscope. A is the simple bi-convex lens held firmly in place. In front of this is the small table, B, with the support, C, on the tip of which the object to be examined was held. This support could be brought nearer to or removed further away from the lens and held firmly in place by the screw, D. E is a second screw for raising or lowering the entire table. A concave mirror that Leeuwenhoek sometimes used to focus more light on the object under examination, is shown at the right.

Leeuwenhoek was a pure observer of facts and made no attempt at speculation, but his discoveries soon started the theorists to discussing the origin of these minute organisms. Most observers, as was probably to be expected, believed that they arose spontaneously. Needham, in 1749, described the development of microörganisms around grains of barley in water. Bonnet, in 1768, suggested that probably Needham’s animalcules came from ova in the liquid. The Abbot Spallanzani, in 1769, called attention to the crudeness of Needham’s methods and later, in 1776, attempted to disprove spontaneous origin by heating infusions of organic material in flasks and then sealing them. His critics raised the objections that heating the liquids destroyed their ability to support life, and that sealing prevented the access of fresh air which was also necessary. The first objection was disproved by the accidental cracking of some of the flasks which thereafter showed an abundant growth. This accident seemed also to support the second objection, and Spallanzani did not answer it. Though Spallanzani’s experiments failed to convince his opponents, they led to important practical results, since François Appert, in 1810, applied them to the preserving of fruits, meats, etc., and in a sense started the modern canning industry.

Fig. 2.—The first drawings of bacteria by Leeuwenhoek. The dotted line C–D indicates the movement of the organism.

Fig. 3.—Schultze’s experiment. The set of bulbs next to the face contained KOH and the other set concentrated H₂SO₄. Air was drawn through at frequent intervals from May until August but no growth developed in the boiled infusion.

From Spallanzani to Schultze, there were no further experiments to prove or disprove spontaneous generation. Schultze, in 1836, attempted to meet the second objection to Spallanzani’s experiment, i.e., the exclusion of air, by drawing air through his boiled infusions, first causing it to bubble through concentrated sulphuric acid to kill the “germs” (Fig. 3.). His flasks fortunately showed no growths, but his critics claimed that the strong acid changed the properties of the air so that it would not support life. This experiment of Schultze’s, though devised for a different purpose, was really the first experiment in the use of chemical disinfectants, though Thaer (page 31) had used chemicals in a practical way. Schwann, in 1837, modified this experiment, by drawing the air through a tube heated to destroy the living germs (Fig. 4). His experiments were successful but the “spontaneous generation” theorists raised the same objection, i.e., the change in the air by heating. This was the first experiment in which the principle of “dry heat” or “hot air” sterilization was used. Similar arguments were brought forward, also to the use of cotton plugs as filters by Schroeder and Dusch in 1859 (Fig. 5). This was the first use of the principle of sterilization by filtration. It remained for Chevreuil and Pasteur to overcome this objection in 1861 by the use of flasks with long necks drawn out to a point and bent over. These permitted a full access of air by diffusion but kept out living germs, since these cannot fly but are carried mechanically by air currents or fall of their own weight (Fig. 6.). Hoffman, the year before (1860), had made similar experiments but these remained unnoticed. The Pasteur flasks convinced most scientists that “spontaneous generation” has never been observed by man, though some few, notably Dr. Charlton Bastian, of England, vigorously supported the theory from the early seventies until his death in November, 1915.

Fig. 4.—Schwann’s experiment. After boiling, as shown in the diagram, and cooling, air was drawn into the flask by aspiration while the coiled tube was kept hot with the flame.

Fig. 5.—Schroeder and Dusch’s experiment. The aspirating bottle drew the air through the flask after it had been filtered by the cotton in the tube.

Fig. 6.—Pasteur’s flask.

Fig. 7.—Tyndall’s box. One side is removed to show the construction. The bent tubes at the top are to permit a free circulation of air into the interior. The window at the back has one corresponding in the front (removed). Through these the beam of light sent through from the lamp at the side was observed. The three tubes received the infusion and were then boiled in an oil bath. The pipette was for filling the tubes. (Popular Science Monthly, April, 1877.).

John Tyndall, in combating Bastian’s views showed that boiled infusions left open to the air in a closed box through which air circulated did not show any growth of organisms provided the air was so free of particles that the path of a ray of light sent through it from side to side could not be seen (Fig. 7). Or if such sterilized infusions were exposed to dust-free air, as in the high Alps, the majority showed no growth, while all infusions in dusty air did show an abundance of organisms. Tyndall’s experiments confirmed those of Pasteur and his predecessors and showed that the organisms developed from “germs” present in the air falling into the liquids and not spontaneously.

While Tyndall’s experiments were of great value as indicated, they probably were harmful in another way. These “germs in the air” were considered by bacteriologists as well as laymen to include necessarily many disease germs and to indicate the very general, if not universal, presence of these latter in the air. This idea led to many erroneous practices in sanitation and disinfection which even to this day are not eliminated.