The Fundamentals of Bacteriology. Charles Bradfield Morrey
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These few instances do not disclose any general principles which may be applied either for the growth or for the distinction of aërobes or facultative anaërobes.
It has been shown that compressed oxygen will kill some bacteria but this method of destroying them has little or no practical value. Oxygen in the form of ozone, O3, is rapidly destructive to bacteria, and this fact is applied practically in the purification of water supplies for certain cities where the ozone is generated by electricity obtained cheaply from water power. The same is true of oxygen in the “nascent state” as illustrated by the use of hypochlorites for the same purpose.
It was stated (p. 74) that certain thermophil bacteria in the soil have an optimum temperature for growth in the air which is much higher than is ever reached in their natural habitat and that they grow at a moderate temperature under anaërobic conditions. It has been shown that if these organisms are grown with aërobes or facultative anaërobes they thrive at ordinary room temperature. These latter organisms by using up the oxygen apparently keep the tension low, and this explains how such organisms grow in the soil.6
OSMOTIC PRESSURE.
Like all living cells bacteria are very susceptible to changes in the density of the surrounding medium. If placed in a medium less concentrated than their own protoplasm water is absorbed and they “swell up”; while if placed in a denser medium, water is given off and they shrink (plasmoptysis or plasmolysis). Should these differences be marked or the transition be sudden, the cell walls may even burst and the organisms be destroyed. If the differences are not too great or if the transition is made gradually, the organisms may not be destroyed, but will either cease to grow and slowly die out, or will show very much retarded growth, or will produce abnormal cell forms. This is illustrated in the laboratory in attempting to grow bacteria on food material which has dried out. A practical application of osmotic effects is in the use of a high percentage of sugar in preserving fruits, etc., and in the salting of meats. Neither the cane-sugar nor the common salt themselves injure the bacteria chemically, but by the high concentration prevent their development. In drying material in order to preserve it there are two factors involved: first, the loss of water necessary for growth and second, the increased osmotic pressure.
In a medium of greater density diffusion of water is outward from the cell and this will continue until an equilibrium is established between cell contents and medium. Food for the organism must be in solution and enter the cell by diffusion. Therefore, growth ceases in a medium too dense, since water to carry food in solution does not enter the cell.
ELECTRICITY.
Careful experimenters have shown that the electric current, either direct or alternating, has no direct destructive effect on bacteria. In a liquid medium the organisms may be attracted to or repelled from one or the other pole or may arrange themselves in definite ways between the poles (galvanotaxis), but are not injured. However, electricity through the secondary effects produced, may be used to destroy bacteria. If the passage of the electric current increases the temperature of the medium sufficiently, the bacteria will be killed, or if injurious chemical substances are formed (ozone, chlorine, acids, bases, etc.), the same result will follow (see Ozone, pp. 77 and 157).
RADIATIONS.
Röntgen or x-rays and radium emanations when properly applied to bacteria will destroy them. The practical use of these agents for the direct destruction of bacteria in diseases of man or animals is restricted to those cases where they may be applied directly to the diseased area, since they are just as injurious to the animal cell as they are to the bacteria, and even more so. Their skilful use as stimuli to the body cells to enable them to resist and overcome bacteria and other injurious organisms or cell growths is an entirely different function and will not be considered here.
PRESSURE.
Hydrostatic pressure up to about 10,000 pounds per square inch is without appreciable effect on bacteria as has been shown by several experimenters and also by finding living bacteria in the ooze dredged from the bottom of the ocean at depths of several miles.
Pressures from 10,000 to 100,000 pounds show variable effects. Some bacteria are readily killed and others, even non-spore formers, are only slightly affected. The time factor is important in this connection. The presence of acids, even CO2, or organic acids, results in the destruction of most non-spore formers.
MECHANICAL VIBRATION.
Vibrations transmitted to bacteria in a liquid may be injurious to them under certain circumstances. Some of the larger forms like Bacillus subtilis may be completely destroyed by shaking in a rapidly moving shaking machine in a few hours. Bacteria in liquids placed on portions of machinery where only a slight trembling is felt, have been found to be killed after several days. Reinke has shown that the passing of strong sound waves through bacterial growths markedly inhibits their development.
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