The Fundamentals of Bacteriology

by Charles Bradfield Morrey

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.

CHAPTER VII.
CHEMICAL ENVIRONMENT.

REACTION OF MEDIUM.

Most bacteria are very susceptible to changes in the degree of acidity or alkalinity of the medium in which they grow. Some kinds prefer a slightly acid reaction, some a slightly alkaline, and some a neutral (with reference to litmus as indicator). The organism which is the commonest cause of the souring of milk thrives so well in the acid medium it produces that it crowds out practically all other kinds, though its own growth is eventually stopped by too much acid. Acid soils are usually low in numbers of bacteria and as a consequence produce poor crops. The disease-producing bacteria as a class grow best in a medium which is slightly alkaline.