There is considerable difference in the resistance to partial drying shown by the different species of bacteria. For example, the staphylococci on the human skin, most of the saprophytic organisms common in the soil and dust, the germ of diphtheria, and particularly the bacillus of tuberculosis can withstand a comparatively high degree of partial drying, but the vast majority of parasitic and pathogenic bacteria die very quickly when deprived of an abundance of moisture.
Spores are much more resistant to drying than vegetative cells. Practically, drying as a method of destroying bacteria is usually combined with action of sunlight or heat, with the result that all types of bacterial cells are completely dried out and quickly destroyed. The preservation of foods by drying is one important application of this method of killing microbes.
2. Sunlight and Ultraviolet Light:
Direct sunlight has a powerful germicidal action. Exposure to directly sunlight for a sufficient time will kill spores as well as vegetative cells. Tuberculosis germs are killed in new hours.
It is often possible to make deliberate use of this germ-killing power of sunlight for the disinfection of clothing, bedding, mattresses, and other materials.
The germicidal property of the sunlight is not due to the ordinary light which we see, but to the very short, invisible, light rays beyond the violet end of the spectrum-the ultraviolet rays. It should be remembered that these rays are filtered out by ordinary glass, and that the sun must shine directly upon &n object in order to exert its germicidal effect.
Ultraviolet rays are easily produced artificially by passing an electric current through vapourized mercury in quartz tubes. The rays pass through the tubes of quartz and so can be applied to any object.
They destroy bacteria in a few seconds or minutes; molds and yeasts are somewhat more resistant. Ultraviolet light has been employed for purification of drinking water and swimming pool water, but it is not very effective for such purposes and is too expensive to justify wide use.
3. Concentrated Solution:
It has already been mentioned that foods may be preserved by immersing them in strong salt or sugar solutions. This is due to the fact that in such concentrated solutions the bacterial cells lose water through osmosis and so shrink and die.
Cold prevents the multiplication of the ordinary bacteria, and refrigeration serves as an admirable method for the preservation of food and other substances which are easily decomposed by microorganisms.
Some of the common disease germs, such as the organisms causing meningitis, gonorrhea, and syphilis are actually killed by cold. But most bacteria are not destroyed even at very low temperatures.
The bacillus of typhoid fever, for example, may be frozen in a block of ice and still be able to grow when carefully thawed out and returned to a favourable temperature.
The application of heat is one form or another is the most widely used method of destroying bacteria. It will be remembered that the thermal death point of the vegetative cells of most of the ordinary bacteria is about 65°C. (Or about 145°F), but that spores and some unusual organisms are very much more resistant to heat. For this reason if we wish to sterilize an object truly, high temperatures continued for considerable periods of time must be applied.