PHYSICAL CONDITIONS REQUIRED FOR GROWTH DURING CULTURING
In order to double its size and divide within 30 minutes, a bacterial cell must be capable of synthesizing its own weight of cell material within that period. Accordingly, it must have a very rapid rate of metabolism together with a correspondingly rapid uptake of nutrients and disposal of waste products. It can do this only because of its small size, which gives it a very large surface for absorption and excretion in relation to its volume. But the very rapid growth that occurs in the log phase is possible only under certain restricted environmental conditions; it is most important to understand these thoroughly. These environmental conditions include:-
3) Influence of Temperature
4) Influence of Moisture and of Desiccation
5) Influence of Hydrogen-ion Concentration
6) Influence of Mechanical and Sonic Stresses
Influence of Temperature
1. On growth.
For each species there is a definite temperature range within which growth takes place. The limits are the 'maximum' and `minimum' temperatures, and an intermediate `optimum' temperature can usually be recognized at which growth is most rapid. In the laboratory, bacteria are grown at this optimum temperature in a thermostatically controlled incubator. The optimum temperature of a bacterium is approximately that of its natural habitat, e.g. about 37°C in the case of organisms that are parasitic on man and warm-blooded animals. These, and many saprophytes of soil and water that grow best at between 25° and 40°C, are termed mesophilic. Some mesophiles have a wide growth temperature range (e.g. 5° to 43°C for Pseudomonas pyocyanea (aeruginosa), whereas others are more restricted (e.g. 30° to 39°C for Neisseria gonorrhoeae). None of them grows appreciably at temperatures below 5°C (as in a domestic refrigerator at 3° to 5°C), and few at temperatures above 45°C. A group of soil and water bacteria, the psychrophiles, grow best at temperatures below 20°C, usually quite well at 0°C and in some cases, slowly, down to about — 7°C on unfrozen media. Their importance lies in their ability to cause spoilage of refrigerated and frozen food, though none is pathogenic. Another group of non-parasitic bacteria, the thermophiles, grow best at high temperatures between 55° and 80°C, and have minimum growth temperatures ranging from 20° to 40°C (facultative thermophiles), or even above 40°C (strict thermophiles). These organisms are important as a cause of spoilage in under-processed canned foods, since many form spores of exceptionally high heat-resistance.
2. On viability.
Heat is an important agent in the artificial destruction of microorganisms, the effect depending under moist conditions on the coagulation and denaturation of cell proteins, and under dry conditions, on oxidation and charring. Among the bacteria that are parasites of mammals, non-sporing forms in the presence of water generally cannot withstand temperatures above 45°C for any length of time. The time of exposure to heat that is necessary for killing is shorter the higher the temperature, and various other factors influence the exact amount of heating required.
Thus, bacteria are more susceptible to 'moist heat', e.g. in hot water or saturated steam, than to 'dry heat', e.g. in a hot-air oven. They are rendered more susceptible to the lethal effect of heat by the presence of acid, alkali or any chemical disinfectant, and less susceptible by the presence of organic substances such as proteins, sugars and fats, and also by their own occurrence in large numbers. The thermal death point of a particular organism may be defined as the lowest temperature that kills it under standard conditions, and within a given time, e.g. 10 min. Under moist conditions, it lies between 50° and 65°C for most non-sporing mesophilic bacteria, and between 100° and 120°C for the spores of most sporing species (e.g. about 105°C for Cl. tetani and 115°C for Cl. botulinum).
The extreme limit of resistance to moist heat is shown by the spores of a non-pathogenic, strictly thermophilic bacillus, B. stearothermophilus, which is killed only after exposure to 121°C for 10 to 35 minutes. With dry heat, the 10-minute thermal death points of the different sporing bacteria are mostly between 140° and 180°C. At low temperatures some species die rapidly, but the majority survive well. Cultures of the latter may be preserved for long periods at between 3° and 5°C in a domestic-type refrigerator, or in the frozen state in a 'deep freeze' cabinet or in liquid nitrogen. The process of freezing kills a proportion of the bacterial cells present, and this is least if freezing is effected rapidly, e.g. by use of solid carbon dioxide or if a stabilizer such as glycerol is added.