Moisture

Water is life, not only for humans, but also for other organisms. Water is necessary for the growth and multiplication of bacteria. Not only is water a major component of the bacterial cell cytoplasm. On an average, 75% to 80% of the bacterial cell is water when examined under a compound microscope, but it also dissolves the food materials in the environment of the bacterial cell so that they can be absorbed.

Drying is highly detrimental to bacterial growth. Delicate bacteria such as the gonococcus resist drying only a few hours, and even highly resistant bacteria such as the tubercle bacillus succumb to drying within a few days. Spores, however, may resist drying for years. As a rule, bacteria with capsules are more resistant in this respect than those with none. Further studies on how moisture affects the growth of the organisms can be observed well with the help of a compound microscope.

Temperature

There exists a minimum, optimum, and maximum temperature for each species of the bacteria, meaning, respectively, the lowest temperature at which the species will grow, the temperature at which it grows best, and the highest temperature at which growth is possible. The optimum temperature, when observed under a compound microscope, for a species corresponds to the average temperature of its usual habitat. For instance, bacteria that naturally live in or attack the human body, live best at 37° C (Celsius). This temperature is considered as the normal body temperature. The lowest temperature at which any of them continue to multiply is around 20°C., and the highest is from 42° to 45° C. In this temperature range, bacteria are mesophiles.
Many bacteria will not grow at a temperature more than a few degrees above or below their optimum, as observed under a microscope. Some pathogens die off rapidly at only 38° C. Most saprophytic bacteria or mesophiles grow best between 25° and 40° C. Some thermophiles or heat-loving species, grow at a temperature above 45° C. or even higher. One species, Sulfolobus acidocaldarius obtains its maximum growth at 85° C. Studies on thermophiles done at Yellowstone National Park and other geothermal areas in the world have demonstrated not only that such heat-living bacteria are there but thriving in waters over 90° C. A few psychrophiles, or cryophiles, or cold-loving species, grow at temperatures just above the freezing point, 20° C or less. They proliferate slowly in the refrigerator. Many psychrophilic organisms have red pigments. Where they grow on the surface of ice and snow, they color it to give red snow.
Cold retards or stops bacterial growth, but when the bacteria are later exposed to a temperature favorable for their growth, multiplication is resumed. Refrigeration which is 4° to 6° C is one of the best methods of preserving bacterial cultures, since bacteria are generally resistant to low temperatures and even to freezing. Prolonged freezing, however, destroys them.
High temperatures are much more injurious to bacteria than are low ones and are used effectively in practical situations where bacteria and their spores must be destroyed. The effect of temperature on the bacterial growth can be studied under a compound microscope.

Hydrogen-ion concentration (pH)

For each species of bacteria there is a certain degree of alkalinity or acidity, a certain pH at which growth is most rapid. The reaction of culture media must be carefully adjusted to the desired hydrogen-ion concentration. The best growth of most microorganisms, as observed under a microscope, is found in a narrow pH range of not less than 6 nor more than 8. Most pathogens grow best in a neutral or slightly alkaline medium. Regardless of the influence of the environment, the reaction of the interior of the cell is fuse at the neutral point of pH 7.

Oxygen

Organisms can also be classified according to their oxygen needs. These different organisms can be viewed under the microscope to distinguish one from the other. Organisms that grow in the presence of free atmospheric oxygen are known as aerobes. If an organism cannot develop at all in the absence of free oxygen, it is an obligate aerobe. Those that cannot grow in the presence of free oxygen but must obtain it from oxygen-containing compounds which includes inorganic sulfates, nitrates, and carbonates or certain organic compounds are anaerobes. Obligate anaerobes are vulnerable to free oxygen. Their enzyme systems are inactivated by atmospheric oxygen. Organisms adaptable either to the presence of atmospheric oxygen or to its absence is facultative. Those growing best in an amount of oxygen less than that contained in the air are microaerophiles. The ones designated capno¬philes need a 3% to 10% increase in carbon dioxide in the environment to initiate growth.

Light

Violet, ultraviolet, and blue lights are highly destructive to bacteria, green light is much less so and red and yellow lights have little bactericidal action. Because of its content of ultraviolet light, direct sunlight kills most bacteria within a few hours. Bright daylight has an effect similar to that of sunlight but one of less potency.

A few species of saprophytic bacteria containing chlorophyll can use sunlight to build up the compounds of which they are composed. This bacterial chlorophyll is scattered throughout the cytoplasm, unlike the chlorophyll of plant cells, which is contained in their chloroplasts. How these bacterial chlorophyll looks like can be best described with the use of the microscope.