2. What kind of cell walls do they have?

On average, bacteria are about 1 micrometre long and 0.5 micrometre in diameter. All bacteria are surrounded by a lipid membrane that regulates the flow of materials in and out of the cell. A rigid cell wall completely surrounds the bacterium and lies outside the membrane. Gram-positive bacteria are stained blue by the gram stain, because their cell walls have a relatively thick and meshlike structure that traps the dye. In gram-negative bacteria, the cell wall is thin and releases the dye readily when washed with an alcohol solution.

3. What are flagellae?

Outside the cell wall, some species of bacteria also have a capsule made up of polysaccharides. Such capsules have many functions, including protecting the bacterium from phagocytes and from dessication (drying). Many species of bacteria swim by means of flagellae, i.e., hairlike structures whose whiplike lashing provide propulsion.

4. What structures are distributed about the bacterial cytoplasm?

The DNA of most bacteria is found in a single circular chromosome and is distributed throughout the cytoplasm rather than in a membrane-bound nucleus. Smaller circular auxiliary DNA strands, called plasmids, are also found in the cytoplasm. There are a number of other structures distributed about the bacterial cytoplasm, including ribosomes.

5. How do bacteria reproduce?

When applied to bacteria, the term growth refers to an increase in the size of a population rather than in that of an individual microorganism. Bacteria usually reproduce through binary fission, an asexual process in which the mother cell increases in size until it divides into two identical daughter cells. There are also bacteria that reproduce through budding, through chains of spores, and through the segmentation of elementary units. Bacteria do not reproduce sexually, but there are several mechanisms by which DNA is exchanged in a one-way transfer between them.

6. What are heterotrophs and autotrophs?

All bacteria require carbon, nitrogen, sulfur, phosphorus, inorganic salts, and micronutri-ents. Bacteria that use an organic compound as their source of carbon are called heterotrophs, while those that use an inorganic source are called autotrophs. In addition, some bacteria use photosynthesis to generate energy in the form of the compound ATP; these are called phototrophs. Some species of bacteria are parasitic and can grow only within a living host cell; examples include the genera Rickettsia and Chlamydia, both of which are parasites in eukaryotic cells. Those bacteria that require oxygen, such as Bacillus, are called aerobes; anaerobes, such as Clostridium, can not survive in the presence of oxygen.

7. What types of bacteria do you know?

Various types of bacteria that are present in water can cause disease in humans, and water-purification plants are designed to destroy these microorganisms. Bacteria from industrial wastes may also act as pathogens, or agents of disease. Conversely, some types of bacteria act as cleansing agents in water, and water-treatment facilities utilize some such bacteria to break down the organic matter that is present in sewage.

Various types of bacteria contaminate foods and can cause food poisoning in humans. Pasteurization is routinely used to neutralize bacteria that may be present in milk, for example. Other sterilization techniques include high temperature, radiation, ethylene oxide, and other antiseptics and germicides.