Reproduction and Growth

Reproduction of bacteria is usually by fission: an individual cell enlarges and splits into two, each of the resulting pair being an exact replica of the parent cell (in the absence of mutation). Filamentous species reproduce by the elongation and branching of their filaments. A few bacteria multiply by a budding process. Methods of reproduction are almost invariably asexual, though a form of ‘sexual^’ mating has been rarely observed.

In the laboratory most bacteria can be induced to ‘grow’ (reproduce) in liquid or on solid culture media. Nutrient requirements, atmospheric conditions correct pH value (acidity or alkalinity) of the medium, osmotic pressures and temperatures are rendered optimum for the multiplication of species in vitro. Some bacteria will grow in a simple defined medium consisting of a few salts in aqueous solution (trace elements are provided by the impurities found in all chemicals, even those described as ‘pure^’). Others, including many pathogenic species, require protein degradation products, such as digested meat, in the constitution of media suitable for their growth. Some fastidious pathogens need media enriched with blood or other highly nutritious substances.

Say whether the following statements are true or false:

1. Methods of reproduction of bacteria are mainly asexual.

2. Bacteria never multiply by budding.

3. Bacteria usually reproduce by dividing into numerous parts.

4. Bacteria can’t be grown in the laboratory.

5. Numerous requirements have to be met in order to make bacteria reproduce in vitro.

6. The medium enriched with nutritious substances is required for some pathogenic bacteria.

T E X T B

Bacteria: their Construction

Bacteria are very small single-celled organisms (microorganisms) that exist in enormous numbers almost everywhere. They live in soil, water, air, and in living and dead animals and plants. A gram of soil can contain up to a thousand million bacteria, and there may be hundreds of thousands in a single drop of milk.

Bacteria differ from each other mainly in where and on what they live, and in the shape of their single cells. There are the spherical coccus types such as Staphylococcus and Streptococcus, which often occur in chains or masses, and the rod-shaped bacillus type such as Mycobacterium, which causes tuberculosis. Other disease-causing bacteria are Eberthella typhi (typhoid), and Vibrio cholerae (cholera). The type of bacterium which forms a coil or spiral is Spirillum.

Although bacteria cells are more complicated than viruses they are still very simple. Their structure has been worked out with optical microscopes which magnify by one hundred thousand times. All bacteria have a tough outer cell wall so their food must be soluble before it can be absorbed into the cell. In some bacteria there is a protecting layer of jelly enclosing the cell wall, and also one or more minute fibres (flagella) used for swimming. Inside the cell there is a coil of DNA and other chemical substances, but there is no definite nucleus or any of the other structures found in plant and animal cells.

Bacteria usually reproduce by simply splitting in two. When temperature conditions are favourable, about 37°C for most bacteria, they can divide about оncе every 30 minutes. In theory, one bacterium could form about 140 000 000 000 000 bacteria at the end of 24 hours. In fact, this does not happen because the supply of food soon runs out, poisonous wastes accumulate, and after a time no more bacteria can survive. Although most bacteria reproduce by dividing in two, some bacteria can reproduce sexually, during which the contents of one bacterium flow into another.

Bacteria are very tough. Different kinds can live in almost every environment, from hot springs to arctic frost. Many can form a type of spore under certain conditions. A spore is capsule inside which a bacterium can survive for years of drying out, intense heat or disinfectant. Few of disease bacteria can make spores.

CRAMMAR EXERCISES

PRACTICE MODALS

Ex. 1. Supply the necessary modal verbs.