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.

1. I was very tired last night but I (не мог спать).

2. I had forgotten to bring my camera so I (не смог сделать) any photographs.

3. It was a great party last night you (следовало прийти).

4. Tom is not at work today. He (возможно болен).

5. You (не надо было спрашивать) him about it.

6. It’s a fantastic film. You (должен посмотреть) it.

7. I think you (следует купить) this dress.

8. Peter (должен был заплатить) a parking fine yesterday.

9. That restaurant (не может быть) very good. It’s always empty.

10. This is a valuable book. You (не должен потерять) it.

11. You have been travelling all day. You (должно быть) very tired.

12. I’d like to read Ann’s letter. The only problem is I (не могу найти) my glasses.

13. You (не должен помогать) him with his homework. It’s important that he does it himself.

14. I just want a general idea of the story so I (не нужно читать) all the book.

15. Why did you wash that suit? It was not dirty, you (не надо было стирать) it.

16. I don’t think you (следует смотреть) TV so much.

17. We (пришлось идти пешком) home last night. There were no buses.

18. I don’t want the children play in this room. They (могут разбить) something.

19. I am sorry that I didn’t take your advice I (следовало отправить) this letter a week ago.

20. Our teacher says that we (сможем говорить) English fluently in a few months.

21. Paul doesn’t like his new job. Sometimes he (вынужден) work at weekends.

22. This restaurant (должно быть) very good. It’s always full of people.

Ex. 2. Translate into Russian.

1. Water has to be thoroughly analysed during the epidemics.

2. Account has to be taken of the elastic properties of the body.

3. The lamprey must have existed at an early period.

4. Attention was to be paid to the climatic conditions.

5. This species cannot have emigrated from the North by two different routes.

6. Care is to be taken not to use methods of growing tissues on the surface of agar.

7. We know that previous to mutation there must have been a normal gene in place of the mutant gene.

8. The majority of the plant names have been taken straight from the chemical literature and taxonomists are bound to find some difficulty in interpretation.

9. Most species comprised in group 3 may have a very complicated origin.

10. It is to be regretted that the deserved attention was not given to their results.

11. It is to be emphasized that some acids are found in very few species.

12. It is to be noted that the aim was to be demonstrated the production of cells.

Ex. 3. Choose and use the appropriate modal verb from given in brackets.

1. These cells (need, can must, to have to be easily found in the roots as well as in the stem of that plant.

2. This plant (must, can, should) grow under such conditions rather well.

3. The roots of this plant (must, can, may, need) be used for human food.

4. Every student (may, can, should) know the difference between the leaves of oak and birch.

5. Such plants (need, must, should) have been cultivated by man long ago.

6. When water (must, may, to be (to), need) to be used for drinking it is necessary that microbes which it (to be (to), to have (to), may) contain (should, must, may) be killed.

7. You will (may, to have (to), to be (to), must) to study the characteristics of both plants as there (may, to be (to), must, can) be a considerable difference between them.

8. One (can, ought to, may, to have (to), should, must) to remember that this reaction is often followed by an explosion.

Ex. 4. Translate the sentences paying attention to Modals.

1. A great series of chemical changes has to occur before food becomes protoplasm.

2. These plant parts may have been found on and below the soil surface.

3. This list could be extended a long way.

4. Living protoplasm must always be thought of as a system in dynamic equilibrium.

5. Different techniques have to be used with microorganisms.

6. Hybrids may show characteristics of both parents.

7. The development of this disease must have been considerable favoured by high moisture conditions.