Are all bacteria harmful?

Although human interest in bacteria frequently focuses on their harmful effects, most bacteria are harmless to human beings, and many of them are actually beneficial. Saprophytic bacteria, for example, perform an ecologically indispensable role in the breakdown of dead organisms and organic wastes; with out such agents of decomposition, the cycling of various elements vital to living organisms (including nitrogen, carbon, and phosphorus) would cease in the biosphere.

How are bacteria used?

Bacteria also form highly beneficial associations with animals. For instance, the bacterial inhabitants of the ruminant stomach break down cellulose; this enables cows, sheep, and other ruminants to digest grass. Humans also harbour beneficial bacteria, such as those in the lower intestine that synthesize vitamin K. Bacteria are also used in various industrial processes, especially in the food industry; the production of buttermilk, yogurt, cheeses, pickles, and sauerkraut are all dependent upon bacterial action.

What is the scientific classification of bacteria?

The scientific classification of bacteria is in transition, particularly at the higher taxonomic levels. The bacteria form the only prokaryotic kingdom, that of the Monera. Within this kingdom, at least two groups have been distinguished, the eubacteria and archaebacteria. DNA hybridization studies of ribosomal RNA have proven useful in defining these groups.

5. Read the text and find English equivalents to the Russian sentences given below.

Fish

Fish are vertebrates (animals with a backbone) that are completely adapted for life in water. They breathe under water and have a streamlined body for swimming. They have fins to propel, balance, and steer themselves. Fish are found in both freshwater and marine habitats. Sharks and rays belong to an ancient group of fish that existed 200 million years before dinosaurs. They are called cartilaginous fish because they have a skeleton made of cartilage, a kind of gristle. Their big, oily liver helps them float, and they have a skin with toothlike scales.

1. У них есть плавники, при помощи которых они передвигаются, держат равновесие и управляют направлением движения.

2. Их называют хрящевыми рыбами, так как у них скелет состоит из своего рода хрящей.

3. Они дышат под водой и имеют обтекаемое тело для плавания.

4. Их большая жирная печень помогает им держаться на воде, а кожа покрыта зубчатой чешуей.

5. Средой обитания для рыб может как пресная, так и морская вода.

6. Read the text, tell about characteristic features of amphibians and the differences between frogs and toads; find English equivalents to the Russian expressions:

Выпученные глаза;

Припадающий к земле корпус;

Перепончатые лапы;

Бородавчатая кожа.