III Read and translate the text about classification of shapes of bacteria

Identify the three basic shapes of bacteria

Bacteria come in a great many sizes and several shapes. Most bacteria range from 0.2 to 2.0 m in diameter and from 2 to 8 m in length. They have a few basic shapes: spherical coccus (plural: cocci, meaning berries), rodshaped bacillus (plural: bacilli, meaning little staffs), and spiral. Cocci are usually round but can be oval, elongated, or flattened on one side. When cocci divide to reproduce, the cells can remain attached to one another. Cocci that remain in pairs after dividing are called diplococci; those that divide and remain attached in chain like patterns are called streptococci.

Those that divide in two planes and remain groups of four are known as tetrads. Those that divide in three planes and remain attached in cubelike groups of eight are called sarcinae. Those that divide in multiple planes and form grapelike dusters or broad sheets are called staphylococci. These group characteristics are frequently helpful in identifying c ertain cocci.

Bacilli divide only across their short axis, so there are fewer groupings of bacilli than of cocci. Most bacilli appear as single rods. Diplobacilli appear in pairs after division, and streptobacilli occur in chains. Some bacilli look like straws. Others have tapered ends, like cigars. Still others are oval and look so much like cocci that they are called coccobacilli.

" Bacillus" has two meanings in microbiology. As we have just used it, bacillus refers to a bacterial shape. When capitalized and italicized, it refers to a specific genus. For example, the bacterium Bacillus allthracis is the causative agent of anthrax. Bacillus cells often form long, twisted chains of cells.

Spiral bacteria have one or more twists; they are never straight. Bacteria that look like curved rods are called vibrios. Others, called spirilla, have a helical shape, like a corkscrew, and fairly rigid bodies. Yet another group of spirals are helical and flexible; they are called spirochetes. Unlike the spirilla, which use propeller-like external appendages called flagella to move, spirochetes move by means of axial filaments, which resemble flagella but are contained within a flexible external sheath.

In addition to the three basic shapes, there are star-shaped cells; rectangular, flat cells of the genus Haloarcula; and triangular cells. The shape of a bacterium is determined by heredity. Genetically, most bacteria are monomorphic; that is, they maintain a single shape. However, a number of environmental conditions can alter that shape. If the shape is altered, identification becomes difficult. Moreover, some bacteria, such as Rhizobium and Corynebacterium , are genetically pleomorphic, which means they can have many shapes, not just one.

IV Remember the following word and word-combinations:

Identify, shape, spherical coccus, rodshape, attached in chain, axis, anthrax, twist, helical shape, rigid bodies, flexible bodies, filaments, external sheath, rectangular, triangular, heredity.