In this article we will discuss about:-1. Size of Bacterial Cell 2. Shape of Bacterial Cell 3. Arrangement.
Size of Bacterial Cells:
Individual bacterial cells are not visible to the unaided eye. In general, bacterial cells do not exceed 1 μm (micrometer or micron) in diameter, though their length may vary widely. Some bacteria discovered in recent years, are much larger than the common ones. For example, a bacterium named Epulopiscium fishelsohnii measuring 80 μm in breadth and 200 μm in length has been discovered in 1991 and another spherical archaebacterium, called Thiomargarita namibiensis has been isolated from sea-bottom in 1999. This organism measures 750 μm in diameter and is visible to the unaided eye. But such giants among bacteria are extremely rare exceptions.
The minute size of bacteria gives certain advantages to them. Due to their small size, bacteria have a much greater surface/volume ratio than most eukaryotic organisms having larger cells. This has important implications on the activity of cells. Because of a greater surface/volume ratio, the bacteria are able to absorb nutrients and gases from the surrounding media very rapidly and this ability is reflected in their faster growth rate in comparison to larger eukaryotic organisms.
For example, the rate of oxygen uptake (μl/mg dry weight/hr) of most aerobic bacteria is approximately 10 times faster than that of another unicellular but larger organism—yeast—and 100 times faster than the cells of animal tissues. This high rate of oxygen uptake reflects the fast rate of metabolic activity of bacteria. Many bacteria under optimal conditions of growth can have a doubling time of 20 to 30 minutes.
The small size also helps bacteria to spread quickly by air currents over great distances. In fact, bacteria are ubiquitous in distribution and they occur in all possible types of environments starting from arctic glaciers to boiling hot springs, and from upper levels of atmospheres to bottom of the seas.
Shape of Bacterial Cells:
Bacterial cells are bound externally by a rigid wall which gives bacteria their characteristic shape. The mycoplasmas are exceptions in this regard, because they lack a cell wall and they do not have also any characteristic shape. That the cell wall is responsible for giving shape to bacterial cells is also shown when the wall is removed by enzymes. A cylindrical bacterial cell on losing the wall assumes a spherical shape.
A bacterium can be spherical or cylindrical. A spherical bacterium is generally known as a coccus and a cylindrical bacterium, when it is straight, goes under the name of a bacillus. If the cylindrical cell is curved, it is known as a vibrio and if helical it is called a spirillum. In all these types, the cell is a rigid structure. Some bacteria have also flexible cells.
Helical bacteria with flexible cells are known as spirochaetes. A group of prokaryotic organisms, known as actinomycetes, characteristically consist of a mycelium, somewhat like that produced by fungi. A mycelium is composed of thin hairy hyphae. An actinomycete hypha is about 1 µm in diameter, whereas a fungal hypha is, on the average, 8-10 μm broad. A hypha is an elongated, often branched structure, divided into cells by formation of cross-walls.
Some hyphae enter into the substratum, while others remain in the aerial part. There are also bacteria which characteristically form filaments or trichomes. A trichome consists of a series of cells arranged one on the other to form a column. The trichome may in some forms be enclosed in a common sheath.
Apart from the variations of the forms, bacteria may also possess various irregular appearances which are, however, characteristic of a group. For example, a group of bacteria produces a narrow extension from the cell which is used either for attachment or for producing daughter cells by budding. Such extensions are called stalks or pros-thecae and the cells are stalked or prosthecate.
Forms of bacteria are shown in Fig. 2.1:
A single species of bacteria generally has a fixed form, but some may change their morphology during their growth cycle. Such bacterial species are called pleomorphic. Mycoplasmas, which lack a rigid cell wall, exhibit many types of cells without a definite shape and size. They are also pleomorphic.
Arrangement of Bacterial Cells:
Bacteria multiply by binary fission which means a bacterial cell divides to form two identical daughter cells. In many bacteria, the daughter cells separate from each other soon after the division. But in many others, the newly formed cells do not separate and adhere to each other to give characteristic cell arrangements. The different types of cell arrangement result from the cell division at different planes.
The cocci show several types of cell arrangements. In some, the cocci remain mostly as single cells. In that case they are called micrococci. When the cocci occur typically in pairs, they are called diplo- cocci. If the daughter cells remain in a chain, they are called streptococci.
If cocci divide in two planes, they produce irregular bunches of cells which are known as staphylococci. In case the cells divide in three planes mutually at right angles to each other, a cubical arrangement results and they are then called sarcinae.
These cell arrangements of spherical bacteria are shown in Fig. 2.2:
Generally, the cylindrical bacteria — bacilli, do not exhibit characteristic cell arrangements as the cocci do. Nevertheless, chains of cylindrical cells occur in many bacterial genera, particularly in the members of Lactobacillus and Bacillus e.g. Lactobacillus acidophilus and Bacillus anthracis. L. acidophilus, a lactic acid bacterium, is present in milk and milk products. B. anthracis is the causal organism of anthrax. The diphtheria bacilli, Corynebacterium diphtheriae, are characteristically arranged side by side like the matchsticks in a match box, or also at an angle to each other.
The cell arrangements of rod-shaped bacteria are shown in Fig. 2.3:
Cyanobacteria, also known as blue-green algae, are prokaryotic organisms. They show a great variation in cell morphology. Many of them are unicellular, while others are filaments. Both unicellular and filamentous forms are often enclosed in a gelatinous sheath.
Some representative forms of cyanobacteria are shown in Fig. 2.4:
