The size, shape, and arrangement of Bacterial cells

The size, shape, and arrangement of Bacterial cells

Among major characteristics of bacterial cells are the size, shape, structure, and arrangement. These characteristic constitute the morphology of the cell. Depending on the species individual cells are spherical rodlike, or helical, although many variations of these three basic shapes occur. Furthermore, in certain species of bacteria the cells are arranged in groups, the most common of which are pairs, clusters, chains, trichomes and filaments. It is important to recognize these patterns of shape and arrangement, since they are often characteristic of a taxonomic group, e.g. a genus. Some bacteria also possesses appendages, which can be made visible by special staining techniques or by electron microscopy. All of these morphological features are regarded as the gross morphological characteristics of the bacterial cell.

The bacterial cell possesses a detailed internal structure. The discovery of this internal structure was made possible by the development of electron microscopy techniques and of instruments for slicing a bacterial cell into extremely thin sections. The terms microbial cytology and bacterial anatomy have become commonplace in microbiological literature.The various structures of a bacterial cell differ from one another not only in their physical features but also in their chemical characteristics and in their functions. Thus biologists today seek to integrate the structural, chemical and functional properties of the bacterial cell. This area of research studied by biologist is sometimes referred to as biochemical cytology.

Size

Bacteria are very small, most being approximately 0.5 to 1.0m in diameter. An important consequence of the small size of microorganisms is that the (surface area/volume ratio ) of bacteria is exceedingly high compared to the same ratio for larger organisms of similar shape.Invisible to the human eye, bacteria are usually measured in micrometer, which is equivalent to 1/1000 nm. Bacterial cells vary in size depending on the species. For example, staphylococci and streptococci are spherical bacteria with diameters ranging from 0.75 to 1.25. Cylindrical typhoid and dysentery bacteria are 0.5 to 1 in width and 2 to 3 in length. Cells of some bacterial species are 0.5 to 2 in diameter but more than 100 in length. Assuming a diameter or length of 1 , 10000 bacteria lying end to end or side by side would span only 1 centimeter.

If one compares bacterial surface area and cell volume, a distinctive feature of bacterial cells becomes evident. The ratio of surface area to volume for bacteria is very high compared with that of larger organisms of similar shape. In practical terms, this means that there is large surface through which nutrients can enter relative to a small volume of cell substance to nourished. This characteristic accounts in part for the high rate of metabolism and growth of bacteria. The rapid growth of bacteria is one of the reasons these microorganisms are so frequently used in molecular biology research. The rapid replication of bacterial cells is used in experiments to provide more information more quickly. For example, the bacterium Escherichia Coli undergoes cell division in about 20 minutes, while a mammalian cell in laboratory culture takes about 13 to 24 hours to divide into two cells.

Shape

Not all bacteria look alike. Individual bacterial cells have one of these basic shapes: they are either spherical, cylindrical, or spiral. Spherical cells are called cocci. They are usually round but they can be ovoid or flattened on one side when they are adhering to another cylindrical or rod-like, the bacterial cell is called bacilli. They are a considerable difference in the length and width of the various species of bacillus. The ends of some are square , others rounded and still other tapered or pointed spiral or helical bacteria look like corkscrews and are called spirilla. There are many modifications of these three basic forms. For instance, Pasteuria has pear-shaped cells, whereas chrysophanol has disk-shaped cells arranged like stacks of coines. Although most bacterial species have cells that are fairly constant in shape, some species can have a variety of cell shapes and are thus termed pleomorphic.

Arrangement

If you look through the microscope at microbial cells, you will see that they are often attached to each other. While spiral-shaped bacteria usually do occur as single cells, other species of bacteria may grow in characteristic arrangements or patterns. For instance, cocci can grow in several arrangements, depending on the plane of cellular division and whether the daughter cells stay together following cell division.

Each of these arrangements is typical of a particular species and can be used in identification. When a coccus divides in one plane it forms a diplococcus or two cells joined together. This typifies some species of Neisseria, including the one that causes gonorrhea. When a coccus divides in one plane and remains attached after several divisions to form a chain, it has the Streptococcal arrangement. Streptococcus species, like those that causes throat and wound infections, shows this pattern during their growth.

Together the size, shape, and arrangement of bacteria constitute their gross morphology, their outside appearance. But a closer look at the individual cell structures gives a better idea of how bacteria function in their environment.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.