The study of bacteria is called bacteriology. Bacteria are unicellular organisms and possess a distinct cell wall. They are included in kingdom Prokaryote. The bacterial cells are capable of independent existence, growth, multiplication and carry other vital activities.
Morphology: There are three basic forms of bacteria, viz. spherical (cocci), rod shaped (bacilli), spiral (spirochaetes) and comma (,) shaped (vibrio) or filamentous. The organisms may be arranged singly, in pairs, small or long chains or clusters.
Chemical composition: The bacterial cell contains nitrogen, carbon, oxygen and hydrogen. The water content of most of the species of bacteria varies from 75% to 85%. Inorganic substances are also found in bacteria. The organic part of the dry matter of bacteria consists of proteins (50%-80%), nucleic acids (10%-30%), carbohydrates (12%-18%), liquids (10% or more) and other compounds. Protein is main basis of their constitution. Bacterial proteins are highly specific in character, so that when introduced into the body of man or animals they act as antigens and stimulate the production of specific antibodies adapted to reacting with the bacterium and modifying its activities. In addition to proteins, lipopolysaccharides also act as distinct antigens. A species may have more than one specific antigen, thus being capable of stimulating production of antibodies corresponding to each antigen. This forms the basis of protection of man and animals through the use of sera or vaccines prepared from bacteria against specific diseases caused by them.
Cell structures: Bacteria have a rigid cell wall except mycoplasmas. The cell wall, an essential structure, protects the delicate cell protoplast from osmotic lysis. The cell protoplasm is surrounded with a thin plasma membrane. Gram-positive cell walls are composed of the amino sugars N-acetylglucosaine and N-acetylmuramic acid (muramic acid). Muramic acid (murein) is a unique component of procaryotic cell walls which serves to distinguish procaryotic from eukaryotic cells. Gram-negative bacteria as compared to Gram positive cell walls are relatively thin and are composed of a thin layer of murein surrounded by a membranous structure called the outer membrane. The outer membrane of Gram-negative bacteria invariably contains a unique component, lipopolysaccharide (LPS or endotoxin), which is toxic to animals and even cause death in diseased animals.
The prokaryotic bacteria differ from eukaryotic cells. Unlike plants and animals, bacteria are unicellular organisms that do not develop or differentiate into multicellular forms. Some bacteria grow in filaments or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remain enclosed in a common sheath or slime secreted by the cells, but typically there is no continuity or communication between the cells. The protoplasm of a cell consists of a watery sap packed with ribosomes (70s), vacuoles and a few convoluted membranous bodies called mesosomes. The nucleus is a simple long molecule of double-stranded deoxyribonucleic acid (DNA) in the form of closed circle. Several extra chromosomal DNA may be present in some bacteria called plasmids. Outside the cell-wall there may be a covering known as capsule. The disease causing ability of some bacteria may depend on the presence of capsule. Some bacteria (Bacillus and Clostridia) develop spores, a resistant state in which organism can survive for a long period of starvation or other adverse environmental conditions.
Surface appendages: Some bacteria have appendages called (i) flagellae, these are filamentous protein structures attached to the cell surface that provide swimming movement for most motile bacterial cells, and (ii) fimbriae or pilus is the organs of adhesion. Adhesion by many bacteria is essential for production of the disease in animals. A specialized type of pilus, the F or sex pilus, in some way facilitates the transfer of DNA between mating bacteria. The transfer of resistance to antibiotics among certain bacteria may be attributed to this structure.
Nutritional requirements: On the basis of oxygen requirement, the bacteria may be (i) aerobes, which need the presence of oxygen, (ii) anaerobes, which cannot survive in the presence of oxygen, (iii) microaerophilic which require slightly higher carbon dioxide tension than normally present in the air, and (iv) facultative anaerobes, which can live both aerobically as well as anaerobically. According to nutritional requirements, the bacteria are divided into two classes : (i) Autotrophic- bacteria synthesize the component parts of their cell by absorbing carbon dioxide, water and simple nitrogen compounds from the environment. These do not cause disease in animals or man. Nitrifying and many sulphur bacteria belong to this class. (ii) Heterotrophic- bacteria require complex organic substances as a source of carbon, various nitrogen compounds, inorganic substances, trace elements and vitamins which are essential for growth.
Reproduction: Bacteria reproduce by simple (binary fission) transverse division, which occurs in different planes and thereby producing many kinds of cells like chains, clusters, pairs and packets. Sometimes bacteria also reproduce by means of cleavage of segmented filaments and by conjugation. The rate of cell division is very fast in bacteria, almost 100 times faster than cells in tissue culture.
Cultivation: The bacteria can be cultivated in artificial medium in the laboratory. The media could be classified into basal media like peptone water, enriched media which contain additional nutrients such as serum broth, blood agar. Liquid enrichment media contain a substance which will selectively promote the growth of a particular organism and prevent others from competing for the nutrients, such as selenite broth and tetrathionate broth. Selective media which are solid enrichment media are MacConkey agar, Brilliant green agar, etc or Indicator media such as triple sugar iron (TSI) agar or brilliant green agar which would indicate the presence of a particular type of organism. Some media could be enriched medium as well as the indicator medium such as blood agar. Similarly, the same medium could be a selective and differential with an indicator medium. The organisms can be isolated in pure culture and identified by their morphology, colony characters, biochemical and serological tests.