There are about 5 billion microbes in the world today and their number is constantly increasing. But, in current list, the diversity of microorganisms like bacteria, fungi, viruses etc are incomplete and deficient in many aspects.
This problem arises due to difficulties of their detection in and recovery from the environment, incomplete knowledge about obligate microbial associations and problem of the species concept of these groups. The smallest unit of microbial diversity is a species. Bacteria, for example, comprise a major division of microbes, which lack sexuality and fossil' records etc.
These are characterized as a group of similar strains distinguished from similar other groups of strains by genotypic, phenotypic and ecological characteristics. Such definitions can be applied to other categories of microbes. To date, more than 69000 species in nearly 5100 genera of fungi and 4760 species of about 700 genera of bacteria have been described in the literature.
These numbers represent a small fraction of these organisms which actually exist in nature. In order to identify and classify the diverse kinds of microorganisms, the principal pre-requisite is to know their characteristics. The major characteristics of the microbes fall into following categories.
1. Morphological characteristics- Cell shape, size and structure; cell arrangement; occurrence of special structures and development of forms; staining reactions and motility and flagella arrangement.
2. Chemical composition- The various chemical constituents of the cells.
3. Cultural characteristics- Nutritional requirements and physical conditions required for growth and manner in which growth occurs.
4. Metabolic characteristics- The way, in which the cells obtain and use their energy, carry out chemical reactions and regulate these reactions.
5. Antigenic characteristics- Special large chemical components i.e. antigens of the cell, distinctive of certain kinds of microorganisms.
6. Genetic characteristics- Characteristics of hereditary material of the cell like DNA and occurrence of other kinds of DNA that may be present such as plasmids.
7. Pathogen city - The ability to cause disease among plants and animals or even on microorganisms.
8. Ecological characteristics - Habitat and distribution of the organism in nature and interaction between and among species in the environment.
Each of the characterizes is elaborated in the following.
1. Morphological characteristics
The determination of the morphological features usually requires studying individual cells of the microbes. Being very minute in size, it requires high resolution microscopy.
The discovery and use of electron microscopes have given added advantage in getting proper morphological features of microbes. Up to the date, the typical microbial structures have been studied using special staining techniques and microscopy.
2. Chemical characteristics
Microbial cells contain variety of compounds. Further, each microorganism has characteristic chemical composition. For example, the occurrence of lip polysaccharide in the cell walls is distinctive of gram negative bacteria.
Fungal and algal cells are very different in composition from those of bacteria. Viruses show major difference in the occurrence of nucleic acids (RNA or DNA).
3. Cultural characteristics
Each microbe has specific growth requirement. Many microorganisms can be grown in culture media, which are a mixture of nutrients used in the laboratory to support their growth and multiplication.
Some can grow in a medium containing only inorganic compounds while others require organic compounds like amino acids, carbohydrates and vitamins etc. Some can grow with natural media like blood cells, malt extracts.
Still others grow in synthetic or semi synthetic media. There are certain microbes, which can not grow in the laboratory media but always need a living host or living cell. Viruses come under such category.
In addition to the nutrients, microorganisms require certain characteristic environmental parameters like temperature, pH, supply of oxygen etc. for their growth in culture media.
4. Metabolic characteristics
The life processes of microbial cells are complex integrated series chemical reactions termed metabolism. The varieties of these reactions provide many opportunities to characterize and identify the organisms. For example, there are certain bacteria, which can independently fix atmospheric nitrogen, yet there are others which require some type of symbiotic association with angiosperms for the same purpose.
Proteins called enzymes catalyze these chemical reactions. The enzyme make up show variability in function of the organisms, some metabolic and physiological characteristics used in the classification of microbes are C and N sources, cell wall constituents, energy sources, fermentation products, general nutrition types, growth temperature optimum and range, luminescence, mechanism of energy conversion, motility, osmotic tolerance, oxygen relationships, pH optima and growth range, photosynthetic pigments, secondary metabolites formed etc,
5. Antigenic characteristics
Certain chemical compounds of the microbial cells are called antigens. When a microbe enters the body of an animal, the animal responds with antibodies. Antibodies are highly specific for antigens, which induce their formation. This can help in the identification of the microbe. Depending on this principle, killer diseases like small pox have been eradicated.
6. Genetic characteristics
The double stranded chromosomal DNA of every kind of microorganism has certain features that are constant and specific for the organism and useful for the classification.
I. DNA base composition- DNA molecule is made up of base pairs: - guanine- cytosine, adenine-thymine. Of the total nucleotides present in DNA ratio percentage of G+C to A+T is characteristic of the species.
II. The sequence of nucleotide bases in the DNA- The sequence of the nucleotide bases is unique for each kind of microorganism and fundamental of all characteristics of an organism.
7. Pathogen city
The ability of an organism to cause disease is otherwise called pathogen city. Certain microbes like bacteria and viruses are pathogenic to humans and still others such as fungi are pathogenic to plants.
8. Ecological characteristics
On the basis of the habitat, microorganisms show variability. The microbes, which occur in the marine environment show difference from those, occur in the fresh water environment.
Some microorganisms are widely distributed in nature; others are restricted to a particular environment. Some only grow in living hosts while others grow initially in living hosts and when they die, the organism converts to saprophytic life, i.e. by drawing nourishment from the dead.
Depending upon the above characteristics, the world of microbes can be classified into various major groups, sub-groups etc. The principle and units of classification is detailed below.