The major characteristics of microorganisms fall into the following categories
Cell shape, size and structure, cell arrangement, occurrence of special structure and developmental form, staining reaction and motility and flagella arrangement.
Unlike other kind of characters, morphological characteristics can be understood by studying individual cell or colonies under microscope. To get it done, we need to get pure culture of that particular microorganisms. So in studying morphological characteristics required more preparation.
The various chemical constituents of the cell.
Microbial cells are made up of variety of organic compounds. When the cell is studied deeply, it is found that every organism have some characteristics chemical composition. Both qualitative and quantitative difference occur in composition of various species of microorganisms.
For example, the occurrence of lipopolysaccharide in cell walls is characteristic of Gram-negative bacteria but not Gram-positive bacteria.
On the other hand many Gram-positive bacteria contain cell wall that contains theichoic acids, compound not found in Gram-negative bacteria.
Fungal and algae cell wall is quite different then those of bacterial cell wall. A major distinction among viruses is made on the basis of the kind of nucleic acid they possess.
Nutritional requirements and physical conditions required for growth, and the manner in which growth occur.
Each kind of microorganisms have specific growth requirements. Many microorganisms can be grown in culture media. Some microorganisms can grow in a medium containing organic compounds. Whereas some can grow on a medium containing organic compounds.
In addition to culture media some microorganisms require special physical conditions as well to grow well. For example, some bacteria can grow well oh higher temperature but cannot grow below 40°C.
Each kind of microorganisms grow in characteristic grows in a characteristic manner. For example, growth in a liquid medium may be abundant or sparse. It may be evenly distributed throughout the medium, or it may occur only as the sediment at the bottom or only as a thin film.
Colonies are characterised by their size, shape, texture, consistency, color and other notable features.
The way in which cells obtain and use their energy, carry out chemical reactions and regulate these reactions.
The life processes of the microorganisms are complex and integrated series of chemical reaction, collectively refer as metabolism. On the bases of different metabolic characteristics microorganisms can be divided into various groups. And can be easily identified as well.
The various chemical reactions of bacterial metabolism are catalysed by different proteins called enzymes. On the bases of presence of particular enzyme or absence of that enzyme can help in further grouping of microorganisms.
Special large chemical components of the cell, distinctive for certain kind of microorganisms.
Antigenic characterization of microorganisms have a great particular importance. Antigens are the chemical compounds or proteins present on the surface of microorganisms. When a certain microorganisms enter in human or animal cell, presence of antigen can trigger antigen antibody reaction in the body of host. And this can be easily identified to analyse further about microorganisms.
Characteristics of the hereditary material of the cell and occurrence and function of other kind of DNA that may be present, such as plasmid.
Every microorganisms have certain genetic material, however most of microorganisms have DNA as a genetic material but some of them may contain RNA as a genetic material. Identification of genetic material in microorganisms may categorised them in different groups.
In addition to chromosomal DNA some bacteria also contain plasmid DNA. Which have its own roal in the bacterial cell. Plasmid are circular DNA molecules those have the capacity to divide within the bacterial cell and can be transferred from one cell to other bacterial cell.
Presence of plasmid DNA in bacterial cell provide them some special features such as ability to make toxins, to become resistant to various antibiotics, or to use unusual chemical compounds as nutritiants.
The ability to cause disease in various plants and animals or even other microorganisms.
Pathogenicity of microorganisms is also a characteristic feature of them. Some of them can cause disease while some other live as symbiotic fashion. Some microorganisms are also opportunistic pathogen. Those may live as symbiotic till there number is not so high to be pathogenic.
On the bases of pathogenicity microorganisms have been classified.
Habitat and the distribution of the organism in nature and the interactions between and among species in natural environments.
Habitat of microorganisms is important in characterizing that organism.
The microbial population of oral cavity is different than those in intestine. Some kind of microorganisms are widely distributed in the nature while other are limited to some specific temperature and nutritional requirement. On studying their behaviour against the environment they may be classified accordingly.
Reference: Microbiology Pelczar