How to identify microorganisms? Here are eight major characteristics followed during identification
Introduction to microorganisms
Microorganisms, the living organism of microscopic size such as bacteria, fungi, protozoa, algae, and viruses, are associated with the health and welfare of humans. Some can be beneficial while others are detrimental. Microorganisms are used to make cheese, wine, yogurt, alcohol, penicillin, interferon, etc. On the other hand, microorganisms can cause disease, spoil food (may result in food poisoning), or cause degradation of materials like iron pipes, wood pilings, and glass lenses.
Most microorganisms are unicellular (all life functions are performed by a single cell) or multicellular. Viruses, simpler in structure and composition than single-celled organisms, are obligate parasites. While multiplying inside the appropriate host cells – plant, animal, or microbe – it directs the host cells to synthesize hundreds of identical viruses.
Identification of microorganisms
Since microorganisms play a significant role in human life, the identification of microorganisms is important. The identification of microorganisms is done on the basis of several major characteristics.
Morphological characteristics
Chemical composition
Cultural characteristics
Metabolic characteristics
Antigenic characteristics
Genetic characteristics
Pathogenicity
Ecological characteristics
Morphological characteristics
Morphological characteristics involve the study of individual cells grown in pure culture.
Since microorganisms are very small, their size is expressed in micrometers (µm), and a high-power microscope is used for visualization.
1 µm = 0.001 mm (millimeter) = 0.00004 in
Microscopes such as bright-field microscopes, dark-field microscopes, fluorescence microscopes, phase-contrast microscopes, and transmission electron microscopes are used for magnification and visualization.
Chemical composition
Each microorganism has a characteristic chemical composition which is different among various species.
Lipopolysaccharide (LPS), which is an important outer membrane component in cell walls, is found in prokaryotic Gram-negative bacteria.
Similarly, an anionic polymer known as teichoic acid is found only in the cell walls of prokaryotic Gram-positive bacteria.
The distinction of viruses is made on the basis of the type of nucleic acid they possess – ribonucleic acid (RNA), and deoxyribonucleic acid (DNA).
Eukaryotic organisms such as Fungai, which may be multicellular or unicellular, have glucans, chitin, and glycoproteins in their cell walls.
Algae, which are photosynthetic organisms have cellulose, hemicellulose, and pectins in their cell walls
Cultural characteristics
Microorganisms have specific growth requirements which can be exploited to culture them on a culture medium in a laboratory
The culture medium, which is a mixture of nutrients for the growth and multiplication of microorganisms, may contain only inorganic compounds, organic compounds (sugars, vitamins, coenzymes, amino acids, purine, pyrimidine), or complex substances (blood cells, peptone, serum, yeast autolysate).
Although most microorganisms are cultured in an artificial medium, some can only be cultured in complex medium i.e. living host/cells.
Eg: Rickettsia can only be cultured in live hosts such as fertilized chicken eggs, mammalian tissue lines, and arthropod
Another cultural characteristic is the physical conditions including temperature, gaseous atmosphere, light, etc.
On the basis of optimum temperature
* psychrophiles = grow at cold temperatures
* mesophiles = grow at moderate temperatures
* thermophiles = grow at high temperatures
On the basis of the optimum gaseous atmosphere
* obligate aerobes = require around 20% atmospheric oxygen
* microaerophiles = require between 5-15% atmospheric oxygen
* aerotolerant anaerobes = can grow in the presence of oxygen but do not use it for ATP production and instead produces ATP via anaerobic respiration or fermentation
* facultative anaerobes = grow equally well in aerobic or anaerobic environments
* obligate anaerobes = die in the presence of O2 as they lack the enzymes required to break down toxic forms of O2 or their intermediate byproducts. Produce ATP via anaerobic respiration or fermentation
Bacteria such as cyanobacteria require light as a source of energy while others may be indifferent to light or maybe even deleterious
On solid media, microorganisms grow as macroscopically visible colonies, which are compact and distinct masses of cells – characterized by their size, shape, color, consistency, texture, etc
In a liquid medium, microbial growth may be abundant or sparse, evenly dispersed or as a thin film, sediment, or as pellicle at the top
Metabolic characteristics
Metabolic byproducts released at the end of metabolism can be used to differentiate and identify groups of microorganisms
The chemical reactions of an organism during metabolism are catalyzed by proteins called enzymes.
Production of such enzymes, which are sometimes possessed by one kind of organism, is used in the identification process. Eg: Coagulase produced by Staphylococcus aureus.
Differentiation can also be made on the basis of the use of substances as a source of energy (organic, inorganic compounds), light absorption, enzyme regulation, cell component synthesization during growth, etc.
Antigenic characteristics
Microbial cells contain unique chemical compounds called antigens.
When antigens are introduced into the host body, the host immune system responds to those antigens by producing specific blood serum proteins called antibodies.
These particular antibodies bind to the antigens that induced its production and thus are used for the identification of microorganisms
Genetic characteristics
The genetic component, either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), is present in all forms of life – including microorganisms
These genetic materials have certain features that are constant and characteristic of that specific organism
Moles % G + C value for different microorganisms may range from 23 to 75
* Guanine-cytosine and Adenine-Thymine
It includes chromosomal DNA, plasmid DNA, autosomal DNA, mitochondrial DNA, messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
The genetic materials also contain the ability of an organism to make products such as toxins, antibiotic resistance, and use of unusual compounds as a source of energy
Pathogenicity
Pathogenicity, which is the ability of microorganisms to cause disease, is an important step in the identification and classification of microorganisms
Microorganisms may be pathogenic to animals, plants, and even other microorganisms
Eg: Bdellovibrios is a bacteria that is pathogenic to other bacteria, Bacteriophages are viruses that are pathogenic to specific bacteria
Ecological characteristics
The habitat in which any microorganism is found is important in characterizing that microorganism
Some microorganisms may be widely distributed while others might be restricted to certain geographical or ecological locations
Eg: marine microorganisms differ from freshwater microorganisms, and microbes in the oral cavity are different from the intestinal tract.