General characteristics of Micro-Organisms based on their occurrence and structure

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Microorganisms: Characteristics, Importance and Control

Microbiology (Greek, mikros—small; bios-life) is the science of small or microscopic organisms. The most important microorganisms relevant to biotechnology include bacteria, fungi, and viruses.

Microorganisms are very widely distributed, and are found almost everywhere in nature. In general, the conditions for their growth and multiplication (food, temperature, moisture etc.) are similar to that of humans. Hence, they are most abundantly present at places where people live.


Bacteriology, the study of bacteria, forms a major part of microbiology. The population of bacteria exceeds all other organisms. For instance, one kg of a soil may contain more bacteria than the entire human population.

Bacteria are very important in biotechnology for the following reasons:

i. Diseases they cause

ii. Domestic uses

iii. Industrial applications

iv. Agricultural processes.

Characteristics of Bacteria:

Bacteria are prokaryotic unicellular organisms. They lack organized nucleus, but possess a rigid cell wall comparable to that found in plants. The average size of a bacterium is around 2 µm. The bacteria may be spherical, rod-like, spirally coiled or filament like. Certain bacteria may occur in more than one form. A typical structure of a rod- shaped bacterium is depicted in Fig. 63.1.

Structure of a Rod-Shaped Bacterium

Gram-positive and Gram-negative bacteria:

Based on the response to Gram’s stain, the bacteria are grouped as Gram +ve or Gram -ve. By Gram’s stain, several distinguishing features of bacteria can be identified. For instance, Gram +ve bacteria possess single-layered cell wall while Gram -ve bacteria have a double-layered one.

Aerobic, anaerobic and facultative bacteria:

On the basis of respiration (i.e. response to O2), the bacteria are grouped into three categories

1. Aerobic bacteria:

These bacteria require O2 for their growth e.g. Pseudomonas sp, Mycobacterium sp.

2. Anaerobic bacteria:

These bacteria do not require O2 to obtain energy, and to grow. In fact, the presence of O2 is toxic to them e.g. Peptococcus sp.

3. Facultative bacteria:

The bacteria that can grow in both aerobic and anaerobic conditions are regarded as facultative bacteria, e.g. Shigella sp, Salmonella sp.

Nutritional aspects of bacteria:

Based on their nutrition, the bacteria are categorized as autotrophic or heterotrophic.

1. Autotrophic bacteria:

These bacteria are capable of synthesizing their own food from inorganic substances. They are compable to higher plants in this aspect. Autotrophic bacteria utilize different hydrogen compounds (not H2O as in the case of higher plants). These include hydrogen, ammonia, hydrogen sulfide and methane e.g. Hydrogenomonas sp, Nitrosomonas sp, Methanomonas sp.

2. Heterotrophic bacteria:

These bacteria cannot synthesize their own food, and are therefore dependent on the outside source. Heterotrophs are of two types—sporophytes and parasites. Sporophytes obtain their food from sources of animal or plant origin. These include organic remains like corpses, animal excreta, meats, fruits and various other products of plant and animal origin. Sporophytes secrete digestive enzymes that break the complex organic molecules into simpler and easily absorbable forms.

These heterotrophic bacteria are useful for the disposal of sewage, cleansing of leather, and manufacture of certain compounds (alcohols, organic acids). Sporophytes can also spoil foods and damage soils (by de-nitrification).

Parasites are the bacteria that obtain their food from living organisms, namely the hosts. They may be either harmless (non-pathogenic) or harmful (pathogenic) to the hosts. E. coli is a good example of non-pathogenic bacteria which has a symbiotic relationship in the human intestine. The pathogenic bacteria may cause serious diseases either by destroying the host’s cells or releasing toxins e.g. Clostridium tetani.

Importance of Bacteria:

Bacteria play a very important and crucial role in the continuous sustenance of life on earth. They are both friends and enemies of humans. Bacteria are important due to the diseases they cause, industrial and agricultural applications.

Diseases caused by bacteria:

The bacteria pose a serious threat to humans, animals and plants due to various diseases they cause. A selected list of the important diseases and the corresponding organisms is given in Table 63.1.

List of Diseases caused by Microorganisms

Industrial uses of bacteria:

An important aspect of biotechnology is the industrial use of bacteria for the production of several compounds. Traditionally, bacteria are in use for the preparation of curd, vinegar, pickles, curing tea and tobacco leaves.

In recent years, large-scale production of amino acids, organic solvents, vitamins and antibiotics is being carried out by employing bacteria. Bacteria play a significant role in the decay and disposal of dead plants and animals. Appropriate sewage disposal requires bacteria. Bacteria are also needed for the disposal of animal dung and human excreta.

Agricultural applications of bacteria:

Nitrogen fixation by converting free nitrogen into nitrogenous compounds is very important in agriculture. This is exclusively carried out by symbiotic nitrogen-fixing bacteria. In addition, nitrifying bacteria (convert ammonia to nitrates) and ammonifying bacteria (convert amino acids to ammonia) also contribute to agricultural practices.

Control of Pathogenic Bacteria:

Bacteria cause several diseases.

There are different approaches to control pathogenic bacteria:

i. Use of antibiotics.

ii. Treatment with antiseptics and disinfectants.

iii. Sterilization and autoclaving.

iv. Radiation treatment.

v. Biological control.

In addition to the above, vaccines have been developed against certain pathogens to control diseases in humans and animals.


Mycology, a branch of microbiology, deals with the study of fungi. Fungi are a group of eukaryotic, heterotrophic organisms. They are dependent on organic compounds for their nutrition. In general, fungi can withstand extreme environmental conditions better than most of the microorganisms.

Characteristics of Fungi:

Fungi cells are usually larger than the bacteria. The sizes may range 1-5 µm in width and 5-35 µm in length. Fungal cells may be elongated or spherical. The fungi are heterotrophic, since they cannot synthesize their own food from the inorganic compounds.

Importance of Fungi:

As is the case with bacteria, fungi are both friends and enemies of humans.

Beneficial aspects of fungi:

The yeasts are useful for the following:

i. Alcohol fermentation e.g. Saccharomyces cerevisiae

ii. Production of vitamins e.g. Ashbya gossypii.

iii. Citric acid fermentation e.g. Candida sp.

iv. Baker’s yeast e.g. S. cerevisiae.

The applications of molds are listed:

i. Production of enzymes e.g. Aspergillus sp.

ii. Citric acid fermentation e.g. Aspergillus niger.

iii. Penicillin production e.g. Penicillium notatum.

iv. Steroid transformation e.g. Rhizopus sp.

v. Gluconic acid production e.g. Aspergillus niger.

Harmful aspects of fungi:

i. Spoilage of foods e.g. moldy bread, rot of fruits and vegetables.

ii. Deterioration of textiles made up of cotton.

iii. Damage to paper.

iv. Diseases caused by fungi e.g. ring worm of the scalp in children caused by Microsporum audouinii.

Control of Fungi:

The fungal growth can be controlled by using phenol and its derivatives e.g. cresol, ethyl phenol, propyl phenol, butyl phenol. Chlorine and chlorine compounds are also useful in this regard. Vegetative cells of yeasts and other fungi can be destroyed by moist heat at 50-60°C for about 5-10 minutes. Spores however require higher temperature 20-80°C.


A virus may be regarded as a small microorganism with a nucleoprotein entity which lives and multiplies in the living cells of other organisms. Thus, viruses as are unable to live in a cell-free environment, and become active and multiply as they enter living cells.

Characteristics of Viruses:

Viruses are the smallest living entities. They do not possess the usual cellular structures. Viruses are composed of either DNA or RNA, and not both of them together. Retroviruses is the name given to viruses containing RNA as the genetic material. Viruses do not possess independent metabolisms, and further they also lack respiratory machinery.

Biological Status of Viruses:

Organismal theory:

Viruses are considered as primitive living microorganisms as they possess genetic material, biosynthetic machinery and certain enzymes, besides being infective.

Molecular theory:

Some people consider viruses are non-living chemical molecules rather than living beings. They argue that viruses do not possess any cellular structures, cannot respire and cannot live independently.

Importance of Viruses:

Viruses are associated with several diseases in humans and plants (Table 63.2). Besides the common diseases such as common cold, and rabies, viruses cause diseases such as AIDS. i.e. human immunodeficiency virus (HIV) is responsible for AIDS, the dreaded and incurable disease.