COMPOSITION OF THE MICROBIAL WORLD

INTRODUCTION

Microbiology is the study of microorganisms. These are organisms too small to be seen clearly by the unaided eyes. Microorganisms include bacteria, fungi, algae, protozoa and entities at the borderline of life that are called viruses. The cell is the fundamental unit of life. Most microorganisms are unicellular, in unicellular organisms all the life processes are performed by a single cell. However, some are multicellular, having more than one cell. This unit examines the definition of microbiology, types of microbial cells, the different groups of microorganisms and the domains in which they are placed and why viruses are not placed in any of the domains

Microorganisms

Microorganisms are organisms too small to be seen clearly by the unaided eyes. They are very small life forms so small that individual microorganisms cannot be seen without magnification. They include fungi, bacteria, algae, protozoa and viruses. Some microorganisms however, like the eukaryotic microorganisms are visible without magnification. Examples are bread moulds and filamentous algae.

 Microorganisms are Cells

The cell is the fundamental unit of life; a single cell is an entity isolated from other cells. Two fundamental different types of cells exist among microorganisms; they are prokaryotic and eukaryotic.

Prokaryotes

These microbial cells lack membrane-bound nucleus and organelles.

· Eukaryotes

Possess a membrane-bound nucleus and organelles.

 Differences between Prokaryotic and Eukaryotic Cells

There are notable differences between prokaryotic and eukaryotic cells.

Fig. 1(a): A Prokaryotic Cell 

Fig. 1(b): A Eukaryotic Cell 

SELF-ASSESSMENT EXERCISE

1.  Define the term, microorganism. 
2. Compare and contrast prokaryotic and eukaryotic cells. 

 Classification Systems for Microorganisms

The Five Kingdom System of Classification

Based on cell type and mode of nutrition, there was an establishment of the five kingdom system of classifying organisms in which we have:

1. Monera 
2. Protista 
3.  Fungi 
4. Planta
5.  Animalia 

Microorganisms except for viruses, which are acellular and have their own classification system, were placed in the first three kingdoms.

The Three Domains System of Classification

Presently, through advances in cell biology, biochemistry and genetics, microorganisms are now placed into three domains, each of which comprises of various kingdoms.

The domains are:

1.  Bacteria (prokaryotic – “true bacteria”) 
2. Archaea (prokaryotic – “ancient bacteria”) 
3. Eucarya (eukaryotic) 

Domain Bacteria

1. They are prokaryotic. 
2. They are single celled organisms. 
3. They lack membrane bound nucleus and organelles. 
4.  Most have cell wall that contains peptidoglycan. 
5. They are found in the soil, water and air and on other living organisms. 
6.  Some are harmful while others are beneficial to man. 

 Domain Archaea

1. They were formerly known as archaeobacteria. 
2. They are prokaryotic. 
3. They are single celled organisms. 
4. They lack membrane bound nucleus and organelles. 
5. They lack peptidoglycan in their cell walls. 
6.  They have unique membrane lipids. 
7.  Some have unusual metabolic characteristics, e.g. methanogens which generate methane gas. 
8.  Many are found in extreme environments. 

Domain archaea is distinguished from bacteria based upon 

1. Differences in ribosomal RNA sequences. 
2. The absence of cell wall peptidoglycan. 
3. The presence of unique membrane lipids. 

 Domain Eucarya

The major groups of microorganism in this domain are protists and fungi.

Protists

These groups of microorganisms are unicellular algae, protozoa, slime moulds and water moulds.

Algae

1. They are simple organisms. 
2. Mostly unicellular. 
3. They are photosynthetic together with cyanobacteria. 
4. They produce about 75% of the plant’s oxygen. 
5. Commonly found in aquatic environment. 
6. They are primary producers in food chains in aquatic habitat. 

Protozoa

1. They are unicellular. 
2. Eukaryotic organisms and animal like. 
3.  They are usually motile. 
4. Some are free living while some are pathogenic. 

Slime Moulds

They are protists which have different forms at different stages of their life cycles. At a stage they are like protozoa and at another stage like fungi.

Water Moulds

These are found on the surface of fresh water and moist soils. They feed on decaying vegetation such as logs and mulch.

Fungi

These are microorganisms that range from unicellular forms like yeasts to moulds and mushrooms which are multicellular with thread like structures called hyphae.

1. They absorb nutrients from their environments. 
2. Many play beneficial roles while others cause diseases in plants, animals and human. 

 Viruses

1. They are acellular entities (non cellular). 
2. They lack the fundamental structure of living cell but only carry out functions of living organisms when in living cells. 
3. They are the smallest of all the microorganisms (10,000 smaller than a typical bacterium). 
4.  They can only be seen by the electron microscope.
5. They cause many diseases of plants, animals and humans. 6. Entities are not placed in any of the domain but are classified on a separate system. They cause many diseases of plants, animals and humans. 

CONCLUSION

Microbiology is the study of microorganisms, most of which are unicellular while some are multicellular. Presently, they are classified under three domains. Viruses are classified under a separate system because they function only as living things when present in living organisms.