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.
3. 2 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; which are prokaryotic and eukaryotic.
. Prokaryotes: These microbial cells lack membrane-bound nucleus
and organelles.
. Eukaryotes: Possess a membrane-bound nucleus and organelles.
Table 1: Difference
Properties
es between prokaryotic and
Prokaryotic cell
eukaryotic cells
Eukaryotic cell
Nuclear
membrane
Absent
Present
Chromosome
Single, closed, circular, dsDNA
Multiple, linear, dsDNA
Ribosomes
70s (50s+30s)
80s (60s+40s)
Mitochondrion
Absent
Present
Examples
Bacteria
Fungi, plant cells, animal cells, algae
Figure 1: A prokaryotic (A) and eukaryotic cells (B).
Source: http://www.thepsimple-homeschool.com/imagefiles/labeled_cell.gif
3. 3 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 kingdoms system of classifying organisms in which we have:
i.
Monera
ii.
Protista
iii.
Fungi
iv.
Planta
v.
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:
i. Bacteria (prokaryotic – “true bacteria”)
ii. Archaea (prokaryotic – “ancient bacteria”)
iii. Eucarya (eukaryotic)
Domain Bacteria
i. They are prokaryotic.
ii. They are single celled organisms.
iii. They lack membrane bound nucleus and organelles.
iv. Most have cell wall that contains peptidoglycan.
v. They are found in the soil, water and air and on other living
organisms.
vi. Some are harmful while others are beneficial to man.
Domain Archaea
i. They were formerly known as archaeobacteria.
ii. They are prokaryotic.
iii. They are single celled organisms.
iv. They lack membrane bound nucleus and organelles.
v. They lack peptidoglycan in their cell walls.
vi. They have unique membrane lipids.
vii. Some have unusual metabolic characteristics, e.g. methanogens
which generate methane gas. viii. Many are found in extreme environments.
The major differences between domain archaea and domain bacteria are:
i. Differences in ribosomal RNA sequences.
ii. The absence of cell wall peptidoglycan.
iii. The presence of unique membrane lipids.
Domain Eucarya
The major groups of microorganisms in this domain are protists and fungi.
Protists
These groups of microorganisms consist of unicellular algae, protozoa, slime moulds and water moulds.
Algae
i. They are simple organisms.
ii. Mostly unicellular.
iii. They are photosynthetic together with cyanobacteria.
iv. They produce about 75% of the plant’s oxygen.
v. Commonly found in aquatic environment.
vi. They are primary producers in food chains in aquatic habitat.
Protozoa
i. They are unicellular.
ii. Eukaryotic organisms and animal like.
iii. They are usually motile.
iv. 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
i. These are microorganisms that range from unicellular forms like
yeasts to moulds and mushrooms which are multicellular with thread like structures called hyphae.
ii. They absorb nutrients from their environments.
iii. Many play beneficial roles while others cause diseases in plants,
animals and human.
Viruses
i. They are acellular entities (non-cellular).
ii. They lack the fundamental structure of living cell but only carry
out functions of living organisms when in living cells.
iii. They are the smallest of all the microorganisms (10,000 smaller
than a typical bacterium).
iv. They can only be seen by the electron microscope.
v. They cause many diseases of plants, animals and humans.
vi. Entities are not placed in any of the domain but are classified on
a separate system.
vii. They cause many diseases of plants, animals and humans.
SELF-ASSESSED EXERCISE
i. Give three examples of diseases caused by each classification of
microorganism
ii. Answer the following questions:
. Define the term, microorganism (LO1)
. Compare and contrast prokaryotic and eukaryotic cells
(LO3)
. List the three domains under which microorganisms are
classified (LO4)
. List three characteristics each of the following domains:
. Bacteria
. Archaea
. Fungi (LO5).
. What are the differences between bacteria and archeae?
(LO5).
. Why are viruses not placed in any of the domains? (LO6).
4.0 CONCLUSION
Microorganisms are life forms that cannot be seen with naked eyes. They include bacteria, viruses, fungi and protozoans placed in three main domains namely Bacteria, Archaea and Eucarya.
5.0 SUMMARY
In this unit, you have learnt about microorganisms, their classification systems and the three main domains where they are placed.
6.0 TUTOR-MARKED ASSIGNMENT
With the aid of a well labelled diagram, distinguish between a bacterial cell and a eukaryotic cell.
7.0 REFERENCES/FURTHER READING
Atlas, R.M. (1995). Microorganisms in Our World. Mosby Year Book. Inc.
Kayser, F.H., Bienz, K.A., Eckert, J. & Zinkernagel, R.N. (2005). Kayser, Medical Microbiology. Thieme, Stuttgart, Germany
Markey, B., Leonard, F., Archambault, M., Cullinane, A. & Maguire, D. (2013). Clinical Veterinary Microbiology. 2nd Edition. MOSBY ELSEVIER
Medigan, M.T. et al. (2009). Brock Biology of Microorganisms. 12th Edition. Pearson Education Inc. Pelczar, M.J., Chan, E.C.S. & Krieg, R.N. (2001). Microbiology. 5th Edition. McGraw-Hill.
Mitchell, R. & Gu, J.-D. (editors) (2010). Environmental Microbiology. 2nd Edition. Hoboken, New Jersey, John Wiley & Sons, Inc, Wiley-Blackwell
Struthers, K. (2017). Clinical Microbiology. 2nd Edition. CRC Press, Taylor & Francis Group.
Talaro, K.P. & Talaro, A. (2002). Foundations in Microbiology. 4th Edition. The McGraw-Hill Companies.
Willey, J.M., Sherwood, L.M & Woolverton, C.J. (2008). Microbiology. 7th Edition. Boston Bur Bridge, IL: McGraw-Hill Higher Education.
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