8.3 - Microorganisms and Viruses

As you would have already known, microorganisms are very small organisms which cannot be seen with our naked eyes. In order to study the structures of the microbes, a microscope is needed. Microorganisms can be categorized into 4 major types which are bacteria, viruses, protozoa, algae and fungi.

The Main Charateristics of Microorganisms and Viruses

Bacteria

• No apparent nucleus because they do not have nuclear membrane. DNA is enclosed in nucleoid. Some bacteria have additional genes in their plasmids.
• Small size between 1 to 10 µm in length.
• Normally exist as a single cell, a diploid (a pair), in filaments, chains or clusters.
• Exist in different shapes such as coccus (sphere), vibrio (comma), bacillus (rod/cylinder) and spirillum (spiral).
• Examples of bacteria include Lactobacillus sp., Streptococcus sp. and Staphylococcus aureus.

Protozoa

• Unicellular animal-like microorganisms.
• Move by using pseudopodia, cilia or flagellum.
• Most of them are found in aquatic habitats.
• Heterotrophic nutrition or autotrophic.
• Example of autotrophic protozoa is Euglena sp.
• Live freely or as parasites.

Algae

• Unicellular microorganism such as Chlamydomonas sp. and multicellular microorganism like Fucus sp.
• Some algae have flagellum for movement in the water.
• Algae are autotrophic organisms because they have chloroplasts. However, algae do not have real leaves, stems and roots like plants.
• Algae can be found in ponds, lakes and sea.

Fungi

• Unicellular or multicellular microorganisms.
• Heterotrophic organisms which can be parasitic or saprophytic. No chlorophylls.
• No roots, stems and leaves.
• Exist in the form of mycelium , made up of hyphae.
• Examples are Saccharomyces cerevisiae (yeast) and Mucor sp.
• Found in dark places or humid areas, dan on top of dead or decomposing organisms.

Viruses

• Not categorized into any of the kingsoms because they are not cellular organisms.
• Requie other living organisms to reproduce.
• Contain DNA or RNA and capsid that is made up of protein.
• Very small size (20 nm to 400 nm). Can only be seen using an electron microscope.
• Examples of viruses include tobacco mosaic virus, bacteriophage T4 dan HIV.

The Role of Microorganisms in the Nitrogen Cycle

Nitrogen is a very important macronutrient required for the growth of plants. Nitrogen in the form of ammonium ion (NH4+) or nitrate ion (NO3-) help in the synthesis of proteins in plants. The nitrogen cycle is a natural cycle that involves the conversion of nitrogen in the atmosphere to nitrate compounds by plants and animals.

Figure 4 - Nitrogen cycle.

1. Nitrogen-fixing bacteria such as Rhizobium sp. and Azobacter sp. that live in the roots of the legume plants will fix the atmospheric nitrogen to form NH 4+ ions via the nitrogen-fixing process.

2. Nitrogen will be oxidized to form nitrogen dioxide (NO2) by lightning in a thunderstorm which will then dissolves in the rainwater to form nitrous acid and nitric acid. Both of the acids will form nitrate salts in the soil. Industrial fertilizers also release ammonium and nitrate fertilizers into the soil.

3. When animals and plants die, decomposing microorganisms such as bacteria and saprophytic fungi will carry out decomposition. Protein compounds in the tissues will be broken down into NH4+ ions viaammonification.

4. Nitrifying bacteria, Nitrosomonas sp. will convert NH4+ ionsinto nitrite ions (NO2-) via nitrification. NO2- ionswill then be converted into nitrate ions (NO3-) by nitrifying bacteria, Nitrobacter sp. Then, NO3- ions will be absorbed by the plants to synthesize proteins.

5. When the plants are being eaten by other organisms, the nitrogen compounds will be transferred to the tissues of the organisms. The denitrifying bacteria will convert nitrate in the soil to nitrogen gas via denitrification process.

The Role of Microorganisms

Producers

• Phytoplankton acts as the producer in the food chain of an aquatic ecosystem.
• Phytoplankton contains chlorophyll and hence can carry out photosynthesis.
• Phytoplankton is commonly found floating on the surface of ponds, lakes or sea.
• Examples of phytoplankton are green algae, blue-green algae (cyanobacteria), dinoflagellates and diatoms.

Decomposers

• Saprophytic fungi and saprophytic bacteria are major decomposers in the ecosystems.
• Complex organic substances such as animal wastes, carcasses and rotting trees will be broken down into simpler compounds such as ammonium.
• During the decomposition process, decomposers will secrete digestive enzymes dan absorb the digestive products.
• The products of decomposition such as carbon, nitrogen and sulphur will be returned to the soil to be used by plants.

Symbionts

• Organisms that have a close relationship with their hosts.
• Two types of symbionts - ectosymbionts and endosymbionts.
• Ectosymbionts live outside the host cells. For example, fungi that live around the plant roots.
• Endosymbionts live inside the host cells. For instance, protozoa Trichonympha sp. lives in the alimentary canal of termintes

Figure 6 - Mutualism is a symbiotic relationship between the rhinoceros and oxpecker (Buphagus erythrorhynchus).

Parasites

• Parasites obtain benefits and nutrients from their hosts whereas the hosts suffer losses and possibly death.
• However, most of the parasites do not kill their hosts.
• For example, Plasmodium sp. is a type of protozoa that lives inside the femlae Anopheles sp. It can be transferred into the blood circulatory of an individual when he/she is bitten by the infected female mosquito.

Figure 7 - Fleas live on the body of animals like cats, dogs and others and suck their blood to obtain nutrients needed

Definition of Pathogens and Vectors

• Pathogen
  • organisms that can cause infectious diseases. examples of pathogens include viruses, bacteria, protozoa and fungi.
  • Pathogens can attack, enter and reproduce inside the body of an individual as well as destroying the body cells.
• Vector
  • Organisms that carry the pathogens and cause specific diseases.
  • Examples of vectors are mosquitoes, flies and cockroaches.

Figure 8 - Aedes aegypti mosquitoes are the vector for dengue virus. Flies are the vectors for bacteria Vibrio cholerae which causes cholera. Cockroaches can transmit the bacteria Salmonella typhi to humans through contaminated food and water.

The Effects of Pathogens on Human Health

Different pathogens will cause different types of diseases.

Bacteria

• Diseases: Tuberculosis
• Symptoms:
  • Weight loss
  • Coughing blood
  • Shortness of breath

Virus

• Diseases: Hepatitis B
• Symptoms:
  • Inflammation (liver cirrhosis)
  • Swollen chest
  • Yellowing of the skin and eyes sclera
  • Possible death

Protozoa

• Diseases: Dysentery
• Symptoms:
  • Stomach ache
  • Diarrhoea
  • Vomiting

Fungi

• Diseases: Tinea versicolor
• Symptoms: White or pink patches on the skin