8.3 - Microorganisms and Viruses

Types of Microorganisms and Viruses

Microorganisms are microscopic organisms that cannot be seen with the naked eye and can only be observed under a microscope.
Most microorganisms are unicellular.
Types: bacteria, protozoa, algae, fungi and viruses.

No distinct nucleus because there is no nuclear membrane.
Genetic material is a chromosomal thread (DNA) free in the cytoplasm; this region is the nucleoid.
Some bacteria have plasmids, small DNA molecules carrying extra genes.
Length: $1$ – $10 μ m$ .
Exist as single cells, pairs, filaments, chains or clusters.
Shapes: coccus (spherical), vibrio (comma), bacillus (rod/cylindrical) and spirillum (spiral).
Examples: Lactobacillus sp., Streptococcus sp. and Staphylococcus aureus.

Animal-like unicellular microorganisms.
Move using pseudopodia (false feet), cilia or flagella.
Usually found in aquatic habitats.
Heterotrophic or autotrophic; Euglena sp. has chloroplasts and can photosynthesise.
Free-living or parasitic.
Examples: Amoeba sp., Paramecium sp. and Euglena sp.

Unicellular microorganisms such as Chlamydomonas sp. or multicellular organisms such as the brown alga Fucus sp.
Some have flagella for movement in water.
Have chloroplasts and are autotrophic.
No true leaves, stems or roots.
Habitats: ponds, lakes and oceans.

No chlorophyll; heterotrophic as parasites or saprophytes.
No roots, stems or leaves.
Cell wall consists of chitin.
Body is a mycelium composed of fine threads called hyphae.
Unicellular, for example Saccharomyces cerevisiae (yeast), or multicellular, for example Mucor sp.
Found in dark or damp places and on decaying or dead organisms.

Not placed in any kingdom because they are not cellular organisms.
Carry out no life processes outside a host cell.
Reproduce only in living cells by injecting genetic material into host cells.
Consist of nucleic acid (DNA or RNA) and a protein capsid.
Size: $20$ – $400 nm$ ; visible only with an electron microscope, not a light microscope.
Examples: tobacco mosaic virus, T4 bacteriophage and Human Immunodeficiency Virus (HIV).

Phytoplankton float on the surfaces of oceans, ponds or lakes.
Examples: green algae, blue-green algae (cyanobacteria), dinoflagellates and diatoms.
Chlorophyll enables photosynthesis.
Phytoplankton are producers in aquatic food chains.

Saprophytic fungi and bacteria decompose organic materials from dead organisms.
They break down animal waste, carcasses and rotting trees into simple compounds such as ammonium.
They secrete digestive enzymes into decaying organic matter and absorb the products of digestion.
Carbon, nitrogen and sulphur are returned to the soil and absorbed by plants.

A parasite benefits while its host is harmed or sometimes dies.
The parasite benefits while the interaction continues; therefore, most parasites do not kill their hosts.
Plasmodium sp. is a protozoan that lives in female Anopheles mosquitoes and causes malaria when transferred into a person’s blood circulation through a mosquito bite.

A symbiont has a very close relationship with another organism called the host.
Ectosymbiont: lives outside host cells; example: ectomycorrhizal fungi around plant roots.
Endosymbiont: lives inside host cells; example: Trichonympha sp. in the alimentary canal of termites.

Plants need nitrogen to synthesise proteins in plant tissues.
Plants obtain nitrogen from soil as ammonium ions ( $NH_{4}^{+}$ ) and nitrate ions ( $NO_{3}^{-}$ ).

Nitrogen fixation
- Rhizobium sp. in legume root nodules and free-living Azotobacter sp. in soil fix atmospheric nitrogen and convert it to ammonium ions.
- Lightning oxidises nitrogen to nitrogen dioxide; this dissolves in rainwater to form nitrous acid and nitric acid, which form nitrate salts in soil.
- Industrial nitrogen fertilisers supply ammonium fertilisers and nitrates to soil.
Decomposition and ammonification
- When plants and animals die, decomposer bacteria and saprophytic fungi break down proteins in body tissues into ammonium ions through ammonification.
Nitrification
- Nitrosomonas sp. converts ammonium ions to nitrite ions ( $NO_{2}^{-}$ ).
- Nitrobacter sp. converts nitrite ions to nitrate ions.
Uptake and transfer
- Plant roots absorb nitrates and use them to synthesise proteins.
- Animals eat the plants; nitrogen compounds are transferred to animal tissues.
Denitrification
- Denitrifying bacteria convert soil nitrates to atmospheric nitrogen gas.

Pathogen: an organism that causes disease, for example a virus, bacterium, protozoan or fungus.
Infection occurs when a pathogen enters the body, divides and multiplies; disease occurs when body cells are damaged and symptoms appear.
Vector: an organism that carries pathogens and causes particular diseases.

Vector	Pathogen	Transmission or disease
Housefly Musca domestica	Vibrio cholerae	Body transfers the bacterium to food; contaminated food causes cholera
Mosquito Aedes aegypti	Dengue virus	Bite transfers the virus to humans
Cockroach Periplaneta americana	Salmonella typhi	Contaminated food and water transfer the bacterium to humans

Viruses or bacteria cause illness by killing body cells or inhibiting cell functions.
Some pathogens secrete toxins that cause paralysis or disrupt the body’s metabolic activities.

Pathogen	Disease	Symptoms
Virus	Hepatitis B	Liver inflammation (cirrhosis), swollen abdomen, yellow skin and sclera; may cause death
Bacterium	Tuberculosis	Weight loss, coughing blood, shortness of breath
Protozoan	Dysentery	Stomach ache, diarrhoea, vomiting
Fungus	Tinea versicolor	White or pink patches on the skin