A Group Of Organisms Biology Essay

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A group of organisms are found on the body at specific sites normal flora. Normal flora is a group of microorganisms that infect the body without necessarily causing disease. These microbes prevent colonisation of pathogens by competing for the nutrients. Pathogens are microorganisms that cause infectious diseases.

White blood cells can ingest the pathogens and destroy the pathogens by phagocytosis.

Microorganisms like bacteria, fungi, yeast and protozoa can be harmful as well as beneficial.

Harmful or Pathogenic Microorganisms (2)

Pathogenicity depends upon:

Virulence : Highly virulent type of microorganisms may cause deadly disease

Invasion: By the mechanism of colonisation and production of substances, microorganisms can overcome the host defence mechanisms. When the skin is broken, Staphylococcus bacteria that can live on many skin surfaces without causing infection; can enter the wound and cause an infection.

Amount of microorganisms-Food contaminated with large amount Staphylococcus bacteria may cause severe food poisoning.

Immune System-When the host is immune, there are less chances of becoming infected.


Bacteria are unicellular prokaryotic microorganisms and are characterised by the lack of a defined nucleus. The bacterial structure consists of DNA, cytoplasm, cytoplasmic membrane, cell wall and capsule. Certain bacteria have flagella for motility.

Beneficial Bacteria:


Different species of Lactobacillus Bacteria are:

Lactobacillus acidophilus

Lactobacillus bulcaricus

Lactobacillus casei

Lactobacillus sporogenes

Lactobacillus acidophilus is naturally found in the digestive, urinary and genital systems. These bacteria help in digestion and absorption of nutrients.

Lactobacillus acidophilus help in the breakdown of lactose to lactic acid in the

digestive system.

Lactobacillae are involved in the fermentation of dairy products such as cheese,

yogurt and buttermilk. The fermented milk improves the taste and increases

the digestibility of the milk. It extends the shelf life of the milk.


Different species of Bifidobacteria are:

Bifidobacteria bifidum: It is found in the intestine and vagina. In the intestine it helps

in digestion and helps to boost the immune system. It is also found in the breast milk

and helps to boost the immune system of the infants.

Bifidobacteria longum: It is found in the gastro-intestinal tract and vagina. It inhibits

the growth of pathogens, helps in digestion and to boost the immune system.

Harmful Effects of Bacteria

Mycobacterium tuberculosis

The species Mycobacterium tuberculosis causes tuberculosis. It affects the lungs. A person with tuberculosis can infect others by coughing and sneezing. The bacteria are propelled in the air and inhalation of these germs can cause infection.

The symptoms of tuberculosis are:

Loss of weight

Poor appetite



Chest pain

Night sweats

Treatment takes long because the bacteria grow very slowly and, unfortunately, also die very slowly. If not treated properly, tuberculosis disease can be fatal. (3)

Streptococcus pneumoniae (4)

A pneumococcal infection is the term used to describe infection caused by the bacterium Streptococcus pneumonia. It is the disease of the upper respiratory tract. The bacteria enter the body by the inhalation of water droplets. The virulence factor of Streptococcus pneumonia is due to the presence of capsule that prevents phagocytosis by the host's immune cells. The bacteria are able to attach to epithelial cells of the upper respiratory tract.

The symptoms of a pneumococcal infection depend upon the type of infection.

The common symptoms are:

Elevated body temperature

Cough accompanied by yellow or green mucous



Body Ach


Shortness of breath

Infections such as tetanus, typhoid fever and diphtheria are caused the pathogenic bacteria.


Viruses are microorganisms that are very small. They are smaller than bacteria. They can't reproduce on their own, but require a host cell for multiplication. (5)

Viruses consist of DNA or RNA which is surrounded by a protein coat. They invade a host by attaching to them to the host cell. They replicate by invading another cell, altering the host's metabolism to allow replication of viral DNA or RNA, so that new viruses can be produced.

Viruses can be beneficial but can also be harmful to the humans.

Beneficial Virus


Viruses can be beneficial in form of vaccines. Vaccines are made by attenuated virus (virulence of the virus is reduced) to protect against infection with the same virus.

Vaccine triggers the production of antibodies and if any time the body comes in contact with the virus, the antibody production is quicker and in higher concentrations.

For example: The human papilloma virus (HPV) vaccine.

Treatment against cancer

Research has been carried out that certain type of viruses can help fight cancer. Clinical trials have been carried out using Reovirus to treat patients with cancer. It kills the cancer cells and makes them to release chemokines and cytokines which trigger an immune response to attack the cancer cells.

Harmful Effects of Viruses


Influenza Virus

The influenza virus causes infection of the respiratory tract in humans. It is commonly known as "the flu". By coughing and sneezing, the infected droplets are dispersed in the air. Inhalation of these droplets can cause infection to others. The flu is contagious and spreads rapidly and affects many people.

Symptoms are:

Fever with chills


muscle aches




The genus Enterovirus causes polio. Poliovirus is an enterovirus. It infects the gastro intestinal tract and then it travels to the nervous system. It causes permanent damage to the motor neurons. It may be fatal or can lead to permanent muscle weakness and paralysis.

The infection is transmitted through faeces or pharyngeal secretions of an infected person. 

Polio virus has been eliminated in many countries; however in some developing countries it is still endemic.

DNA-containing Virus: 

Hepatitis B

Hepatitis B is a type of virus that infects the liver. The infection caused by this virus can be both acute and chronic.

The virus is transmitted through infected blood or body fluids. A small amount of blood can transmit the virus as it is very infectious. Hepatitis B can be spread during unprotected sex and sharing of contaminated needles by drug users.

Symptoms can include:


lack of appetite

flu-like symptoms, such as weakness, aches , headaches




It takes 40 to 150 days for the symptoms to develop after exposure to the virus.


Fungi are group eukaryotic organisms that are characterised by a defined nucleus. This group includes microorganisms such as yeasts and molds. Fungi are found in every habitat.

Beneficial fungi


Penicillum is a beneficial fungus.

Penicillum, is mold that is commonly found on old bread. Penicillium is used to produce antibiotic Penicillin. Penicillin was discovered to have anti-bacterial properties. Many diseases such as bacterial pneumonia can be treated quickly and effectively with penicillin. Penicillin is widely used as an antibiotic, although its effect has been diminished by some bacteria developing resistance to it.

Saccharomyces cerevisiae

Saccharomyces cerevisiae is known as brewers or baker's yeast. It used in many baked goods and in beer. It is used for leavening of breads and in the manufacture of beer (beer ferments and turns bubbly).

Harmful Effects of Fungi

Ringworm is an infectious skin infection. It causes ring like rashes on the skin. The circular growths may appear darker or lighter than the normal skin colour. Ringworm is highly contagious. The growths may be scaly or itchy.

Athlete's foot (ringworm of the foot) is an  infection of the skin that causes scaling and itching of affected areas. It is caused by fungi Trichophyton . It affects the feet.

Ringworm infections are divided into different categories:

Tinea capitis: Ringworm of the scalp and eyebrows

Tinea corporis: Ringworm of the body.

Tinea pedis: Ringworm of the feet. Example: athlete's foot.

Tinea manuum: Ringworm of the hand.


Protozoa are unicellular eukaryotic organisms that cause infections. They come in many different shapes and sizes. Amoeba and Paramecium are protozoa. They normally live in moist habitats including soil, water and marine environment.

Beneficial Protozoa

Most protozoa are helpful and play a major role in the food chain.

Protozoa are part of the soil food chain. As predators upon algae, bacteria and fungi, a process that produces nitrogen in the soil. It protects the plant by consuming disease causing fungi and bacteria. Some protozoa attack harmful nematodes. Protozoa help to convert organic matter into minerals. As predators they help to aerate the soil

Protozoa play an important role in controlling bacteria population. Protozoa eat harmful bacteria and are food for fish and other animals.

Harmful Protozoa

Malaria is caused by the following four species of Plasmodium:

Plasmodium falciparum

Plasmodium vivax

Plasmodium malariae 

Plasmodium ovale

Plasmodium falciparum can cause severe form of malaria which can be life-threatening.

Malarial fever is caused by an infected mosquito, or by receiving infected blood from someone during a blood transfusion. 

The symptoms are:





Children may have additional symptoms of cough and shivering.

Trypanosomiasis is caused by Trypanosoma .This protozoa is parasitic.

Human African Trypanosomiasis (sleeping sickness) is transmitted by the bite of tsetse fly infected with Trypanosoma brucei. The tsetse fly bite causes a red sore followed by fever after few weeks. Fever is accompanied by swollen lymph glands, muscle aches and headaches. In advanced stages, the disease affects central nervous system and can be fatal.

American trypanosomiasis (Chagas disease) transmitted by contamination with faeces infected with Trypanosoma cruzi of assassin bug. The bug drops the faeces and urine on the host after feeding. The parasites enter the body when the person smears the bug faeces into the bite, eyes, mouth, or in any skin break. The parasite can also be transmitted by blood transfusion using blood from an infected donor.