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Unit 7

Microbes and Human Welfare

Microorganisms are the most abundant life form on Earth. Most are essential for our survival and have developed mutualistic relationships with the body. If the balance is broken or microbes break though the primary defences, harmful consequences may occur. Only a minority of microbes are harmful to humans, which are responsible for infectious diseases. These harmful effects, affect other life forms as well.

Vast majority of microbes are vital to life on Earth. Microbes managed to form mutualistic relationships with all organisms. Bacterial flora digest cellulose in the stomach and nitrogen-fixing bacteria on the roots of leguminous plants provide nitrogen for protein production. Normal flora occupy skin and intestines, competing with pathogens for physical space and secreting antibodies. Most recent evidence suggest that normal flora regulates immune system, as a loss of them is linked with rise in hypertension and autoimmune diseases.

Microbes have other positive contributions:

1)      Major portion of biomass on Earth.

2)      Cyanobacteria and Algae are responsible for majority of photosynthetic activity.

3)      Degrade dead organisms and recycle chemical elements.

4)      Bacteria decompose organic residue in sewage.

5)      Contribute to energy production.

6)      Biological pesticides, controlling pests in agriculture.

7)      Used for production of foods.

8)      Production of chemicals – including medicine.

9)      Gene therapy and genetically modified food.

Virulence Factor

Ability of a microorganism to cause a disease is called pathogenicity. The degree of the pathogenicity is called virulence. It is the relative ability to invade and cause disease. The severity of the disease doesn’t matter. Highly virulent strains always cause disease, less virulent strains may only cause a disease in immune-comprised patients, or when the microbes are in high numbers. Microbes have tactics to spread to new hosts, adhere to cells, access nutrients, cause damage and avoid the immune system. These are called virulence factors:

1)      Adhesion Factors – allow the pathogen to attach to the target cell. The first step in invading cells. Pathogens have specific ligands on their surface to bind to the complementary receptors.

2)      Biofilm Formation – bacterial cells are capable of forming a biofilm, helping bacteria adhere to each other and to other surfaces. Bacteria in the biofilm can communicate and exchange information, by passing plasmids using Pili. Biofilm can also entrap nutrients and block antimicrobial agents.

3)      Extracellular Enzymes – virulent pathogens use these enzymes to digest specific body tissues in order to maintain infection, invade new sites or avoid the immune system.

4)      Toxins – chemicals produced by bacteria that is either toxin to body cells and tissues or triggers over reaction of the immune system. Subdivided into two categories.

a.       Exotoxins – secreted by viable bacteria to manipulate host metabolic processes, to kill host cell. Three categories depending on effect; cytotoxins (kill host cell), neurotoxins and enterotoxins (GIT cells).

b.      Endotoxins – fragments of gram negative bacterial cell walls, lipopolysaccharide (LPS). Released when gram negative cells


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