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Bacteria as pathogens

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Bacteria can cause disease (become pathogenic) when they invade the interface between an organism and their environment. This could be the skin, lungs, digestive system, etc. There are two ways which you need to know about in which pathogens cause disease: 1) damage to the cells, and 2) producing toxins.

The damage caused can be a byproduct of their normal growth and division, as they destroy host tissue and disrupt the host's healthy structure and function. For example, bacteria that cause tuberculosis destroy lung tissue and therefore compromise the host's ventilation ability. Cholera bacteria on the other hand produce toxins which cause excessive water to be drawn into the intestine from cells by osmosis, casing great water loss through diarrhoea.

Let's read a few more details about the specific kinds of bacteria toxin and tissue damage in 3 case studies: Staphylococcus aureus, commonly found on skin, Salmonella sp. (species) that can be the source of food poisoning, and the aforementioned Mycobacterium tuberculosis which is passed between infected individuals.


Exotoxins e.g. Staphylococcus aureus

Staph. is commensally (safely) found on around a third of all people. The most common pathogenic breakouts are in the form of skin or tissue infection such as boils of the skin, infection of the deep layer of skin called cellulitis (not to be confused with cellulite which is a normal condition of fatty tissue) and impetigo. More rarely, these superficial infections can penetrate deeper, infect blood (bacteremia) and cause organ infection and damage (endocarditis).

Exotoxins are substances actually excreted by bacteria actively to invade the host tissue and wreak havoc. There are many different kinds by Staph. aureus, such as protein barrels that integrate into the plasma membrane of host cells and cause their contents to leak out through these huge pores created by the barrel shape.




Others include protease toxins that can cause skin peeling (occurs in infants and children) as well as superantigens that can cause toxic shock syndrome associated with tampon use. Superantigens, unlike simple antigens that serve to actually fight the infection by activating the host T cells to find and destroy the bacterial invader, trigger a massive immune response (activating a fifth of all of the host body's T cells as opposed to the normal response of just 0.001% or less) that overwhelms the body and is dangerous enough to be lethal. Shock and multiple organ failure can occur.

Methicillin-resistant Staphylococcus aureus (MRSA) is an antibiotic resistant form of Staph. that poses particular danger in hospitals as the environment consisting of a very populated area of ill and immune susceptible patients is an opportunity for quick spread. This makes it difficult to treat and eliminate. As its name indicates, this is a form resistant to the antibiotic methicillin.


Endotoxins e.g. Salmonella sp.

Salmonella are gram-negative bacteria that reside in farm animal guts, and can therefore spread to items of food used by humans, directly in meat, eggs and milk, or indirectly via leaks in soil to non-animal food like lettuce. Eating contaminated food that has not been cooked properly or stored properly can cause food poisoning. This entails dehydration through diarrhoea, which in itself can be threatening. If kept under control no further treatment is required before the infection clears up.

Since these are gram-negative bacteria, they have a thinner murein (a.k.a. peptidoglycan) layer in their cell wall, as well as an outer membrane on top of it. This is the site of their endotoxins which are lipid-based and do not get secreted out of the cell. Instead, endotoxins remain part of the bacterial outer membrane up to their death. Then, the endotoxins are released.




They trigger an immune response, and as the bacteria invade the cells lining the intestine epithelium, proinflammatory cytokines are released by the host. Acute inflammation then causes diarrhoea.


Invasion of host tissue e.g. Mycobacterium tuberculosis

Tuberculosis (TB) is an infectious disease of the lungs which causes constant coughing with blood, shortness of breath, fever and weight loss over the years. Every year 2 million people die from TB out of 8/10 million who get the disease. Far more people, around 2 billion, carry the TB bacteria on them without having the disease.

TB is passed on between people by inhaling droplets from the air infected with the bacteria Mycobacterium tuberculosis. When the infection is confirmed, patients are isolated for up to 4 weeks and put on a course of antibiotics for up to 6 months.




When bacteria reach the alveoli, the immune system reacts by surrounding them with white blood cells, which results in the formation of scar tissue. The shortness of breath symptom is caused by less oxygen reaching the circulatory system due to a decreased surface area for diffusion in the lungs, as many alveoli are damaged.

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