The pathogenic effect of bacteria

In a nutshell

Some bacteria are pathogenic, this means they cause disease. They can do this by producing either endotoxins or exotoxins, or by directly invading host tissues.

Commensals

Definition

Commensal bacteria are harmless. They form an association with a host organism that benefits the bacteria; the host species neither benefits nor is harmed.

Example

Commensal Escherichia coli and Lactobacillus species are found in the guts of many mammals, including humans. The collection of commensal microorganisms in the gut is known as the gut microbiota.

Pathogenic bacteria

Some bacteria are pathogens, meaning they invade tissues and cause damage by releasing toxins or directly destroying host cells.

Example

Shigella species are a type of pathogenic bacteria that cause gastrointestinal infections.

Toxins

Toxins produced by pathogenic bacteria can be classified into two types: exotoxins and endotoxins.

	Endotoxin	Exotoxin
Type of bacterium	Gram negative	Gram positive and gram negative
Type of molecule	Lipopolysaccharide (lipid and sugar)	Polypeptide/protein
Size of molecule ( $kDa$ )	$\approx 10$	$\approx 1000$
Relation to cell	Part of cell surface membrane	Secreted by the cell
Potency ( $\mu g$ needed to cause symptoms)	$\approx 100$	$\approx 1$
Denatured at $100\degree C$ ?	No	Yes

Endotoxins

Endotoxins are made up of three key components.

Biology; Microbiology and pathogens; KS5 Year 12; The pathogenic effect of bacteria

Component	Description
Lipid-A component	This is the part of the endotoxin that is embedded in the outer phospholipid layer of a bacterium. The lipid-A component is toxic.
Core polysaccharide	This is the core part of the endotoxin.
O-specific polysaccharide	This faces outside of the cell and allows the bacterium to invade the host. It is sometimes called the O-specific antigen as hosts immune systems often produce antibodies against it.

Example

Salmonella enterica is a Gram negative bacterium that produces an endotoxin. S. enterica invades intestinal cells and causes them to lyse, when it does this the endotoxin is released. This causes inflammation and these intestinal cells will have a reduced ability to stop the movement of water and ions into the gut lumen. This causes diarrhoea.

Exotoxins

Staphylococcus species are Gram positive bacteria that are able to secrete two types of endotoxin.

Haemolysins: These are polypeptides that become integrated in the cell surface membranes of host red blood cells; this creates pores. These pores cause the cells to lose water and ions; this causes lysis.
Superantigens: These are polypeptides that stimulate many host immune cells and cause a large influx of cytokines known as a 'cytokine storm'. This results in toxic shock syndrome which is incredibly dangerous as it can progress to a coma and/or multiple organ failure.

Tissue invasion

Some bacteria induce pathogenic effects by invading host tissue directly.

Mycobacterium tuberculosis

M. tuberculosis causes disease in the lungs. Upon initial infection, it is engulfed by macrophages in the alveoli and bronchioles. These macrophages then become surrounded by immune cells and form a structure known as a granuloma. Often, the macrophages kill the bacteria. However, some are able to survive as a latent infection that could get reactivated years later.

Biology; Microbiology and pathogens; KS5 Year 12; The pathogenic effect of bacteria — 1. T cells, 2. Macrophage infected with tuberculosis, 3. Crumbly centre of granuloma called caseous necrosis.

This is only the case in healthy hosts. If the host has a weakened immune system (e.g. by HIV infection or due to malnourishment) they may develop a chronic tuberculosis infection. During chronic infection, M. tuberculosis is able to secrete hydrolytic enzymes that cause the digestion of host cells. This digestion of host tissues by the bacterium causes infected people to cough up blood.