Infection for higher education

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For bacteria to successfully cause disease they must pass through several stages of infection: colonisation, invasion, proliferation and transmission. Whilst in the body, bacteria must also evade constant attack from the immune system. bacterial genomes contain a wide variety of genes encoding virulence determinants that facilitate these processes.

Infection is the colonisation of human tissues by pathogens, followed by rapid multiplication, often leading to the onset of disease. This can occur in many different parts of the body, for example: the urinary tract (UTI), the skin and the respiratory tissue of the lung. The site of infection along with the characteristics of the invading pathogen determine how a disease will manifest itself.

For bacteria to successfully  cause disease, they must follow a particular sequence of events. These are colonisation of the host tissue, expressing the appropriate molecule for attachment, invasion of the host cells, this is often the way they cause damage to the host, and acquisition of the appropriate nutrients to proliferate.

Throughout the process of infection, bacteria need to evade attack by the cells and effectors of the immune system (immune evasion). Many virulence factors govern the above processes, and their expression are often under clever, and stringent control.

Course of infection

Above- the progression of infection. During this time bacteria are constantly under the attack of the immune system.


The ability of bacterial pathogens to colonise us relies on their ability to migrate to an appropriate environment, and the expression of appropriate molecules on their surface for attachment to the host cell.

These processes are facilitated by:

  • Adhesion molecules, which are complimentary to the target host cell and allows the bacteria to 'stick'.
  • The ability to migrate to an appropriate environment in the host (chemotactic motility). Over on the motility page you can read about how motility contributes to pathogenesis using the food-borne pathogen Campylobacter jejuni as a model organism.


Once bacteria have successfully colonised their host, they need a way to invade these tissues in order to proliferate and cause disease.

Invasion can be mediated by:

  • Toxins which penetrate and damage cells, usually to aid the producing bacterium. You can read about the different types of bacterial toxins and their role in pathogenesis here.


In order to proliferate (rapidly multiply), bacteria need to obtain the right nutrients. These include iron, sources of carbon such as sugars and phosphates to name a few.

Some mechanisms for bacteria to proliferate include:

  • Acquisition of iron. Iron is an essential nutrient for bacteria to proliferate and as such, they have evolve several mechanisms to obtain it.

Evasion of the immune system

Throughout the process of infection, bacteria are constantly under attack from cells of the immune system and other environmental stresses.

Mechanisms for evasion of these defences include:

  • Phase variation of immunogenic surface antigens. Read about phase variation and the consequences of these process in Campylobacter and Neisseria on the phase variation page.


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