Infectious Diseases - Pathogens

Page 9 of 10

  • Infectious Diseases: Pathogens
  • Bacteria
  • Fungi
  • Viruses
  • Protozoa
  • Parasites
  • How pathogens are spread
  • How pathogens cause disease
  • Growth of pathogen populations
  • Quiz

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Growth of pathogen populations

Bacteria grow by splitting in two. In ideal conditions bacteria can divide every 20 minutes, so the size of the colony doubles every 20 minutes. This is known as exponential growth. In theory a colony of bacteria can continue to grow like this without stopping.

The conditions needed for ideal bacterial growth:

  • Plenty of food
  • A suitable temperature
  • A suitable pH
  • No build up of poisonous waste products
  • In many cases, oxygen is needed

If conditions are not ideal each division can take several days.

bacteria under colourised scanning electron micrograph

bacteria under colourised scanning electron micrograph
Photo provided by CDC/ Margaret Williams, PhD; Claressa Lucas, PhD;Tatiana Travis, BS

Fungi also grow very fast in ideal conditions. Their needs are very similar to those of bacteria although many common fungi such as yeast do not need oxygen to grow. The rate of growth of a colony of yeast doubles for every 10°C rise in temperature until an optimum temperature is reached. If the temperature goes up higher the enzymes in the yeast don't work properly and they stop growing.

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This rapid exponential growth of bacteria explains how diseases develop so fast. The bacteria grow and cause the symptoms of disease as the body reacts to the bacteria or the toxins they make. The immune system cannot react quickly enough to prevent this happening the first time a bacterium gets into the body. The second time, the immune system is ready and the bacteria are destroyed before they can grow and cause the symptoms of disease.

In fact bacteria don't show exponential growth – in the real world ideal conditions do not last long either in the lab or in the body. Factors which slow down or even stop bacterial growth can include:

  • Lack of food
  • Lack of oxygen
  • Build up of poisonous waste products
  • Change in pH as carbon dioxide levels build up

Graph to show a typical natural growth curve of a bacterial colony

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Growing bacteria in the lab

You can culture bacteria in the lab and observe the growth of different colonies. It is important to keep everything aseptic – clean and uncontaminated by bacteria from the world around you – so that you only grow the cultures that you want to investigate.

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Probiotics and prebiotics – helping bacteria grow?

Your gut contains about 100 trillion bacteria which weigh around 1.5kg. Many of these are very useful – the so called 'good bacteria' which help break down your food and protect you from pathogens. But some of the bacteria are not so healthy, and they can cause problems. It is important to have plenty of the good bacteria in your gut for it to work properly and to prevent the other bacteria from causing health problems. A number of different foods are sold which claim to encourage the growth of healthy gut bacteria.


Probiotics contain live, healthy bacteria usually Lactobacillus and Bifidobacteria. They produce lactic acid in your gut. Many different probiotic foods are sold, often as yoghurts or yoghurt drinks.

The manufacturers claim that they will make you healthier in many different ways, improving your digestive system, helping your immune system and reducing allergies.

In 2009 scientists at the European Food Safety Agency looked at the scientific evidence for 180 health claims for probiotics. They rejected ten completely and said there was not currently enough evidence to support the other 170 claims.

Probiotics are big business – millions of pounds worth of probiotic drinks and yoghurts are sold every year.

Probiotics are big business – millions of pounds worth of probiotic drinks and yoghurts are sold every year
© iStock


Prebiotics act as food for the 'good' bacteria in your digestive system and encourage their growth. Surprisingly they are mainly substances which you can't digest in your body. You can find them in some fruit and vegetables such as asparagus, bananas, onions and tomatoes. They are also sold in specially made dairy products or prebiotic capsules. The scientific evidence is growing that prebiotics really can increase the 'good' bacteria in your gut and help maintain good health.

Fruit and veg.

© iStock

Medicine that acts against bacterial infections. Penicillin is an example of an antibiotic.
Protein that is produced by lymphocytes (white blood cells) and that attaches to a specific antigen.
Molecule on the surface of a pathogen that identifies it as a foreign invader to the immune system.
Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.
The use of biological organisms or enzymes to create, break down or transform a material
To cut apart, or separate, tissue especially for anatomical study.
Exponential growth
If something is growing exponentially the larger the quantity gets, the faster it grows
Micro-organism that can grow in long tubes called hyphae or as single cells. Fungi have a nucleus, cytoplasm and a cell wall.
Herd immunity
If a high percentage of a population is immune to a disease the disease cannot be passed on because it cannot find new hosts.
Infection caused by the human immunodeficiency virus (HIV). It attacks and destroys the immune system.
Hybridoma cells are formed by fusing a specific antibody-producing cell with a type of cancer cell that grows well in tissue culture
Immune system
The body's natural defence mechanism against infectious diseases.
A process which gives immune resistance to a particular disease. The human or animal is exposed to a harmless antigen in order to raise antibodies and provide an immune memory.
A type of white blood cell that make antibodies to fight off infections.
A type of white blood cell that consumes dead pathogens that have been killed by antibodies.
Organism that feeds off another living host and causes it some damage. An example of a parasite is a tapeworm that lives in the digestive system of a host organism.
A micro-organism that causes disease.
Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells.
A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.
Protozoa are one-celled animals
A spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavourable conditions.
A poisonous or toxic substance - produced by pathogens.
A small amount of dead or weakened pathogen is introduced into the body. It prepares the immune system to prevent future infections with the live pathogen.
Medicine that contains a dead or weakened pathogen. It stimulates the immune system so that the vaccinated person has an immunity against that particular disease.
The smallest of living organisms. Viruses are made up of a ball of protein that contains a small amount of the virus DNA. They can only reproduce after they have infected a host cell.
Opportunistic Infections
An infection that would not normally cause disease in a healthy person but which can take hold when a person's normal immune defences are reduced.