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Bacteria and Disease


We often assume that all bacteria are harmful, but there are many uses of bacteria, and every healthy person has a natural flora of bacteria lining the skin and the gut. Those bacteria that cause harm are described as pathogenic.

There are numerous ways that pathogenic bacteria can be transmitted to us. In the table below we will take a two examples.

SalmonellaFood - mainly eggs and egg products.This bacteria infects the colon and lower intestine. The response to this is to increase fluid production which results in diarrhoea. This can cause dehydration so sufferers must drink alot of water and replace salts, it also makes absorbing antibiotics difficult.
Escherichia coliWater contaminated with faeces.E. coli is a bacteria naturally occuring in the human gut, but some strains are pathogenic. This also results diarrhoea, and can be fatal. In developing countries over 5 million children die a year from diarrhoea because of poor sanitation.

Contamination of food and water can be prevented by basic hygiene. Food-borne infection is prevented by thoroughly cooking food, inspecting farms to ensure animals do not carry disease and by using protective clothing where food is produced on a large scale.

Water-borne infection is prevented by treating it before and after it is used by people. To reduce the millions of deaths in developing countries, infrastructure to provide clean water is needed.


Pathogenicity is the ability of the bacteria to cause disease: the most dangerous bacteria have a high pathogenicity. There are a number of features that can make a bacteria more pathogenic.

How easily the bacteria can attach and enter the host is important. Molecules in their cell wall can bind to molecules on the host's cell membranes. Receptors vary between individuals, so some people may be susceptible to diseases than others.

Many pathogenic bacteria produce toxins: chemicals that have a harmful effect on the body. Exotoxins are produced by living bacteria and can have varying affects. Tetanus is caused by Clostridium tetani which releases toxins in to the nervous system that cause involuntary muscle contractions. Endotoxins are compounds in bacterial cell walls that only cause effect when the cell dies and is broken down - these are what cause fever and aches.

Large numbers of bacteria may be required to cause infection, as is the case with Salmonella enteritidis (10 million), however another species of salmonella only need be present in small numbers to cause thyphoid fever.

The invasiveness of a bacteria is its ability to spread within the host. A bacterium that can break in to one of the body's major transport systems (blood or lymph, say) will be very invasive.


Milk is very vulnerable to contamination during collection and storage, and is an excellent medium for growth.

The flash pasteurisation method is the most common for destroying any micro-organisms. Milk is heated to 72o C for 15 seconds, then rapidly cooled to 3oC

The heat kills many pathogens and the rapid cooling prevents those resistant to heat from dividing rapidly. The milk now must be kept refridgerated and can last for several days. The flash pasteurisation process has little effect on the flavour or nutritional value of milk.

Another method of heat treating milk is ultra-high temperature processing where the milk is heated to 135°C for 1 or 2 seconds. This is more effective and means the milk can be stored for several months. This treatment however has more of an effect on the nutritional content of the milk (mainly vitamins).