Tuberculosis is caused by two bacteria, > Mycobacterium tuberculosis and M. bovis. These pathogens live inside human cells, particularly the > lungs. Although the lungs are the first site of the infection, the bacteria can spread throughout the whole body, and can even infect the bone tissue. An electron micrograph of the bacteria is shown below. Go here for a great Flash animation of the infection process!

Some people very quickly develop signs of infection, whilst others demonstrate no symptoms for years and the bacteria remain inactive and uninfectious to others. It is quite possible however for the bacteria to become active at any time, and this commonly happens during times of malnutrition, HIV infection, or if weakened by other diseases. TB is frequently the first opportunistic infection to strike HIV+ people, perhaps because of increased susceptibility to infection or due to reactivation of dormant M tuberculosis present from childhood. It is reckoned that approx. 30% of the population of the world is infected but symptomless. Active sufferers may have the illness for an extended period of time, with a persistent cough and are often feverish with suppressed appetite and consequent extensive weight loss .

TB was once thought to be nearly eradicated from the world, but has shown a resurgence and there are high rates of incidence across the developing world, see the figure below.

Very high rates are also found within many inner cities where people are destitute. The worldwide resurgence may be partially explained by the following factors:
  1. Drug- resistant strains
  2. The AIDS pandemic
  3. Poor housing and rising homelessness
  4. The breakdown of TB treatment programmes (which also increases the chances of drug resistance developing)
  5. Migration from E Europe and developing countries to large cities, eg London


TB is spread by the coughs and sneezes of infected people. The bacteria are carried in the air in tiny droplets of liquid.Uninfected people inhale the droplets and the bacteria within them. TB is spread msot rapidly therefore in overcrowded conditions. Sleeping close together in large numbers is particularly rsiky, such as in overnight shelters for the homeless.
The disease primarily attacks > the homeless, those in poor, substandard housing, those with low immunity, the malnourised, those with HIV infection.

How TB develops

If you cannot see the Flash Movie playing then you may not have the flash player installed. The latest version of the Flash player can be downloaded free from Macromedia
More information and help with installing the Flash Player can be foundon the BBC's Webwise pages.

If you can see the Flash movie then please ignore this message.

You will need Flash 4 or above to see this animation, which you can download for free..

Diagnosis and Treatment
A doctor will have samples of sputum analysed microscopically for the presence of mycobacterium. If they are found to be in the most infectious stage (the first 2-4 weeks) they are isolated. It is reckoned that one personcan infect ten to fifteen others, especially in over-crowded conditions.
Treatment involves the use of several different drugs to ensure all bacteria are killed and to remove the chance of drug-resistant strains developing. The drugs must be consumed for 9 months to one year as the bacteria are slow growing and are not very sensitive to the drugs used. Many people do not complete the course of drugs as they believe themselves to be cured, and such people may be harbouring drug resistant forms. Antibiotics kill drug-sensitive strains and leave resistant ones behind. Drug resistance occurs as a result of mutation in the bacterial DNA and occurs about once in every 1000 bacteria. If four drugs are used the chances are reduced to about 1 in a thousand million. Stopping treatment early can mean that the bacteria develops resistance to all the drugs used.
Multi drug resistant forms of TB (MDR-TB) now exist.

The WHO now promote a scheme called DOTS. What is it ? > Direct Observation Treatment, Short Course, in which health workers or responsible family members make sure that the patient takes the drugs for 6-8 months.This approach cures 95% of all patients.
Contact tracing is also essential in the effective combat of TB. Among children the spread is controoled well through vaccination- the BCG vaccine given to school students at age 13-14 years.

Badgers and Cattle
TB can be transmitted between humans and cattle. Cattle are routinely tested and if infected they are killed. Pasteurisation kills TB bacteria so milk cannot carry the disease. In many areas, badgers are killed in the area of a TB infected herd of cattle. There are vociferous arguments from supporters and protesters to this practise.

Should badgers be culled to see if they spread TB?
Yes No
In recent years, new cases of TB in UK cattle have doubled TB is still relatively rare, affecting only 0.4% of UK herds
Many people think increasing badger numbers are to blame for the rise in TB According to a government committee there is no proof that badgers spread TB
In all areas where cattle are infected with TB, badgers are often infected as well 30,000 badgers have already been killed since 1975 and 80% of them didn't have TB
Farmers, already hard hit by outbreaks of Foot and Mouth Disease and BSE, are worried that TB could add to their burden Unless a link can be proven, culling badgers might turn out to be an ecological and economical mistake
Only by culling badgers can scientists prove whether badgers spread cattle TB or not Culling as many as 12,500 badgers over the next 5 years is not justified. The £34 million might be better spent in other ways

Can you complete the summary table below?
Pathogen> Myobacterium tuberculosis and M. bovis
Method of transmission> airborne droplets; unpasteurised milk
Global distribution> Worldwide
Incubation period> few weeks or months
Site of action> primary infection in lungs; secondary infection
in lymph nodes, bones and gut
Clinical features> racking cough, coughing blood, chest pain,
shortness of breath, fever, sweating, weight loss
Method of diagnosis> microscopic examination of sputum for bacteria.
Chest X-ray
Worlwide annual incidence> 8 million (6000+ UK)
Worldwide annual mortality> 2 million

Back to the A level Bio notes menu