Tonsillitis, pneumonia, cystitis, school sores and conjunctivitis all have one thing in common – they are all caused by bacteria.

Bacteria are not all bad. They are essential for the production of many foods, from wine and beer to mature cheese and yoghurt.

Bacteria are microscopic single celled organisms that are between 0.3 and 10 microns in length. A thousand microns make one millimetre, or a micron is 0.0000001 of a metre. Bacteria are everywhere in the environment in extraordinarily vast numbers. Every gram of soil contains between 1,000,000,000 and 20,000,000,000 bacteria, as well as 10,000,000 to 50,000,000 fungi, about 20,000 algae and 100 to 1000 protozoa and other single celled organisms. Amazingly, eight out of every ten cells in our bodies is actually a bacterium, and there are between 500 and 1000 different types of bacteria in a person’s body at any time. That means that we are more a bacteria than a human. The ratios of these bacteria vary from one person to another, and can be as identifying as a fingerprint. It is obvious that humans evolved with these bacteria and could not survive without them.

Illustrations of bacteria

Human life would be impossible without bacteria as they are essential for our digestive systems, the manufacture of some essential vitamins, and the good symbiotic bacteria even fight of the pathogenic ones. Sometimes the beneficial bacteria multiply excessively or move to different areas of the body where they become pathogenic (harmful). For example, the Escherichia coli bacterium is very common, and usually harmless in the gut, but in the bladder it can cause a urinary infection. Other bacteria (e.g. Mycobacterium tuberculosis that cause tuberculosis) are always pathogenic.

Pathogenic bacteria can penetrate into healthy tissues and start multiplying into vast numbers. When they do this they damage the tissue that they are infecting, causing it to break down into pus. Because of the damage they cause the involved area becomes red, swollen, hot and painful. The waste products of the damaged tissue, along with the bacteria, spread into the blood stream, and this stimulates the brain to raise the body temperature in order to fight off the infection. Thus a fever develops.

The body is invaded by millions of pathogenic bacteria every day, but very few ever cause problems because the body’s defence mechanisms destroy the majority of the invading organisms. The white blood cells are the main line of defence against infection. They rapidly recognise an unwanted bacteria, and large numbers move to the area involved to engulf the bacteria and destroy them. It is only when these defences are overwhelmed that a noticeable infection develops.

Hundreds of bacteria are known to microbiologists (the doctors and scientists who study them), but only a few dozen cause significant infections in mankind. All these bacteria have specific names and can be identified under a microscope by experts who can tell them apart as easily as most of us can identify different breeds of dogs.

Every species of bacteria (and fungi, but not viruses) has two names – a family name (e.g. staphylococcus) which uses a capital initial letter and comes first, and a specific species name (e.g. aureus) that uses a lower case initial letter and comes second. The golden staph bacteria which causes many serious throat infections is thus called Staphylococcus aureus but may be abbreviated to S.aureus.

When an infection occurs, the patient usually consults a doctor because of the symptoms. If the infection is bacterial, the appropriate antibiotics can be given to destroy the invading bacteria. Because different types of bacteria favour different parts of the body and lead to different symptoms, a doctor can make an educated guess about the antibiotic to use. When there is any doubt, a sample or swab is sent to a laboratory for expert analysis so that the precise organism can be identified, together with the appropriate antibiotic to kill it.

Many bacteria, particularly those in the gut, are beneficial to the normal functioning of the body. They can aid digestion, and prevent infections caused by fungi (e.g. thrush) and sometimes viruses. Unfortunately, antibiotics can kill off these good bacteria too, and so common side effects of the use of antibiotics are diarrhoea, and fungal infections of the mouth and vagina.

The most common bacteria that attack humans, and the diseases they cause, or organs they attack, are listed below.

Bacillus Anthrax, tuberculosis.
Bacteroides Pelvic organs.
Bordetella pertussis Whooping cough.
Brucella abortus Brucellosis
Chlamydia tracholatis Venereal disease, pelvic organs, eye
Clostridium perfringens Gas gangrene, pseudomembranous colitis.
Clostridium tetani Tetanus
Corynebacterium diphtheriae Diphtheria
Escherichia coli Urine, gut, Fallopian tubes, peritonitis
Haemophilus influenzae Ear, meningitis, sinusitis, epiglottitis
Helicobacter pylori Peptic ulcers
Klebsiella pneumoniae Lungs, urine
Kingella kingae Mouth, throat, joints, bone
Legionella pneumophilia Lungs
Moraxella Nose, ears, eye, lungs
Mycobacterium leprae Leprosy
Mycobacterium tuberculosis Tuberculosis
Mycoplasma pneumoniae Lungs
Neisseria gonorrhoea Gonorrhoea, pelvic organs
Neisseria meningitidis Meningitis
Proteus Urine, ear
Pseudomonas aeruginosa Urine, ear, lungs, heart
Salmonella typhi Typhoid fever
Shigella dysenteriae Gut infections
Staphylococcus aureus Lungs, throat, sinusitis, ear, skin, eye, gut, meningitis, heart, bone, joints
Streptococcus pneumoniae  Throat, ear, sinusitis, lungs, eye, joints
Streptococcus pyogenes Sinuses, ear, throat, skin
Streptococcus viridans Heart
Treponema pallidum Syphilis
Vibrio cholerae Cholera
Yersinia pestis Plague

As a side curiosity, sharks never catch bacterial infections and medical scientists are still trying to work out why.

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