Bacteria are the dominant form of life on Earth, with more of them here than any other creature - not a bad achievement for a single-cell organism.
Part of their success stems from their ability to divide rapidly, which, among other things, allows them to evolve beyond a threat before it overwhelms them.
Doctors see control of bacteria as an important goal, and thought they may have achieved it when the potential of penicillin was acknowledged throughout the medical world.
But, thanks to their ability to adapt and survive, bacteria have managed to maintain a position as a significant threat to human health right up into the close of the twentieth century.
Threat to humans
Pathogenic bacteria - those that cause disease - are responsible for conditions from food poisoning and tetanus to meningococcal meningitis and tuberculosis.
However, most are harmless to humans and many contribute to good health, while a large proportion of those that can cause disease are only potentially-pathogenic - meaning they only affect people who are already at risk or have a weakened immune system.
Escherichia coli provide an example of the double nature of bacteria.
While the O157 strain regularly grabs headlines as killer bug picked up from food stored in unsanitary conditions, many other strains live quite naturally in the human digestive system and help process food as it makes its way through the body.
The advent of the microscope allowed scientists to uncover the existence of bacteria in the seventeenth century, although it was Louis Pasteur who made the link between them human disease in the nineteenth.
They come in three shapes - spherical, rod-shaped or spiral-shaped - and can be either aerobic - needing free oxygen to function - or anaerobic - able to live and survive in the absence of oxygen or air.
They can move in various ways including gliding and pushing themselves along using flagella - tiny whip-like attachments.
On average they measure one or two millionths of a metre and multiply by dividing in two. Some can divide as fast as once every 12 to 20 minutes.
Their exponential rate of division means that a single bacterium can produce billions of descendants a day, and this is the key to their ability to develop resistance to antibiotics.
It only takes one bacterium to prove resistant to an antibiotic and, as the drug or immune system wipe out the others, the resistant one will multiply to replace them.
Another key to their survival is their very simplicity. With a protective membrane to distinguish them from the outside world, some are thought to be as simple as strings of protein molecules around an RNA molecule (a simpler form of DNA).
They can feed on a wide range of substances and achieve this by sending enzymes out into their surroundings and breaking down anything from sugar to iron nails.
The enzymes reduce the food to tiny pieces that are then soaked up through the membrane, and this action can be exploited - for example, the effect of bacteria on milk turns it into yoghurt.
And they can live anywhere from human skin - an average of 100,000 per square centimetre - to undersea volcanoes and in oil.