Bacteria are micrometers small unicellular organisms that, with a few exceptions, are only visible under the microscope but, unlike viruses, have their own metabolism. Although thousands of different bacterial species have been described and systematically classified in the approximately 350 years since the discovery of these microorganisms, it can be assumed that the vast majority – 95 to 99 percent – of all bacterial species existing on earth are not yet known.
In nature, bacteria occur in large numbers in practically every conceivable environment, in soil, earth crust, air, fresh and salt water. Humans, too, are colonized by numerous bacterial species. It is estimated that on its outer and inner surfaces it contains about 1014 to 1015 bacterial cells as so-called “normal flora” or “microbiome”. While about 1,000 germs per cm2 can be detected on the skin, significantly higher bacterial densities can be found on the mucous membranes of other parts of the body, for example in the mouth, throat or vagina. However, the largest concentration of bacteria can be found in the large intestine. It is estimated that there are up to 1012 bacteria per gram of stool, belonging to 400 to 500 different species.
Humans benefit in many ways from their microbiome: digestive processes and detoxification processes in the intestine are supported, enzymes and vitamins are provided and colonization with harmless bacteria, for example on the skin, in the intestine, in the oral cavity and in the vagina, prevents the settlement of pathogenic bacteria.
However, bacteria can also cause damage to their host. This is possible if they succeed in breaking through natural barriers. If, for example, the protective layer of the skin is damaged during an injury, germs from the skin or the environment can get into deeper tissue layers and cause a wound infection. A similar situation occurs with the perforation of a hollow organ in the abdomen (“burst appendix”), in which masses of intestinal microorganisms are flushed into the sterile abdominal cavity. Since numerous types of bacteria have mechanisms that enable them to settle on artificial surfaces and organize themselves there in a so-called “biofilm”, implants inserted into the body, for example joint endoprostheses, artificial heart valves, bone-stabilizing metal plates and screws (osteosyntheses) as well as vascular catheters represent another not negligible source of infection.