Bacteria are found simply everywhere - in soil, water, air, plants, animals, and humans. Approximately one hundred thousand billion bacteria inhabit our skin, mouth, throat, intestines, the urethra and vagina. That means for each single cell in the body, there are one thousand bacteria. Every human being has a specific natural microbial flora of more than 1000 different species. Most of them are beneficial and only a few species can cause diseases. An intact normal flora prevents possible pathogenic bacteria from multiplying by utilizing available resources for growth themselves. Still, beneficial bacteria leaving their natural environment and gaining access to other organs or tissues can cause infections as well. Thus, usually harmless Escherichia coli bacteria present in the intestinal flora may cause urinary tract infections.

Bacteria are small single-cell organisms that can be distinguished by size, shape, type of cell wall or whether they need oxygen to live or not. The basic shapes are spherical (cocci), cylindrical (rods) and spiral. Many bacteria have special appendages, called flagella or pili, which enable them to move actively or to attach to surfaces. Others are surrounded by a viscous capsule protecting them from phagocytosis or antibiotic actions. Using a staining process, most bacteria can be classified by the structure of the cell wall as "gram-positive" (appearing blue) or "gram-negative" (appearing red). The cell wall of gram-positive bacteria consists of a thick layer of murein, whereas the murein layer of gram-negative bacteria is thin but covered by a protective outer membrane. Obligate aerobic bacteria can only live in the presence of oxygen, whereas obligate anaerobic bacteria are better off without oxygen. The majority of bacteria is facultatively anaerobic; they can live under both conditions. Bacteria are adaptable organisms. Bacteria inevitably have to develop resistance mechanisms and constantly new strategies for survival against host immune responses, other germs, or medical treatment.