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. 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. Obligate aerobic bacteria can only live in the presence of oxygen, whereas obligate anaerobic bacteria are better off without oxygen.
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 .