n. 1. the infolding of the wall of a solid structure to form a cavity. This occurs in some stages of the development of embryos. 2. see intussusception.
Bacteria are classi?ed according to their shape: BACILLUS (rod-like), coccus (spherical – see COCCI), SPIROCHAETE (corkscrew and spiral-shaped), VIBRIO (comma-shaped), and pleomorphic (variable shapes). Some are mobile, possessing slender hairs (?agellae) on the surfaces. As well as having characteristic shapes, the arrangement of the organisms is signi?cant: some occur in chains (streptococci) and some in pairs (see DIPLOCOCCUS), while a few have a ?lamentous grouping. The size of bacteria ranges from around 0.2 to 5 µm and the smallest (MYCOPLASMA) are roughly the same size as the largest viruses (poxviruses – see VIRUS). They are the smallest organisms capable of existing outside their hosts. The longest, rod-shaped bacilli are slightly smaller than the human erythrocyte blood cell (7 µm).
Bacterial cells are surrounded by an outer capsule within which lie the cell wall and plasma membrane; cytoplasm ?lls much of the interior and this contains genetic nucleoid structures containing DNA, mesosomes (invaginations of the cell wall) and ribosomes, containing RNA and proteins. (See illustration.)
Reproduction is usually asexual, each cell dividing into two, these two into four, and so on. In favourable conditions reproduction can be very rapid, with one bacterium multiplying to 250,000 within six hours. This means that bacteria can change their characteristics by evolution relatively quickly, and many bacteria, including Mycobacterium tuberculosis and Staphylococcus aureus, have developed resistance to successive generations of antibiotics produced by man. (METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS (MRSA)) is a serious hazard in some hospitals.
Bacteria may live as single organisms or congregate in colonies. In arduous conditions some bacteria can convert to an inert, cystic state, remaining in their resting form until the environment becomes more favourable. Bacteria have recently been discovered in an inert state in ice estimated to have been formed 250 million years ago.
Bacteria were ?rst discovered by Antonj van Leewenhoek in the 17th century, but it was not until the middle of the 19th century that Louis Pasteur, the famous French scientist, identi?ed bacteria as the cause of many diseases. Some act as harmful PATHOGENS as soon as they enter a host; others may have a neutral or benign e?ect on the host unless the host’s natural immune defence system is damaged (see IMMUNOLOGY) so that it becomes vulnerable to any previously well-behaved parasites. Various benign bacteria that permanently reside in the human body are called normal ?ora and are found at certain sites, especially the SKIN, OROPHARYNX, COLON and VAGINA. The body’s internal organs are usually sterile, as are the blood and cerebrospinal ?uid.
Bacteria are responsible for many human diseases ranging from the relatively minor – for example, a boil or infected ?nger – to the potentially lethal such as CHOLERA, PLAGUE or TUBERCULOSIS. Infectious bacteria enter the body through broken skin or by its ori?ces: by nose and mouth into the lungs or intestinal tract; by the URETHRA into the URINARY TRACT and KIDNEYS; by the vagina into the UTERUS and FALLOPIAN TUBES. Harmful bacteria then cause disease by producing poisonous endotoxins or exotoxins, and by provoking INFLAMMATION in the tissues – for example, abscess or cellulitis. Many, but not all, bacterial infections are communicable – namely, spread from host to host. For example, tuberculosis is spread by airborne droplets, produced by coughing.
Infections caused by bacteria are commonly treated with antibiotics, which were widely introduced in the 1950s. However, the con?ict between science and harmful bacteria remains unresolved, with the overuse and misuse of antibiotics in medicine, veterinary medicine and the animal food industry contributing to the evolution of bacteria that are resistant to antibiotics. (See also MICROBIOLOGY.)... bacteria
Health Encyclopedia