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
Among the smallest and simplest microorganisms are the viruses. First described as ?lterable agents, and ranging in size from 20–30 nm to 300 nm, they may be directly visualised only by electron microscopy. They consist of a core of deoxyribonucleic or ribonucleic acid (DNA or RNA) within a protective protein coat, or capsid, whose subunits confer a geometric symmetry. Thus viruses are usually cubical (icosahedral) or helical; the larger viruses (pox-, herpes-, myxo-viruses) may also have an outer envelope. Their minimal structure dictates that viruses are all obligate parasites, relying on living cells to provide essential components for their replication. Apart from animal and plant cells, viruses may infect and replicate in bacteria (bacteriophages) or fungi (mycophages), which are damaged in the process.
Bacteria are larger (0·01–5,000 µm) and more complex. They have a subcellular organisation which generally includes DNA and RNA, a cell membrane, organelles such as ribosomes, and a complex and chemically variable cell envelope – but, unlike EUKARYOTES, no nucleus. Rickettsiae, chlamydia, and mycoplasmas, once thought of as viruses because of their small size and absence of a cell wall (mycoplasma) or major wall component (chlamydia), are now acknowledged as bacteria; rickettsiae and chlamydia are intracellular parasites of medical importance. Bacteria may also possess additional surface structures, such as capsules and organs of locomotion (?agella) and attachment (?mbriae and stalks). Individual bacterial cells may be spheres (cocci); straight (bacilli), curved (vibrio), or ?exuous (spirilla) rods; or oval cells (coccobacilli). On examination by light microscopy, bacteria may be visible in characteristic con?gurations (as pairs of cocci [diplococci], or chains [streptococci], or clusters); actinomycete bacteria grow as ?laments with externally produced spores. Bacteria grow essentially by increasing in cell size and dividing by ?ssion, a process which in ideal laboratory conditions some bacteria may achieve about once every 20 minutes. Under natural conditions, growth is usually much slower.
Eukaryotic micro-organisms comprise fungi, algae, and protozoa. These organisms are larger, and they have in common a well-developed internal compartmentation into subcellular organelles; they also have a nucleus. Algae additionally have chloroplasts, which contain photosynthetic pigments; fungi lack chloroplasts; and protozoa lack both a cell wall and chloroplasts but may have a contractile vacuole to regulate water uptake and, in some, structures for capturing and ingesting food. Fungi grow either as discrete cells (yeasts), multiplying by budding, ?ssion, or conjugation, or as thin ?laments (hyphae) which bear spores, although some may show both morphological forms during their life-cycle. Algae and protozoa generally grow as individual cells or colonies of individuals and multiply by ?ssion.
Micro-organisms of medical importance include representatives of the ?ve major microbial groups that obtain their essential nutrients at the expense of their hosts. Many bacteria and most fungi, however, are saprophytes (see SAPROPHYTE), being major contributors to the natural cycling of carbon in the environment and to biodeterioration; others are of ecological and economic importance because of the diseases they cause in agricultural or horticultural crops or because of their bene?cial relationships with higher organisms. Additionally, they may be of industrial or biotechnological importance. Fungal diseases of humans tend to be most important in tropical environments and in immuno-compromised subjects.
Pathogenic (that is, disease-causing) microorganisms have special characteristics, or virulence factors, that enable them to colonise their hosts and overcome or evade physical, biochemical, and immunological host defences. For example, the presence of capsules, as in the bacteria that cause anthrax (Bacillus anthracis), one form of pneumonia (Streptococcus pneumoniae), scarlet fever (S. pyogenes), bacterial meningitis (Neisseria meningitidis, Haemophilus in?uenzae) is directly related to the ability to cause disease because of their antiphagocytic properties. Fimbriae are related to virulence, enabling tissue attachment – for example, in gonorrhoea (N. gonorrhoeae) and cholera (Vibrio cholerae). Many bacteria excrete extracellular virulence factors; these include enzymes and other agents that impair the host’s physiological and immunological functions. Some bacteria produce powerful toxins (excreted exotoxins or endogenous endotoxins), which may cause local tissue destruction and allow colonisation by the pathogen or whose speci?c action may explain the disease mechanism. In Staphylococcus aureus, exfoliative toxin produces the staphylococcal scalded-skin syndrome, TSS toxin-1 toxic-shock syndrome, and enterotoxin food poisoning. The pertussis exotoxin of Bordetella pertussis, the cause of whooping cough, blocks immunological defences and mediates attachment to tracheal cells, and the exotoxin produced by Corynebacterium diphtheriae causes local damage resulting in a pronounced exudate in the trachea.
Viruses cause disease by cellular destruction arising from their intracellular parasitic existence. Attachment to particular cells is often mediated by speci?c viral surface proteins; mechanisms for evading immunological defences include latency, change in viral antigenic structure, or incapacitation of the immune system – for example, destruction of CD 4 lymphocytes by the human immunode?ciency virus.... microbiology
Some toxins are not set free by bacteria, but remain in the substance of the latter. They are known as endotoxins and are not capable of producing antitoxins.... toxins
Poisons may also originate in the body, as when bacteria produce endotoxins, or when metabolic disorders produce poisonous substances or allow them to build up.
Poisoning may be acute (a large amount of poison over a short time) or chronic (gradual accumulation of poison that is not eliminated quickly).
Unintentional poisoning occurs mainly in young children.
Adults may be poisoned by mistaking the dosage of a prescribed drug (see drug poisoning), by taking very high doses of vitamin or mineral supplements, by exposure to poisonous substances in industry, or by drug abuse.
Poisoning may also be a deliberate attempt to commit suicide.... poisoning
(See also poison; poisoning; toxaemia.)... toxic shock syndrome
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