Microbiology, Clinical
Clinical microbiology is concerned with infectious microorganisms. Various bacteria, algae and fungi are capable of causing disease.
Disease causing microorganisms have been present for millennia. Examples include anthrax, smallpox, bacterial tuberculosis, plague, diphtheria, typhoid fever, bacterial diarrhea, and pneumonia. While modern technological advances, such as mass vaccination, have reduced the threat of some of these diseases, others remain a problem. Some illnesses are re-emerging, due to acquisition of resistance to many antibiotics. Finally, other diseases, such as the often lethal hemorrhagic fever caused by the Ebola virus, have only been recognized within the past few decades.
Many bacterial diseases have only been recognized since the 1970s. These include Legionnaires' disease, Campylobacter infection of poultry, toxic shock syndrome, hemolytic uremic syndrome, Lyme disease, peptic ulcer disease, human ehrlichiosis, and a new strain of cholera. Clinical microbiology research and techniques were vital in identifying the cause of these maladies, and in seeking treatments and ultimately a cure for each malady.
Clinical microbiology involves both the detection and identification of disease-causing microorganisms, and research to find more effective means of treating the infection or preventing infections from occurring. The symptoms of the ailment, and the shape, Gram stain reaction (in the case of bacteria), and biochemical reactions of an unknown organism are used to diagnose the cause of an infection. Knowledge of the identity of the microbe suggests means of treatment, such as the application of antibiotics. Many clinical microbiologists are also researchers. In many cases, the molecular basis of an organism's disease-causing capability is not clear. Unraveling the reasons why a disease is produced can help find ways to prevent the disease.
There are several groups or categories of bacteria that are of medical importance. They are grouped into five categories based on their shape and reaction to the Gram stain. These criteria apply to the light microscope, as typically a first step in the identification of bacteria in an infection is the light microscope examination of material obtained from the infection or from a culture. The groups are Gram-positive bacilli (rod-shaped bacteria), Gram negative bacilli, Gram positive cocci (round bacteria), Gram negative cocci, and bacteria that react atypically to the Gram stain.
A group of spiral shaped bacteria called spirochetes are responsible for leptospirosis in dogs, and syphilis and Lyme disease in humans. These bacteria are easily identified under the light microscope because of their wavy shape and corkscrew movement (courtesy of rigid internal filaments that run the length of the bacterium). A related group (a genus) of spiral shaped bacteria is Spirilla. These bacteria move by means of external flagella, not by means of the internal filaments. Two members of Spirilla are important disease-causing bacteria. The first is Campylobacter jejuni, which frequently contaminates raw meat such as poultry and drinking water, and which is the cause of diarrhea, especially in children. The second bacterial type is Helicobacter pylori, which grows in the stomach and has been demonstrated to be the principle cause of stomach ulcers.
Another group of clinically relevant bacteria is termed pseudomonads. This group contains many different types of bacteria. They all are similar in shape and biochemical behavior to a species called Pseudomonas aeruginosa. Most pseudomonads, like Pseudomonas aeruginosa live in water and the soil. They cause a variety of ailments. Bordetella pertussis causes whooping cough, Legionella pneumophila causes Legionnaires' disease, Neisseria gonorrhoeae causes gonorrhea, and Neisseria meningitides causes bacterial meningitis. Pseudomonas aeruginosa is the quintessential so-called opportunitistic pathogen; a bacteria that does not normally cause an infection but can do so in a compromised host. Examples of such infections are the chronic lung infections in those who have certain forms of cystic fibrosis, and infections in burn victims.
Yet another group of bacteria of medical importance live in the intestinal tracts of humans, other mammals and even in birds and reptiles. These are the enteric bacteria. The best-known enteric bacteria is Escherichia coli, the cause of intestinal illness and sometimes even more severe damage to the urinary tract and kidneys from ingestion of contaminated water or food ("hamburger disease"). Other noteworthy enteric bacteria are Shigella dysenteriae (dysentery), Salmonella species gastroenteritis and typhoid fever), Yersinia pestis (bubonic plague), and Vibrio cholerae (cholera).
Bacteria including Staphylococcus and Streptococcus, which normally live on the skin, can cause infection when they gain entry to other pasts of the body. The illnesses they cause (such as strep throat, pneumonia, and blood infection, as examples), and the number of cases of these illnesses, make them the most clinically important disease-causing bacteria known to man. Staphylococcus aureus is the leading cause of hospital acquired infections of all the gram-positive bacteria. Ominously, a strain of this organism now exists that is resistant to many antibiotics. As this strain increases its worldwide distribution, Staphylococcus infections will become an increasing problem.
Bacteria that normally live in the mouth are responsible for the formation of dental plaque on the surface of teeth. Protected within the plaque, the bacteria produce acid that eats away tooth enamel, leading to the development of a cavity.
A few examples of other clinically important bacteria are Bacillus anthracis (anthrax), Clostridium tetani (tetanus), Mycobacterium tuberculosis (tuberculosis), Corynebacterium diphtheriae (diphtheria), various Rickettsias (Rocky Mountain Spotted Fever, Q fever), Chlamydia trachomatis (chlamydia).
Fungi and yeast are also capable of causing infection. For example, the fungal genus Tinea comprises species that cause conditions commonly described as "jock itch" and "athlete's foot." Scalp infections are also caused by some species of fungus.
Viruses are also the cause of a variety of infections. Inflammation of the coating of nerve cells (meningitis) and brain tissue (encephalitis), and infections of tissues in the mouth, bronchial tract, lungs and intestinal tract result from infection by various viruses.
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