Although the vast majority of bacteria are harmless or beneficial, a few bacteria are pathogenic. The most common bacterial disease is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which kills about 2 million people a year, mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter and Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis and leprosy.
Koch's postulates, proposed by Robert Koch in 1890, are criteria designed to establish a causal relationship between a causative microbe and a disease. A pathogenic cause for a known medical disease may only be discovered many years after, as was the case with Helicobacter pylori and peptic ulcer disease.
Each pathogenic species has a characteristic spectrum of interactions with its human hosts. Some organisms, such as Staphylococcus or Streptococcus, can cause skin infections, pneumonia, meningitis and even overwhelming sepsis, a systemic inflammatory response producing shock, massive vasodilation and death. Yet these organisms are also part of the normal human flora and usually exist on the skin or in the nose without causing any disease at all. Other organisms invariably cause disease in humans, such as the Rickettsia, which are obligate intracellular parasites able to grow and reproduce only within the cells of other organisms. One species of Rickettsia causes typhus, while another causes Rocky Mountain spotted fever. Chlamydia, another phylum of obligate intracellular parasites, contains species that can cause pneumonia, or urinary tract infection and may be involved in coronary heart disease. Finally, some species, such as Pseudomonas aeruginosa, Burkholderia cenocepacia, and Mycobacterium avium, are opportunistic pathogens and cause disease mainly in people suffering from immunosuppression or cystic fibrosis.
Bacterial infections may be treated with antibiotics, which are classified as bacteriocidal if they kill bacteria, or bacteriostatic if they just prevent bacterial growth. There are many types of antibiotics and each class inhibits a process that is different in the pathogen from that found in the host. For example, the antibiotics, chloramphenicol and tetracyclin inhibit the bacterial ribosome, but not the structurally-different eukaryotic ribosome, and so exhibit selective toxicity. Antibiotics are used both in treating human disease and in intensive farming to promote animal growth. Both uses may be contributing to the rapid development of antibiotic resistance in bacterial populations. Infections can be prevented by antiseptic measures such as sterilizating the skin prior to piercing it with the needle of a syringe, and by proper care of indwelling catheters. Surgical and dental instruments are also sterilized to prevent contamination and infection by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection. Most bacteria in food are killed by cooking to temperatures above 60 °C (140 °F).
|Genus||Important species||Gram staining||Shape||Capsulation||Bonding tendency||Motility||Respiration||Growth medium||Intra/Extracellular|
|Bacillus||Gram-positive||Blunt-ended bacilli||Antiphagocytic capsule||singly, in pairs or frequently in long chains||Nonmotile||Facultative or strictly aerobic||Blood agar||extracellular|
|Bordetella||Gram-negative||Small coccobacilli||Encapsulated||singly or in pairs||aerobic||Regan-Lowe agar||extracellular|
|Borrelia||Gram-negative, but stains poorly||Long, slender, flexible, spiral- or corkscrew-shaped rods||higly motile||(difficult to culture)||extracellular|
|Brucella||Gram-negative||Small coccobacilli||Unencapsulated||singly or in pairs||aerobic||Blood agar||intracellular|
|Campylobacter||Gram-negative||Curved, spiral, or S-shaped
with single, polar flagellum
|characteristic darting motion||microaerophilic||Blood agar inhibiting other fecal flora||extracellular|
|Chlamydia||(not Gram-stained)||Small, round, ovoid||motile||Facultative or strictly aerobic||Obligate intracellular|
|Clostridium||Gram-positive||Large, blunt-ended rods||mostly motile||Obligate aerobic||Anaerobic blood agar||extracellular|
|Corynebacterium||Gram-positive (unevenly)||Small, slender, pleomorphic rods||unencapsulated||clumps looking like Chinese characters or a picket fence||nonmotile||Mostly facultative anaerobic||Aerobically on Tinsdale agar||extracellular|
|Enterococcus||Gram-positive||Round to ovoid||pairs or chains||6.5% NaCl, bile-esculin agar||extracellular|
|Escherichia||Gram-negative||Short rods||Facultative anaerobic||MacConkey agar||extracellular|
|Francisella||Gram-negative||Small, pleomorphic coccobacillus||strictly aerobic||(rarely cultured)||Facultative intracellular|
|Haemophilus||Gram-negative||Ranging from small coccobacillus to long, slender filaments||Chocolate agar with hemin and NAD+||extracellular|
|Helicobacter||Gram-negative||Curved or spiral rods
pultiple polar flagella
|rapid, corkscrew motility||Medium containing antibiotics against other fecal flora||extracellular|
|Legionella||Gram-negative, but stains poorly||Slender rod in nature, cocobacillary in laboratory.
|unencapsulated||motile||Specialized medium||facultative intracellular|
|Leptospira||Gram-negative, but stains poorly||Long, very slender, flexible, spiral- or corkscrew-shaped rods||highly motile||Specialized medium||extracellular|
|Listeria||Gram-positive, darkly||Slender, short rods||diplobacilli or short chains||Distinct tumbling motility in liquid medium||enriched medium||intracellular|
|Mycobacterium||(none)||Long, slender rods||nonmotile||aerobic||M. tuberculosis: Lowenstein-Jensen agar
M. leprae: (none)
|Mycoplasma||(none)||Plastic, pleomorphic||singly or in pairs||(rarely cultured)||extracellular|
|Neisseria||Gram-negative||Kidney bean-shaped||diplococci||aerobic||Thayer-Martin agar||Gonococcus: facultative intracellular|
N. meningitidis: extracellular
|Pseudomonas||Gram-negative||rods||encapsulated||motile||Obligate aerobic||MacConkey agar||extracellular|
|Rickettsia||Gram-negative, but stains poorly||Small, rod-like coccobacillary||(rarely cultured)||Obligate intracellular|
|Salmonella||Gram-negative||Facultative anaerobic||MacConkey agar||acellular|
|Shigella||Gram-negative||rods||Facultative anaerobic||Hektoen agar||extracellular|
|Staphylococcus||Gram-positive, darkly||Round cocci||in bunches like grapes||Facultative anaerobic||enriched medium (broth and/or blood)||extracellular|
|Streptococcus||Gram-positive||ovoid to spherical||pairs or chains||nonmotile||Facultative anaerobic||blood agar||extracellular|
|Treponema||Gram-negative, but stains poorly||Long, slender, flexible, spiral- or corkscrew-shaped rods||highly motile||none||extracellular|
|Vibrio||Gram-negative||Short, curved, rod-shaped with single polar flagellum||rapidly motile||Facultative anaerobic||blood- or MacConkey agar. Stimulated by NaCl||extracellular|
|Yersinia||Gram-negative, stains bipolarly||Small rods||encapsulated||nonmotile||MacConkey or CIN agar||extracellular|
This is a further description of the species presented in the previous section, containing transmission, diseases, treatment, prevention and laboratory diagnosis, which all can differ substantially among the species of the same genus.
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- Unless else specified then ref is: Fisher, Bruce; Harvey, Richard P.; Champe, Pamela C. Lippincott's Illustrated Reviews: Microbiology (Lippincott's Illustrated Reviews Series). Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-8215-5. Pages 332 to 353