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|Classification and external resources|
|Classification and external resources|
Although most bacteria are harmless or often beneficial, several are pathogenic. One of the bacterial diseases with the highest disease burden 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 are criteria designed to establish a causal relationship between a causative microbe and a disease.
Conditionally pathogenic bacteria are only pathogenic under certain conditions, such as a wound that allows for entry into the blood, or a decrease in immune function.
For example, Staphylococcus or Streptococcus are also part of the normal human flora and usually exist on the skin or in the nose without causing disease, but can potentially cause skin infections, pneumonia, meningitis, and even overwhelming sepsis, a systemic inflammatory response producing shock, massive vasodilation and death.
Some species of bacteria, such as Pseudomonas aeruginosa, Burkholderia cenocepacia, and Mycobacterium avium, are opportunistic pathogens and cause disease mainly in people suffering from immunosuppression or cystic fibrosis.
Other organisms invariably cause disease in humans, such as obligate intracellular parasites (e.g. Chlamydophila, Ehrlichia, Rickettsia) that are able to grow and reproduce only within the cells of other organisms. Still, infections with intracellular bacteria may be asymptomatic, such as during the incubation period. An example of intracellular bacteria is Rickettsia. One species of Rickettsia causes typhus, while another causes Rocky Mountain spotted fever.
Following is a list of bacterial infections classified by location in the body:
Iron is required for humans, as well as the growth of most bacteria. To obtain free iron, some pathogens secrete proteins called siderophores, which take the iron away from iron-transport proteins by binding to the iron even more tightly. Once the iron-siderophore complex is formed, it is taken up by siderophore receptors on the bacterial surface and then that iron is brought into the bacterium.
Once pathogens attach to host cells, they can cause direct damage as the pathogens use the host cell for nutrients and produce waste products. As pathogens multiply and divide inside host cells, the cells usually rupture and the intercellular bacteria are released. Some bacteria such as E. coli, Shigella, Salmonella, and Neisseria gonorrhoeae, can induce host epithelial cells to engulf them in a process resembling phagocytosis. The pathogens can then disrupt host cells as they pass through them and be extruded from host cells by a reverse phagocytosis process, enabling them to enter other host cells. Some bacteria can also penetrate host cells by excreting enzymes and by their own motility; such penetration can can itself damage the host cell.
Toxins are poisonous substances that are produced by certain microorganisms and are often the primary factor contributing to the pathogenic properties of the microorganisms. Endotoxins are the lipid portions of lipopolysaccharides that are part of the outer membrane of the cell wall of gram negative bacteria. Endotoxins are released when the bacteria lyses, which is why after antibiotic treatment symptoms can at first worsen as the bacteria are killed and they release their endotoxins.Exotoxins are proteins produced inside pathogenic bacteria as part of their growth and metabolism, most common in gram positive bacteria. The exotoxins are released when the bacteria die and the cell wall breaks apart. Exotoxins are highly specific in the effects on body tissues and work by destroying particular parts of the host cell or by inhibiting certain metabolic functions. Exotoxins are among the most lethal known substances, only 1mg of the botulinum exotoxin is enough to kill one million guinea pigs. Diseases caused this way are often caused by minute amounts of exotoxins, not by the bacteria themselves.
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, so they 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. Phage therapy can also be used to treat certain bacterial infections. Infections can be prevented by antiseptic measures such as sterilizing 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 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. Bacteria in food are killed by cooking to temperatures above 73 °C (163°F).
|Genus||Important species||Gram staining||Shape||Capsulation||Bonding tendency||Motility||Respiration||Growth medium||Intra/Extracellular|
|Bordetella||Gram-negative||Small coccobacilli||Encapsulated||Singly or in pairs||Non-motile||Aerobic||Regan-Lowe agar||Extracellular|
|Borrelia||Gram-negative, but stains poorly||Spirochete||Unencapsulated||Long, slender, flexible, spiral- or corkscrew-shaped rods||Highly motile||Anaerobic||(Difficult to culture)||Extracellular|
|Brucella||Gram-negative||Small coccobacilli||Unencapsulated||Singly or in pairs||Non-motile||Aerobic||Ruiz Castaneda medium||Intracellular|
|Campylobacter||Gram-negative||Curved, spiral, or S-shaped|
bacilli with single, polar flagellum
|Unencapsulated||Singly||characteristic darting motion||microaerophilic||Blood agar inhibiting other fecal flora||extracellular|
|Chlamydia and Chlamydophila||(not Gram-stained)||Small, round, ovoid||Unencapsulated||motile||Facultative or strictly aerobic||Obligate intracellular|
|Clostridium||Gram-positive||Large, blunt-ended rods||Normally encapsulated||mostly motile||Obligate anaerobic||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 (cocci)||pairs or chains||Non-motile||Facultative Anaerobic||6.5% NaCl, bile-esculin agar||extracellular|
|Escherichia||Gram-negative||Short rods (bacilli)||Encapsulated and Unencapsulated||Normally motile||Facultative anaerobic||MacConkey agar||extracellular or intracellular|
|Francisella||Gram-negative||Small, pleomorphic coccobacillus||Encapsulated||Non-motile||strictly aerobic||(rarely cultured)||Facultative intracellular|
|Haemophilus||Gram-negative||Ranging from small coccobacillus to long, slender filaments||Encapsulated or Unencapsulated||Non-motile||Chocolate agar with hemin and NAD+||extracellular|
|Helicobacter||Gram-negative||Curved or spiral rods|
with multiple polar flagella
|rapid, corkscrew motility||Microaerophile||Medium containing antibiotics against other fecal flora||extracellular|
|Legionella||Gram-negative, but stains poorly||Slender rod in nature, cocobacillary in laboratory.|
|unencapsulated||motile||aerobic||Specialized medium||facultative intracellular|
|Leptospira||Gram-negative, but stains poorly||Long, very slender, flexible, spiral- or corkscrew-shaped rods||Highly motile||Strictly aerobic||Specialized medium||extracellular|
|Listeria||Gram-positive, darkly||Slender, short rods||diplobacilli or short chains||Distinct tumbling motility in liquid medium||Facultative Anaerobic||enriched medium||intracellular|
|Mycobacterium||(none)||Long, slender rods||Unencapsulated||nonmotile||aerobic||M. tuberculosis: Lowenstein-Jensen agar|
M. leprae: (none)
|Mycoplasma||(none)||Plastic, pleomorphic||Encapsulated||singly or in pairs||Mostly facultative anaerobic; M.pneumoniae strictly aerobic||(rarely cultured)||extracellular|
|Neisseria||Gram-negative||Kidney bean-shaped||Encapsulated or Unencapsulated||diplococci||Non-motile||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||Slime/microcapsule||Non-motile||Aerobic||(rarely cultured)||Obligate intracellular|
|Salmonella||Gram-negative||Bacilli||Encapsulated||Normally motile||Facultative anaerobic||MacConkey agar||Facultative intracellular|
|Shigella||Gram-negative||rods||Unencapsulated||Non-motile||Facultative anaerobic||Hektoen agar||extracellular|
|Staphylococcus||Gram-positive, darkly||Round cocci||Encapsulated or Unencapsulated||in bunches like grapes||Non-motile||Facultative anaerobic||enriched medium (broth and/or blood)||extracellular, facultative intracellular|
|Streptococcus||Gram-positive||ovoid to spherical||Encapsulated or Unencapsulated||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||Aerobic||none||extracellular|
|Vibrio||Gram-negative||Short, curved, rod-shaped with single polar flagellum||Unencapsulated||rapidly motile||Facultative anaerobic||blood- or MacConkey agar. Stimulated by NaCl||extracellular|
|Yersinia||Gram-negative, stains bipolarly||Small rods||encapsulated||nonmotile||Facultative Anaerobe||MacConkey or CIN agar||Intracellular|
This is a rather clinical description of the species presented in the previous section, containing the main examples of transmission, diseases, treatment, prevention and laboratory diagnosis, which all can differ substantially among the species of the same genus.
|Bacillus anthracis||In early infection:|
|Borrelia burgdorferi||Ixodes ticks|
reservoir in deer, mice and other rodents
|Combination therapy of:||-|
|Campylobacter jejuni||No available vaccine |
|Chlamydia pneumoniae||Community-acquired respiratory infection||None||None for routine use|
|Chlamydia trachomatis||No vaccine |
|Chlamydophila psittaci||Inhalation of dust with secretions or feces from birds (e.g. parrots)||Psittacosis||-|
|Clostridium botulinum||Spores from soil and aquatic sediments contaminating vegetables, meat and fish|
|Clostridium perfringens||Gas gangrene: |
|Appropriate food handling|
|Corynebacterium diphtheriae||(no rapid)|
|Enterococcus faecalis and Enterococcus faecium||No vaccine |
|Escherichia coli (generally)||UTI:|
(resistance-tests are required first)
|(no vaccine or preventive drug) |
|Enterotoxigenic Escherichia coli (ETEC)|
|Enteropathogenic E. coli|
|E. coli O157:H7|
|Francisella tularensis||(rarely cultured) |
(resistance-tests are required first)
|Helicobacter pylori||(No vaccine or preventive drug)|
|Legionella pneumophila||(no vaccine or preventive drug) |
|Leptospira interrogans||(no vaccine) |
Prevention of exposure
|Listeria monocytogenes||(no vaccine) ||Isolation from e.g. blood and CSF|
|Mycobacterium leprae||Tuberculoid form: |
|Tuberculoid form: |
(difficult, see Tuberculosis treatment for more details)
Standard "short" course:
|Mycoplasma pneumoniae||(difficult to culture)|
|Neisseria gonorrhoeae||Uncomplicated gonorrhea:|
|Pseudomonas aeruginosa||Infects damaged tissues or people with reduced immunity.|
Localized to eye, ear, skin, urinary, respiratory or gastrointestinal tract or CNS, or systemic with bacteremia, secondary pneumonia bone and joint infections, endocarditis, skin, soft tissue or CNS infections.
|(no vaccine) |
|Rickettsia rickettsii||(no preventive drug or approved vaccine) |
|Salmonella typhi||Human-human |
|Salmonella typhimurium||(No vaccine or preventive drug) |
|Staphylococcus aureus||Coagulase-positive staphylococcal infections:||(no vaccine or preventive drug) |
|Staphylococcus epidermidis||Human flora in skin and anterior nares||None|
|Staphylococcus saprophyticus||Part of normal vaginal flora||None|
|Streptococcus agalactiae||Human flora in vagina or urethral mucous membranes, rectum||None|
|Streptococcus pyogenes||No vaccine |
|Treponema pallidum||No preventive drug or vaccine ||Cannot be cultured or viewed in gram-stained smear|