Pathogenic bacteria

Pathogenic bacteria are bacteria that cause bacterial infection. This article deals with human pathogenic bacteria.

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.


Each pathogenic species has a characteristic spectrum of interactions with its human hosts.

Conditionally pathogenic

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.

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.

Salmonella, Neisseria, Brucella, Mycobacterium, Listeria, Francisella, Legionella, and Yersinia pestis can exist intracellularly, though they are facultative (not obligate) intracellular parasites.

By location

Following is a list of bacterial infections classified by location in the body:

  • Bacterial vaginosis is a disease of the vagina caused by an imbalance of naturally occurring bacterial flora and is often confused with yeast infection (candidiasis), or infection with Trichomonas vaginalis (trichomoniasis), which are not caused by bacteria.
  • Bacterial meningitis is a bacterial inflammation of the meninges, that is, the protective membranes covering the brain and spinal cord.
  • Bacterial pneumonia is a bacterial infection of the lungs.
  • Urinary tract infection is almost exclusively caused by bacteria. Symptoms include frequent feeling and/or need to urinate, pain during urination, and cloudy urine. The main causal agent is Escherichia coli. Although urine contains a variety of fluids, salts, and waste products, it does not usually have bacteria in it, but when bacteria get into the bladder or kidney and multiply in the urine, they may cause a UTI.
  • Bacterial gastroenteritis is caused by pathogenic enteric bacteria. Such pathogenic enteric bacteria are generally distinguished from the usually harmless bacteria of the normal gut flora, but the distinction is often not fully clear, and Escherichia, for example, can belong to either group.
  • Bacterial skin infections include:
    • Impetigo is a highly contagious bacterial skin infection most common among pre-school children. It is primarily caused by Staphylococcus aureus, and sometimes by Streptococcus pyogenes.
    • Erysipelas is an acute streptococcus bacterial infection of the deep epidermis with lymphatic spread.
    • Cellulitis is a diffuse inflammation of connective tissue with severe inflammation of dermal and subcutaneous layers of the skin. Cellulitis can be caused by normal skin flora or by exogenous bacteria, and often occurs where the skin has previously been broken: cracks in the skin, cuts, blisters, burns, insect bites, surgical wounds, intravenous drug injection, or sites of intravenous catheter insertion. Skin on the face or lower legs is most commonly affected by this infection, though cellulitis can occur on any part of the body.

How Bacterial Pathogens Damage Host Cells

Using the Host’s Nutrients

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.

Direct Damage

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.

Toxin production

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. Diseased caused this way are often caused by minute amounts of exotoxins, not by the bacteria themselves.


See also overview list below

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).

List of pathogenic bacteria by basic laboratory characteristics

Following are the genera that contain the most important human pathogenic bacteria species:

List of pathogenic bacteria by clinical characteristics

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.


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