Pathogenic

The pathogenic term (of the Greek παθογένεια ! “birth of the pain”) means: who involves a disease. The pathogenic germs or the pathogenic Bactérie S are responsible for diseases.

The pathogenic capacity of a bacterium is thus its capacity to cause disorders at a host. It depends on its invasive capacity (capacity to be spread in the fabric S and to establish un/des infectious hearths there), and on its toxicogenic capacity (capacity to produce Toxine S). There exists also many pathogenic viruses .

One distinguishes two categories of pathogenic bacteria:

  • Strict or specific: These bacteria cause disorders whatever the patient, except in the case of the healthy carriers . For example: Salmonella typhi and Vibrio cholerae .
  • Opportunist : These bacteria cause disorders when immunizing defenses of the host are weakened (one also speaks about immunodéprimés subjects ). For example: Pseudomonas aeruginosa .

In the hospitals, the infections caused by opportunist bacteria are called Infections nosocomiales .

Invasive capacity

The invasive capacity is the capacity of a bacterium to be multiplied and spread themselves in a organization-host, in spite of immunizing defenses.

Factors related to the bacterium

The constitution and the Métabolisme of the bacterium partly define the invasive capacity of this one.

  • the factors of adhesion : The presence of fimbriae (or pili ), of adhésines, and/or Glycocalyx facilitates the fixing of the bacterium on its cell-target.

  • the resistance to the Phagocytosis thanks to the presence of a Capsule, but also resistance to the enzymes lysozomiales (judicious to destroy the phagocytée bacterium).
  • production of enzymes:
    • the collagénases , which degrade the collagen of conjunctive fabrics (at Clostridium perfringens for example).
    • the coagulases , which allow the formation of a clot around the bacterial body, which protects it from the cells of the immune system (at S.aureus for example).
    • the hyaluronidases , which degrade the hyaluronic acid, constituting conjunctive fabrics
    • the DNases ( or ADNases ) which degrade DNA infected cells.
    • the kinases (or fibrinolysines ).

Factors related to the host

The ground infected, i.e. the whole of the factors independent of the bacterium, which will determine the gravity of the infection under the conditions given: age, tiredness, diseases,… and physicochemical factors of the environment.

Toxic capacity

A Toxine is a molecule synthesized by a living organism, having a harmful or lethal effect for the organisme-hôte.
See also : Properties of bacterial toxins The proteinic toxins are the poisons more actifs.
100 toxin Mg (tetanic or botulinic) would be enough to kill all the human population.

Quantification of the toxic capacity

The pathogenic capacity can be quantified by three data : the Minimal Amount Mortal (DM) , the Lethal Amount 50 (DL50) and the Infecting Minimal Amount (DMI) .

  • the DM is the weakest amount which kills within a given time an experimental group.
  • DL50 is the amount where the number of disease-causing agents is able to kill 50% of the hosts of an experimental group in a given time.
  • DMI is the minimal amount of pathogenic agents given allowing the contamination and the development of the disease.

Mode of action of toxins

The toxins can act according to several modes: according to the bacterium in question and the mode of contamination, the production and the action of toxin will be done differently.

  • At the time of a wound , the bacterium multiplies and releases its proteinic toxin, which will act on the level of the spinal-cord , by decreasing the quantity of released neurotransmitters, and on the level of the neuro-muscular synapses by increasing the acetylcholine release. It causes a paralysis diminishes .

Example: Tetanic toxin
  • At the time of a ingestion , the toxin passes in the Sang and decreases the quantity of acetylcholine on the level of the neuro-muscular junctions. It causes a flask paralysis .
Example: Botulinic toxin
  • Following a ingestion , the bacterium adheres to the intestinal epithelium and produces the toxin which is fixed on the entérocytes. It prevents the absorption of the ions Na+ and Cl-, and thus causes a mineral-water escape .
Example: Choleraic toxin

Antigenic capacity of toxins

The proteinic toxins have a very high toxic capacity. They cause the appearance of antibody in the organization: the antitoxins . They can be transformed into anatoxines by a treatment formol, and an incubation with 40°C ( Méthode of Ramon ). These anatoxines is used for:

  • to manufacture vaccines ;
  • to manufacture serums used in serotherapy.

Pathogenic examples of bacteria

Pathogenic examples of bacteria are Escherichia coli (diarrheas, infections urinary, infections nosocomiales: septicaemias, meningitis of the newborn, haemolytic-uremic syndrome: EHEC O157), Staphylococcus aureus , Pseudomonas aeruginosa (infections of the Eyes, the Wound S and acute gastro-enteritis), Salmonella, Yersinia enterocolitica (gastro-enteritis), Campylobacter jejuni Vibrio cholerae (cholera) Shigella dysenteriae (dysentery) .

The purified water must thus be removed from these pathogenic germs when it is rejected into the natural environment not to contaminate and cause a epidemic being able to cause the death of the populations downstream.

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