Pathogenicity is a complex and multi-factorial process that encompasses all biochemical mechanisms whereby micro-organisms produce infections. In other words, pathogenicity is the ability to produce the disease in a host body.

There are several types of bacteria:

• Saprophytic Bacteria – those that live in nature but are set in the body: on the skin, mucous membranes.
• Commensal Bacteria – those that develop in contact with the host.
• Pathogenic bacteria/high pathogens – those that produce an apparent or clinically apparent disease:

N. Gonorrhoeae, T. pallidum.

• Pathogen-opportunistic bacteria: those which are commensal but become pathogenic under certain conditions: E. coli, K. pneumoniae.

Two great qualities of pathogenic bacteria are the basis for the means of causing a disease:

1. Invasiveness is the ability to invade the tissues.
2. To be toxic is the ability to produce toxins.

Bacteria can produce two types of toxins called exotoxins and endotoxins depending on the place of toxins action.

• Exotoxins are released from bacterial cells and sent outside permanently, not only after the death of the bacterium.
• Bacterial endotoxins are basically structural components of the bacterium and are localized intraparietal or intracellular.

After activity, Exotoxins can be:

• Toxins that block the synthesis of cellular proteins: – Kill the cell in which they penetrated; – Toxin Bacillus pseudomonas (Pseudomonas aeruginosa)
• Neurotoxins: – tetanus toxin (Clostridium tetani) and botulinum toxin (Clostridium botulinum).
• Toxins that change cell metabolism. These toxins do not lead to the death of the cell, but to a chenge in function and metabolism, making it inactive (example: Thermolabile toxin of E. coli)
• Toxins that destroy the fundamental substance of connective tissue (example: S. pyogenes – Hyaluronidase)

Endotoxins – are released into the environment only after the cell death, as they are part of lipidic cellular  wall structure of the pathogenic G (-) bacteria.

Endotoxins structure:

• Lipid A (the active part of all endotoxins) which is a glycophospholipid;
• Core (core) consisting of sugars, ethanolamin and phosphoric acid;
• Antigen O, a long chain of saccharides, specific to the species.

The effects of Endototoxins are not specific depending on the bacteria species that synthesize them (such as in the case of Exotoxins), but are identical regardless of which gram negative bacteria originate. Such:

• Bacterial endotoxin causes the febrile reaction by its action on macrophages and stimulation of the secretion of endogenous Pirogeni: Interleukins and TNF (tumour necrosis factor);
• Complement factors are directly activated alternatively by opsonized bacteria and toxic products released by them and will support phagocytosis NPM (neutrophils polymorphonuclear) and macrophages on bacteria that have spread to the infectious outbreak. The NPM will accumulate under the C5a action (powerful factor chemotactic and chemokinetic for NPM that will be attracted in the inflammatory outbreak) and will phagocyte C3b opsonized bacteria. Anaphylotoxins C3a and C5a are generated in vivo, and their release in physiological quantities leads to mastocytes degranulation with the release of vasoactive products. This makes the inflammatory response.

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