Pathogenicity of microorganisms

2021-08-24 11:10 PM

Pathogenic microorganisms are the cause of infections. Without pathogenic microorganisms, there is no infection. The pathogenicity of microorganisms depends on their virulence

General perception

For bacteria, the human body is a suitable habitat for many microorganisms, this environment has the right temperature, humidity, and food for them to grow. Many bacteria reside (colonization) on the surface of the human body without harming the host organism, they live in normal commensal with the host body and form a normal microflora component. ) of the body, however, these bacteria will become pathogens when they cross the body's barriers (skin, mucous membranes) to enter the host's body. Virulent bacteria such as diphtheria, typhoid, cholera, dysentery bacteria .. they always have a mechanism that makes it easy for them to grow in the host body and cause damage to organs or organs of the body. and cause infection.

When pathogenic microorganisms enter the host's body, under certain natural and social conditions, they cause a complex, interactive process called infection. In this process microorganisms are the cause, the human body is the object of perception. The objective circumstances directly or indirectly influence the infection. When microorganisms have not yet entered the body, environmental factors have a direct effect on humans and pathogenic microorganisms create favorable or unfavorable conditions for the infection process. When microorganisms have entered the body, the environment only affects humans and affects microorganisms.

Pathogenic microorganisms are the cause of infections. Without pathogenic microorganisms, there is no infection. The pathogenicity of microorganisms depends on the virulence, the number of invading microorganisms, and the route of entry.

Microbial virulence

As strong or weak pathogenicity of a microorganism, Salnonella typhi and Salmonella paratyphi both cause typhoid fever, but Salmonella typhi presents with more severe disease symptoms.

To evaluate the virulence of a bacterial strain, a lethal dose of 50 (LD 50) was used. LD 50 is the dose of microorganisms or their toxins that kill 50% of the experimental animal population in a given time period. There is now a tendency to use this unit rather than the minimum lethal dose (MLD). MLD is the smallest dose of a microorganism or its toxin that kills a test animal of a given weight, within a given experimental period. MLD is less accurate because it depends on the state and function of the immune and nervous systems of each experimental animal.

The virulence of a microorganism is not fixed. When newly isolated in the patient's body, the microorganism is usually highly virulent, but through the inoculation process, the virulence gradually decreases and sometimes disappears, due to the selection of strains adapted to new conditions. On this basis, many live attenuated microbial vaccines have been prepared such as the polio vaccine, BCG vaccine. In the BCG vaccine, the first source of bacteria was a very virulent strain of bovine tuberculosis (M. bovis), but through culture on bovine bile for many years and repeated subcultures, Calmette and Guerin created a complete strain of bacteria. completely non-toxic bacteria called BCG bacteria used as tuberculosis vaccines (Bacille de Calmette et Guerin). On the other hand, an attenuated strain of microorganisms when injected into animals can increase their virulence.

The virulence of microorganisms includes their toxins, adhesion, and infectivity.

Adhesion ability

Adhesion is the first step in infection, many bacteria have on their surfaces macromolecules called adhesion factors (adhesin) that mediate bacterial adhesion to specific receptors. The signal was found in some animal cells but not in others. The adhesion components in bacteria can be taken up by pili, E.coli adheres to intestinal cells or bladder epithelial cells more than to the oral mucosa. Shigella pathogens adhere to the intestinal epithelium. Likewise, Neisseria gonorrhea adheres to the urethral epithelium.

Antibodies of bacterial surface components not only have the effect of opsonizing bacteria but also neutralize antigens, preventing bacteria from adhering to the epithelium. More than half of the glycoproteins in body secretions compete with bacterial adhesion receptors and prevent bacterial adhesion; surface epithelial cell shedding is another defense mechanism as it eliminates bacteria. adhere to cells

Possibility of infection

The ability to enter the host's tissues, multiply there, and then spread to other regions.

The ability to create shells

The envelope of many bacteria confers resistance to phagocytosis, the most obvious example being pneumococcus: compare an enveloped pneumococcal strain (S-type colony) and an unencapsulated pneumococcal strain (colony) form R), the enveloped pneumococcal strain (S) causes disease in mice while the unencapsulated strain (R) does not. In Streptococcus pyogenes the anti-macrophage factor is located on the outer surface of the cell wall.


Many pathogenic bacteria produce many enzymes that contribute to their ability to infect.

The enzyme hyaluronidase secreted by staphylococcus and streptococcus hydrolyzes hyaluronic acid, a basic component of connective tissue, making it easier for bacteria to disperse into the tissue.

Collagenase breaks down the collagen component of muscle tissue.

Streptokinase dissolves blood clots.

Leucocidin kills white blood cells.

Hemolysin dissolves red blood cells.

Mucinous is secreted by some bacteria in the digestive tract, breaking down the mucous layer covering the intestinal mucosa, creating conditions for bacteria to come into direct contact with cells and penetrate inside the epithelium.

Possibility to produce toxins

Toxins are formed during the metabolism of bacteria. Bacterial toxins are divided into exotoxins and endotoxins.


Because some gram-positive bacteria and gram-negative bacteria form and secrete into the surrounding environment, its chemical nature is protein, has high antibiotic properties, and is easily destroyed by temperature (600C). Many bacterial exotoxins such as diphtheria, tetanus, and botulism are extracted in pure form. Exotoxins are highly toxic, for example, 0.02mg of diphtheria exotoxin can kill a person.

Exotoxins, when treated with forcmol or by temperature, after a while lose their toxicity but retain their antigenic properties completely, this preparation is called an antitoxin used to make a specific preventive vaccine.


This toxin is closely related to the cell wall of gram-negative bacteria, it does not diffuse to the outside environment, only when the bacteria die and the cells are destroyed, the endotoxin is released to the outside.

The chemical structure of the wall of gram-negative bacteria is a mixture of glucit, lipid, and protein in which the toxic component of the endotoxin is mainly the lipid A component of the lipopolysaccharide layer of the cell wall. Endotoxin is highly heat-resistant, and its toxicity is weaker than that of exotoxins. It is now well known that endotoxins from gram-negative bacteria (-) have biological effects that are both beneficial to the host organism by stimulating an immune response, on the other hand, it is also toxic to cause shock and death.

The immunogenicity of the endotoxin is weak, and it is not able to transform into an antitoxin when treated with forcmol.

Virulence of the virus

Unlike bacteria, viruses are obligate intracellular invaders. Viruses have a simple structure, viruses do not contain the enzymes and mitotic machinery necessary for metabolism and growth. Viruses can only multiply when they enter the host's cells, they use the available materials of the cell to synthesize and multiply on the basis of their genetic information. New viruses formed in the host cell will break the cell to release new viruses into the environment and continue the replication cycle in adjacent cells. Thus, the pathogenic nature of the virus is related.

Disruption of infected cell populations.

Change in morphology, structure and make cells lose function.

Number of microorganisms

The human body has effective defense mechanisms. Microorganisms cause disease only when the number of invaders reaches a certain level. If the amount is too small, the microorganisms are easily eliminated by the host body. Experiments in volunteers showed that the amount of Salmonella typhi ingested to cause disease was about 106 or to cause experimental cholera, the volunteers had to drink sodium bicarbonate solutions to alkalize the gastric juices. at the same time drinking an amount of about 109 cholera bacteria.

Path of entry

The route of entry also affects the likelihood of infection. Many microorganisms have organotrophic properties, which selectively target the tissue surface they infect. Meningococcal and pneumococcus are commonly found in the nasopharynx, but pneumococcus infects the lower respiratory tract causing pneumonia, while meningococcal infects the upper respiratory tract and causes meningitis. Microorganisms cause disease only when they enter the body through suitable routes: Oral typhoid bacteria, genital gonorrhea, wound tetanus bacilli, hepatitis A virus, poliovirus through orally, hepatitis B virus through injection...