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The host-parasite relationships

 College of Medicine

Dept. of Microbiology

 Assit. Prof. Dr .Ilham alsaedi

The Host-Parasite Relationships

1-       Mutalism:- The relationship between host and parasite that to be beneficial to both organisms.

2-       Parasitism:- The relationship may be in which one organism damages the other. Parasitism is the condition that is taken place when human are invaded by infectious microorganism.

3-       Commensalism:- The relationship in which parasite deriving nourishment from a host without causing any harm or benefit to host.

4-       Apportunistic:- Commensal microorganism that invade the tissue of the host when the normal immune responses have been compromised.

5-        Saprophyte:- The microorganisms that found in nature, not parasite and living on dead organic matter.

*Carrier:- A person or animal with a symptomatic infection that can be transmitted to another susceptible person or animal.

*Infection:- Is condition in which microorganisms penetrate the host defenses, colonize its tissues and multiply. The microbe may be eliminated by the body or become part of its normal flora.

*Disease:- Refers to those conditions where the host tissues are damaged or their function are altered by those microorganisms.

*Pathogenicity:- The ability of an infectious agent to cause disease or result in production of progressive lesion.

*Virulence:-Degree of pathogenicity of microorganisms virulent agents cause disease when introduced in to the host in small numbers. Virulence involves invasion and toxigenicity.

a-Invasiveness:-Ability to enter host tissue multiply there and spread.

b-Toxogenecity:- Ability to produce toxic substances.

The ability of pathogenic bacteria to cause disease is done by 2 means:-

1-       Ability to invade tissues that started with

a-       Colonization (Adherence and initial multiplication).

b-       Ability to by pass or overcome host defense mechanism.

c-       Production of extracellular substances (invasions) which facilitate the actual invasive process.

2-       Ability to produce toxins that may cause damage to the tissue at site of invasion.

Invasion:- Penetration of pathogens into host cells and tissues that may enhance by:

a-       production of invasions:

many types of enzymes that produced by many species of bacteria are not toxic but do play important roles in the infectious process.

b-       Certain surface components that protect bacteria from phagocytosis and destruction like capsule, LPS, fimbriae.

 (( Types of Invasions))

Extracellular Enzymes:

1-       Hemolysin and Lucocidins:

      Many bacteria produce substances that are (Cytolysin); ie, they dissolve red blood cells (hemolysin) or kill tissue cells or leukocytes (leukocidin), by insertion in to membrane forming a pore that results in cell lysis. e.g. group A Streptococci produce two types of Streptolysin, Streptolysin O and Streptolysin S.

2-       Coagulase:

     Which is produced by Staphylococcus aureus and accelerates the formation of fibrin clot from its precursor, fibrinogen, coagulase contribute to either:

a-       prevent phagocytosis by forming fibrin barrier around bacteria.

b-       fibrin barrier around Staphylococci lesion which help in persist.

3-       IGA protease: Which degrades IGA, allowing the organism to adhere to mucous membrane and is produced chiefly by Neisseria gonorrhoeae, Streptococcus pneumonia.

4-       Spreading factors:

Bacterial enzymes that help in promotion of spread of pathogen these are:-

a-       Collagenase and Hyaluronidase:

    Which degrade their respective intracellular substances, thereby allowing the bacteria to spread through tissue, and are especially important in skin infections, caused by Streptococcus pyogenes

b-       Streptokinase(fibrinolysin):

     A substances that activates a proteolytic enzyme of plasma. This enzyme is then able to dissolve coagulated plasma and probably aids in the rapid spread of Streptococci through tissues.

c-       Neuroaminidase:

     Enzyme that break down neuramic acid a type of intracellular cement of epithelial cells of intestinal mucosa. Produced by Vibrio cholerae

1-       Toxin production:

      The second major mechanism by which bacteria cause disease is the production of toxins.

Toxins: Are defined as microbial products that damage host cells or hast tissues. toxins are generally classified into two groups Exotoxins and Endotoxins.

Exotoxins: Are produced by several G+ve and G-ve bacteria the essential characteristic of exotoxins is that they are secreted by the bacteria. Exotoxins are polypeptide consist of 2 subunits, one for the binding to the cell membrane and entry in to the cell and the other possessing the toxic activity.

Exotoxins are among the most toxic substances known. They are good antigens and induce the synthesis of protective antibodies called antitoxins.

When treated with formaldehyde (or acid heat) the exotoxin polypeptides are converted in toxoids, which are used in protective vaccines because they retain their antigenicity but have lost their toxicity.

Endotoxins:-

     They are integral parts of the cell walls of both G-ve rods and cocci, in contrast to exotoxins which released from the cell. Endotoxins are lipopolysaccharide. The toxicity of endotoxin is low in comparison with that of exotoxins. Endotoxins are weakly antigenic, no toxoid have been produced from endotoxins, and endotoxins are not used as antigens in available vaccine.