Host-parasite interactions

Host-parasite interactions

The microorganism as a pathogen

The capability of a microorganism to cause disease is called pathogenicity. When a microbe invades a host (i.e., when it enters the body tissues and multiplies), it establishes an infection. If the host is susceptible to the infection and its functions are implied, this is called disease. Thus a pathogen is any microorganism or larger organism capable of causing disease. An example of a larger pathogen is the Trichinella worm that causes trichinosis, a parasitic disease affecting muscle tissue.

The ability of a pathogenic microorganism to cause disease (its pathogenicity) is influenced not only by the properties inherent in the microbe but also by the ability of the host to resist infection. In recent years, increasing numbers of infections have been caused by microorganism previously considerable nonpathogens; most are members of the normal flora. These infections develop in people whose resistance factors are compromised by another disease or by prolonged antibiotic and immunosuppressive therapy. Such microorganisms are called opportunistic pathogens; they are distinguished from primary pathogens, which can initiate disease in healthy individuals.

The degree of the ability of a pathogen, opportunistic or primary, to cause disease is termed virulence. Thus microbial properties that enhance a microorganism’s pathogenicity are called virulence factors. If one microbe is more capable of producing a disease, it is said to be more virulent than another. The virulence factors of some pathogens are easily identified. For example, cells of Streptococcus pneumonia that have capsules are virulent and can cause pneumonia, whereas those without capsules are virulent; virulent strains of Corynebacterium diphtheria produce a toxin that causes diphtheria. For most pathogens, however, the virulence factors are not so obvious.

In order to cause infectious disease, a pathogen must accomplish the following complex steps:

1. It must infect the host (a primary pathogen must enter the host).
2. It must metabolize and multiply in the host tissue.
3. It must resist host defenses, for a time.
4. It must damage the host.

Microbial virulence factors

Some bacteria secrete substances that contribute to their virulence, while others have special structures. Unfortunately, in many microorganisms, the properties conferring virulence are either unclear or unknown. Several well-known microbial virulence factors are considered here.

Toxins

Some microorganisms produce poisonous substances known as toxins. The capability of a microorganism to produce a toxin- a substance that has a damaging effect on cells and tissues in a host and the potency of the toxin are important factors in the ability of the microorganism to cause disease. The toxins produced by microorganisms may be exotoxins, toxins excreted into the surrounding medium; or endotoxins, toxins retained within the cell as part of the cell.

Exotoxins are excreted from the microbial cells into a culture medium or into the circulatory system and tissues of a host. The medium might be an improperly processed can of vegetables contaminated with Clostridium botulinum, for example; ingestion of the vegetables containing botulinum toxin results in food poisoning called botulism. In botulism food poisoning, the exotoxin that is ingested by an individual causes paralysis by affecting the person’s nervous system; thus the toxin is called a neurotoxin. Infection of the host by the bacterium is not necessary for the disease to occur. Therefore, strictly speaking, botulism is not the infectious disease, but a toxemia caused by a bacterial toxin produced outside of the host. When the bacillus Corynebacterium, diphtheria grows in the human throat and secretes its exotoxin, the toxin is absorbed into the bloodstream and causes the disease diphtheria. When the tetanus bacillus Clostridium tetani is introduced into a wound, it may grow and form tetanus exotoxin (which is also a neurotoxin). Thus, unlike botulism, tetanus and diphtheria are caused by organisms that produce their toxins while growing within the host.

Exotoxins are proteins; they can be produced by either Gram-negative or Gram-positive bacteria. Their effects on human tissues are usually highly specific. For example, the toxins of botulism and tetanus are neurotoxins. The cholera vibrio secretes an exotoxin called an enterotoxin which decreases the retention of fluid by the intestine, thereby causing diarrhea. Thus exotoxins usually have an affinity for a particular tissue to which they can cause damage. Exotoxins lose their toxicity when they are heated or treated chemically. Phenol, formaldehyde, -propiolactone, and various acids can modify exotoxins chemically so that they lose their toxicity. They are then called toxoids.

Both toxins and toxoids have the ability to stimulate the production of antitoxins, antibodies which neutralize the toxicity of toxins in the body of the host. This ability is important in the protection of susceptible hosts from diseases caused by bacterial toxins. Antitoxins can be produced commercially as well. Some of the well-known diseases caused by exotoxins producing bacteria.

Many microorganisms, particularly the Gram-negative bacteria, do not secrete a soluble toxin but make an endotoxin that are liberated only when the cells disintegrate. The endotoxins of Gram-negative bacteria are structural components of the outer membrane of Gram-negative bacteria cell walls. These components are lipopolysaccharides (specifically, the lipid A portion). Endotoxins are effective poisons booth in the bound state (when still part of the intact cell wall) and when released as lytic products upon cell disintegration. Compared with exotoxins, endotoxins are highly heat-stable, do not form toxoids, and are less toxic. Endotoxins are responsible for many disease symptoms such as fever and shock.

The detection of endotoxin in various body fluids is helpful in the laboratory diagnosis of infections caused by Gram-negative bacteria. A laboratory method called the Limulus amebocyte lysate (LAL) assay can detect tiny amounts of endotoxin. Limulus polyphemus is the Atlantic coast horseshoe crab, and the hemolymph (blood) of this crab contains a single type of while cells called amebocytes. Each amebocyte is packed with proteins responsible for clotting. When the crab blood contacts endotoxin in a purified form as a component of Gram-negative bacteria, the clotting proteins of the amebocytes are released and the blood forms a gel or clot. This clotting reaction is supposed to be advantageous to the crab because it localizes as infection in a walled-off area and prevents blood loss.

Extracellular enzymes. The virulence of some microorganisms is partly due to the production of extra-cellular enzymes. Although no single extracellular enzyme has been proved to be the sole factor responsible for virulence, there is no doubt that such enzymes do play some role in the pathogenic process. The ability of pathogenic bacteria to penetrate tissues or invasiveness, is due to same these enzymes, lists some extracellular bacterial enzymes that contribute to the disease process.

Hyaluronidase may help the pathogen to penetrate the tissues of the host by hydrolyzing hyaluronic acid, an essential “tissue cement” that helps hold living cells together. Because of this, the enzyme is referred to as the “spreading factor”. Staphylococcus aureus, Streptococcus pyrogens, and Clostridium perfringens produce hyaluronidase. Lecithinase is an enzyme that lyses various tissue cells, especially red blood cells, by hydrolyzing membrane lipids. For example, the virulence of Clostridium Perfringenss (cause of gas gangrene) is due, at least in part, to the production of lecithinase. Collagenase, also produced by Clostridium perfringens, destroys collagen. A tissue fiber found in muscle, bone, and cartilage. Collagen provides the meshwork within the tissue cells lie. Without it, tissue is more susceptible to invasion by pathogens.

Some virulent staphylococci produce the enzyme called coagulase. It acts as a substance in the plasma to transform fibrinogen to fibrin. This causes the deposition of fibrin around the bacterial cells, thus protecting them from the action of host phagocytes (cells that eat foreign objects). There is some evidence that coagulase is also involved in the walling-off probes (formation of peripheral insoluble protein) in boils caused by staphylococci.

Hemolysins are substances that lyse red blood cells, releasing their hemoglobin. Generally speaking, hemolytic strains of pathogenic bacteria are more virulent than non-hemolytic strains of the same species. Bacterial hemolysins from various species differ in their chemical nature and Rhoeae (which causes gonorrhea) and Escherichia coli (which can cause urinary tract infection) possess pili.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.