MODE OF TRANSMISSION AND MECHANISMS

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 Mode of transmission, mechanisms by which microorganisms cause disease and diagnosis

                    


 Definitions of Terms   
  • Endemic-An acute outbreak of a disease in community or region.(cholera, menegitidis)
  • Pandemic-Infection that spread to several country and affect large number of people e.g HIV
  • Epidemic-Constant presence of disease in an area
  • Sporadic-A disease that break out only occasionally in area
  • Pathogen- Is an organism which is capable to causing a disease to man
  • Commensal- Bacteria that live naturally on our body parts but do not cause an harm.They benefit from the host.
  • Opportunistic-Bacteria  that can cause an infection if a suitable opportunity arises
  • Symbionts-Organisms that benefits the infected person.
  • Capsule-A gelatinous(slimy layer) secretions produced by virulent microorganism around the cell wall.e.g.pneumococci,klebsiella.
  • Toxins-Harmful substances produced by bacteria
  • Spore-Athick layer produced by some bacteria when the conditions are un favourable.This is a resting stage  of bacteria.
  • Carriers –A person who harbours a specific infection/bacteria but does not show signs and symptoms.He serves as source of disease
  • Aerobic bacteria-Bacteria that depend on oxygen  for metabolism
  • Anaerobic bacteria - Bacteria that depend on carbon dioxide for metabolism
  • Strict(obligate) aerobes-Bacteria that requires free oxygen only to survive.e.g Psedomonas aeruginosa
  • Facultative anerobes- Bacteria that can live with or without free oxygen to survive.eg.Strep. pyogenes
  • Obligatory anaerobes- Bacteria that requires carbon dioxide only to survive. eg.Clostridium tetani
  • Microaerophilic bacteria-Bacteria that grows well in only traces of oxygen.eg.Campylobacter jejuni
  • Virulence-Ability of pathogen to cause disease even in small number.
  • Infection-The invasion of the body by pathogenic organism.
  • Invasiveness-Ability of microorganisms to penetrate tissue and spread.
  • Pathogenicity-Ability of an organism to cause disease.

 LIST THE TRANSMISSION ROUTES/MODES 
  • Inhalation
  • Ingestion
  • Direct contact
  • Maternal transmission
  • Blood transfusion

 EXPLAIN THE MECHANISMS BY WHICH BACTERIA CAUSE DISEASE
This are the ways and means in which bacteria establish, multiply and be able to cause a disease. Microorganism that cause disease are called pathogens. They include:-

  • Capsule secretion
  • Possession of pili
  • Production of extra cellular enzymes
  • Endotoxin production
  • Eexotoxin production
1)Capsule secretion; Bacteria such as streptococcus pneumoniae, Klebsiella pneumoniae, and Haemophilus influenzae are able to secrete around their cell walls a thick protective capsule. This helps to prevent such organisms being engulfed and destroyed by the phagocytic cells of the host. Capsules are also thought to contribute in other ways to the virulence of bacteria
2)Possession of Pili; Virulent strains of organisms such as Neisseriae gonorrhea and some Escherichia coli are covered in Pili. These help the organisms to adhere to one another and to the tissue of the host
3)Production of extracellular enzymes; many bacteria produce extra cellular enzymes that contribute to their pathogenicity. These include.
-       Kinases, which include streptokinase produced by streptococcus and staphylokinase by staphylococcus. These Kinases assist the organisms to spread in the body by breaking down fibrin, which is formed by the host as a protective barrier.
-       Coagulase produced by staphylococcus aureaus. By coagulating plasma, this enzyme forms fibrin, which is thought to protect staphylococcus aureus from phagocytosis by host cells. It was also thought that when the organisms are held inside fibrin dots, they are less likely to be removed from the circulation by reticulo – endothelial cells.
-       Hyaluronidase possessed by clostridium perfringens and some streptococci and staphylococci. This enzyme helps the organisms to spread through the body by breaking down the hyaluronic acid of connective tissues.
-       Beta (β)- lactamases produced by many bacteria including some strains of staphylococcus aureus and Neisseria gonorrhea. These enzymes are able to destroy penicillin’s and cephalosporins
4)Exotoxin production; certain organisms secrete powerful poisons called exotoxins that are capable of destroying or injuring host cells. They tend to be specific in their action for example the Exotoxin of clostridium tetani is a neurotoxin, that is it affects the cells of the central nervous system. Produced mainly by gram positive other important Exotoxin producing pathogens include clostridium botulism, clostridium perfringens, corynebacterium diphtherial, enterotoxigenic Escherichia coli (ETEC) Shigella dysenteric and Vibrio cholerae .The pathogenicity of such organisms is completely dependent on their Exotoxin production. The toxin produced by enteric pathogens is known as enterotoxin.
 Staphylococcus species and some streptococcus species produce the extoxin leucocidin, which is capable of destroying white blood cells.
-       Exotoxins are highly antigenic by special chemical preparation, exotoxins can be made into non – toxic toxioids, which can be used to immunize and protect individuals against specific diseases.
-       The term antitoxin refers to the antibody used to neutralize the harmful effects of exotoxins.
-       Antitoxins are prepared by immunization of animals with toxin or toxiod.
5)Endotoxin production: the cell walls of Gram negative organisms contain endotoxin. Unlike exotoxin, endotoxin is not usually secreted by an organism but is released only when the organisms are destroyed. Endotoxin, therefore, does not contribute directly to the invasiveness of bacteria.
When endotoxin is released into a host blood circulation, the resulting toxaemia may cause rigor, chills and shock.
The endotoxin from some pathogens may also have clotting properties and lead to disseminated intravascular coagulation (DIC). Endotoxin release may also lead to a marked Leucocytosis. In contract to exotoxin, endotoxin is only weakly antigenic. It is also more heat stable then exotoxin.
Bacterial Associations:
  • An organism living and multiplying within the living human body is termed a parasite, the body in this instance being a host. When harmless to the host, the parasite is termed a commersal, when harmful a pathogen, under certain conditions, commersals may become pathogens, and pathogens may assume a commersal role.
  • Organisms living on dead matter are termed saprophytes.
  • When both host and parasite mutually benefit the association is often called symbiosis. Some authorities irrespective of use this same term whether benefit occurs to either partner, but Satellitism is the more correct term in these instances other factors, which contribute to pathogens causing disease.
  • Other factors, which determine whether a pathogen will cause disease include:
-       Transmission route
-       Number of bacteria that invade
-       State of health of the person infected.
  • Transmission route for a pathogen to cause disease it must enter the body by a route, which will enable it to reach a site where it can establish itself and multiply.
  • For example, an organism which causes gas gangrene must reach deep tissues to find the aerobic condition’s necessary for its growth.
-       Other organisms such as staphylococcus aureus can cause several different diseases depending on whether the organism is ingested (e.g. food poisoning), enters the skin (e.g. boils), or reaches the lung (e.g. Pneumonia).
-       Certain organisms may require a vector for their development and transmission. For example, reckettsiae develop in arthropod bites and the organism is injected into the blood.
  • Bacteria numbers and hosts state of health: for some bacteria, the entry of large number of organisms may be necessary before a healthy persons defence mechanisms are overcome, where as only a few organisms may be required to produce disease in a person, already in poor health or in a malnourished person, especially a child.
  • Particularly virulent bacteria, however, need only be present in very small numbers to cause disease, for example Shigella dysenteriae.


 Key Points 

Mechanisms how disease are caused.
Some of the ways, which assist pathogens to become established in the tissue, multiply and cause disease include; -
  • Capsule secretion
  • Possession of pili
  • Production of extra cellular enzymes
  • Endotoxin production
  • Eexotoxin production
 LIST THE TRANSMISSION ROUTES/MODES 

  • Inhalation
  • Ingestion
  • Direct contact
  • Maternal transmission
  • Blood transfusion

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