Infection and its mechanism

Infection and its mechanism

1. Infection and its mechanism MAN BAHADUR RANA BPH,ACAS,NEPAL

2. Definition: Infection may be defined as lodgement and multiplication of an infectious agent in the body. Mechanism of Infection: The process through which microorganisms cause disease involves several or all of the following stages. Infection

3. . Encounter . Colonisation . Penetration . Spread . Damage Contd…

4. The initial contact with a given microbial species is critically important. Depending on whether the source of infection, it may be endogenous-due to organisms of the normal flora and exogenous-due to the organisms derived from a source outside the body. The source of infection are: - Humans - Animals Encounter

5. - Insects - Environment The major routes of transmission are: Contacts (direct and indirect) including intimate sexual contact e.g soft tissue infection, gonorrhoea etc. Inhalation / droplet infection e.g common cold ,pneumonia Ingestion /fecal oral route e.g gastroenteritis Contd..

6. Inoculation / e.g tetanus, malaria Congenital / e.g congenital toxoplasmosis. Contd..

7. The initial encounter with a new microbial species may result in nothing more than short lived contact with an external body surface. The microorganisms needs to survive and multiply under local conditions( e.g temperature and ph) to establaish itself in its new habitat. It most successfully compete against an established indigenous microbial and resist local defence mechanism. Colonisation

8. Some species are capable of producing mucolytic enzyme to help them penetrate the layer of mucus coating internal body surface. Some species have specific adhesins that enable binding with receptor site on human cell.( e.g gonococcal pilli attachment to urethral epithelium). Bacteria such as H. influenza, streptococcus pneumoniae and Neisseria meningitidis produce Ig A protease which can inactivate locally active Ig A produce by some mucosal surface. Contd..

9. Once established on a body surface, an organism is said to have colonised that site. Contd..

10. A microorganisms must breach surface barrier in order to invade living human tissue. In the case of the skin, bacteria probably do not penetrate intact surface. Infection thus acquires due to trauma, surgical wounds, chronic skin diseases or insect bites. The respiratory tract is continuously exposed to air borne organism. However, the upper respiratory tract functions as an inertial filtration system and protects the lungs from exposure to inhaled particles. Penetration

11. The cough reflex and muco ciliary escalator provide back up, expelling any particle inhaled into the airways. Infective particle may reach the alveoli and establish infection. In the GIT, some diseases causing organism damage the mucosal surface by releasing cytotoxin( e.g those causing dysentry ). Contd..

12. An invading microorganism may spread by one or more routes. Direct extensions through surrounding tissue, along tissue planes or via the veins and lymphathic vessels. Organisms may play an active part in spread by destroying cells or even by self propulsions. Spread

13. Microorganisms damage tissue by a variety of mechanism. - Bulk effect - Toxin mediated - altered function of host system - host response to infection Damage

14. Bulk effect: - The sheer bulk of organism may obstruct a hollow organ e.g some helminth infection of intestine. - swelling of infected tissue can cause pressure on adjacent hollow organ or neurovascular bundles. Contd..

15. Toxin: - Bacteria produce two types of toxin, . Exotoxin . Endotoxin Exotoxin : These are heat labile proteins which are secreted by certain species of bacteria and diffuse readily into the surrounding medium. - They are highly potent in minute amounts and constitute some of the most poisionous known. Contd …

16. - 1mg of tetanus or botulinum toxin is sufficient to kill more than one million guinea pigs and it has been estimated that 3kg of botulinum toxin can kill all the inhabitants of the world. Endotoxin : - These are heat stable LPS which form an integral part of the cell wall of Gram- ve bacteria. Contd..

17. These are not secreted outside the bacterial cell and are released only by the disintegration of the cell wall. They are active only in relatively large dose. e.g tetanospasmin : tetanus botulinum toxin: botulism Cholera toxin: cholera diphtheria toxin: diphtheria Contd..

18. Altered function of organs, tissue or cells: - Microbial invasion can change the function of organs, tissue or cells. e.g increased bowel motility leading to diarrhoea , coughing and sneezing. The host response to infection: - The host response usually begins with an inflammatory reaction and is followed by a humoral or cell mediated immune response. Contd..

19. - This may cause damage due to swelling , increased fragility of tissue, formation of pus, scarring or necrosis. Contd..