Manoj Mahato

1. CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA
Infection and its mechanism

2. INFECTION: Infection may be defined as lodgement and multiplication of an infectious
agent in the body.
Mechanism of Infection:
The process through which micro organisms cause disease involves several or all the
following stages.
Encounter
Colonisation
Penetration
Spread
Damage.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

3. 1. ENCOUNTER:
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
Microorganisms
Animals
Insects
Environment.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

4. The major routes or transmission are:
Contacts( direct and indirect) including intimate sexual contact e.g. soft tissue
infection, gonorrhea etc.
Inhalation/Droplet infection eg common cold,pneumonia
Ingestion/fecal oral route eg gastroenteritis.
Inoculation eg tetanus, malaria.
Congenital eg. Congenital toxoplasmosis.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

5. 2. COLONISATON
The initial encounter with a new microbial species may result in nothing more than
short lived contact with a external body surface. The microorganisms needs to
survive and multiply under local conditions(eg.temp and ph) to establish itself in its
new habitat.
It most successfully compete against an established indigenous microbial and resist
local defense mechanism.
Some species are capable of producing mucolytic enzyme to help them penetrate
the layer of mucus coating internal body surface .
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

6. Some species have specific adhesions that enable binding with receptor site on
human cell.(eg. Gonococcal pilli attachment to urethral epithelium)
Bacteria such as H. influenza, streptococcus pneumoniae and Neisseria
meningitides having IgA protease which can inactivate locally active IgA
produce by some mucosal surface.
Once established on a body surface, an organism is said to have colonised that
site.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

7. 3. PENETRATION
A microorganisms must breach surface barrier in order to invade living human
tissue.
In the case of the skin, bacteria probably do not penetrate the 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 a inertial filtration system and
protects the lungs from exposure to inhaled particles.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

8. The cough reflex and mucociliary escalator provide back up, expelling
any particle inhaled into the airways. Infective particle may reach the alveoli and
establish infection.
In the GIT, some disease causing organism damage the mucosal surface
By releasing cytotoxin ( eg those causing dysentery).
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

9. 4. Spread:
An invading microorganisms may spread by one or more routes.
Direct extensions through surrounding tissue, among tissue planes or via the
veins and lymphatic vessels.
Organisms may play inactive part in spread by destroying cells or even by
propulsions.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

10. 5. Damage
Microorganisms damage tissue by a variety of mechanism.
I. Bulk effect
II. Toxin mediated
III. Altered functions of host system
IV. Host response to infection
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

11. A. Bulk effect:
- The sheet bulk of organism may obstruct a hollow organ eg. Some helminth
infection of intestine.
-Swelling of infected tissue can cause pressure on adjacent hollow organ or
neurovascular bundles.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

12. B. 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 protein in minute amounts and constitute some of the most
poisonous known.
-1mg of tetanus or botulinum toxin is sufficient to kill more than one million
guinea pigs and it has been estimated that 3 kg of botulin toxin can kill all the
inhabitants of the world.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

13. Eg tetanospasmin: tetanus
Botulinum toxin : botulism
Cholera toxin: Cholera
Diphtheria toxin: diphtheria
Endotoxin:
These are heat stable LPS which form an integral part of the cell wall of Gram
negative bacteria.
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
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

14. C. Altered function of organs, tissue, or cells:
Microbial invasion can change the function of organs,tissue,cells.
Eg.Increased bowel motility leading to diarrhea, coughing and sneezing
D. The host response to infection:
The host response usually begins with an inflammatory reaction and is
followed by a humoral or cell mediated response.
This may cause damage due to swelling increased fragility of tissue, formation
of pus scarring or necrosis.
Infection and its mechanism
CLINICAL MICROBIOLOGIST MR. MANOJ MEHTA

15. THANK YOU