Transcript disc diffusion method

Introduction to Lab Ex. 14: Antibiotic Sensitivity
Disc Diffusion Method
Introduction to Lab 10: Ex. Antibiotic Sensitivity
Antibiotics are chemicals that are produced by other bacteria/fungi
that have the ability to prevent other organisms (bacteria)
from growing or killing them.
These have been used widely in controlling bacterial infections
in humans that cause diseases in humans.
There have been many antibiotics that have been developed.
These have been found to be effective in controlling various
infectious bacteria in humans and thus used extensively
in disease control (treatment of bacterial infections)
While there are many different kinds of antibiotics and still
many more kinds of bacteria, not all antibiotics are
effective in inhibiting all bacteria.
Hence, it is essential to determine whether an organism is
sensitive to the inhibitory action of the antibiotic or not.
Many procedures have been developed to determine
antibiotic sensitivity.
The most common method is the Disc diffusion method:
Filter paper discs are saturated with specific amounts of
certain antibiotics and placed on a growth of the particular
bacterial culture to be tested.
If growth is inhibited then sensitivity is seen
If growth is seen then the culture is resistant to the
antibiotic action and is not inhibited
(which will mean that it cannot be used to treat an infection
by the bacterium)
It has also been discovered that antibiotics inhibit bacteria
by stopping their growth (bacteriostatic) or
by killing them (bactericidal).
Antibiotics have been found to have different modes of action
to inhibit bacteria:destruction of cell wall
destruction of cell membrane
inhibition of protein synthesis
inhibition of DNA synthesis
inhibition of intermediary metabolism
Also, most antibiotics are effective only against bacteria
(which have a prokaryotic cell structure and processes different
from eukaryotic cells – resulting in antibiotics being effective
primarily against prokaryotic cells i.e. other bacteria)
The disc diffusion method
- E.coli representing the Gram negative organisms
- S.aureus representing the Gram positive bacteria
Antibiotics that are effective against a wide range of bacteria
(G+ and G-) are called broad spectrum
Those that have effect against a small specific group of bacteria
(either G+ or G-) are called narrow spectrum.