Antibiotic sensitivity test





Antibiotic is a medicine that is used to kill or prevent bacterial infections. It inhibits the
development of cell wall. Not all type of antibiotics is effective against all bacteria. Some
are effective for a certain bacterium and some are not. Antibiotic sensitivity test can help
to choose which antibiotic is effective to fight a certain or specific type of bacteria. When
bacteria develop the ability to defeat the drugs designed to kill them, it is called resistance.
Some bacteria can resistant to antibiotic as they mutate for example, methicillin-resistant
Staphylococcus aureus (MRSA), penicillin-resistant Enterococcus, and multidrugresistant Mycobacterium tuberculosis (MDR-TB). Example of antibiotic sensitivity test is
Kirby-Bauer disc diffusion. We can know which one is effective by observing the size of
inhibition zone. Antibiotic sensitivity test can also be used to treat a certain fungal

1. To determine type of antibiotic that are suitable to kill or inhibit the growth of bacteria.


Sterile cotton swab
Six disc pre-soaked with different antibiotic
A plate with single colony
Blank plate
A container for waste

1. Gloves were put on.
2. The bottom of the plate was labelled.
3. Sterile swab was taken out and a black dot next to the tip was drawn.
4. A single colony were picked.
5. A lawn of blank plate was inoculated by spreading the bacteria evenly.
6. The swab then was discarded in a waste container.
7. The plate was divided in six
8. One antibiotic disc was picked by a tweezer and placed in its own section of the
9. The process was repeated 5 times for each of the antibiotic disc.
10. The label was checked to make sure they are different.
11. The plates were placed upside down in a 37*c incubator for 24 to 48 hours.


AMC : Intermediate
CF : Sensitive
C : Sensitive
CIP : Sensitive
CC : Resistant
E : Resistant
OX : Resistant
P : Resistant S : Sensitive
TE : Intermediate
TM : Sensitive
SXT : Sensitive

(S. Aureus)
Penicillin G: 2.7cm: Sensitive
CH: 1.5cm: Intermediate
OF: 0cm: Resistant

(E. coli)
Penicillin G:
Gentamycin: 0.8cm: Intermediate

Antibiotics are antibiotics that are used to treat infections caused by bacteria. Antibiotics
come in a variety of forms. Each one is exclusively effective against a certain type of
bacteria. An antibiotic sensitivity test might help you figure out which antibiotic will work
best for your ailment. The test may also aid in the discovery of an antibiotic-resistant
infection’s therapy. When regular antibiotics become less effective or useless against
some bacteria, this is known as antibiotic resistance. Antibiotic resistance can turn
diseases that were once easily curable into serious, even life-threatening conditions.
Antibiotic sensitivity testing is used to determine the most effective treatment for bacterial
infections. It can also be used to determine which medication is most effective for specific
fungal infections. Sensitivity testing is usually done in a medical laboratory and involves
either culture procedures that expose bacteria to antibiotics or genetic methods that check
for resistance genes in bacteria. The diameter of areas lacking bacterial growth, known
as zones of inhibition, around paper discs containing antibiotics on agar culture dishes
that have been equally injected with bacteria is frequently measured in culture
procedures. The size of the zone of inhibition can be used to estimate the minimum
inhibitory concentration, which is the lowest dose of antibiotic that stops bacteria from

To conclude, the type of antibiotic that are suitable to kill or inhibit the growth of bacteria
is the sensitive antibiotic that have the large diameter of inhibition zone for example,
Cephalothin, Ciprofloxacin, and Trimethoprim Sulfa which is sensitive to E. coli. \


Antibiotic Sensitivity Test by MedlinePlus

Antibiotic Resistance, written by Douglas Morier

Antimicrobial Susceptibility Testing by L. Barth Reller

Antibiotic susceptibility tests by Theranostics

Kirby-Bauer (Antibiotic Sensitivity) by Jackie Reynolds

1. Would a doctor ever prescribe an antibiotic without or before performing a
sensitivity test?
– Yes, in some cases, a healthcare practitioner may choose a therapy while a culture
is incubating, and in other cases, the practitioner may prescribe therapy based on
knowledge and experience without ever ordering a culture.
2. What factors affect antibiotic susceptibility testing?
– The factors that affect reproducibility of susceptibility testing include inoculum, media
composition and depth, delay between application of the disc and incubation,
temperature, atmosphere and duration of incubation, generation time, the antibiotic
concentration of the disc and the method of reading zone.
3. What media is used for antibiotic testing?
– Media that is used for antibiotic testing is Mueller-Hinton agar (MHA)
4. What is the common type of bacteria tested during antibiotic susceptibility
– The common type of bacteria tested during antibiotic susceptibility testing is
Staphylococcus aureus and Pseudomonas aeruginosa.
5. What is the optimum temperature used to incubate the growth of the bacteria
in antibiotic sensitivity test?
– 35°C ± 2°C is the optimum temperature used to incubate the growth of the bacteria
in antibiotic sensitivity test.
6. What type of bacteria does tetracycline kill?
– Tetracycline are broad-spectrum antibiotics that may kill gram-positive and gramnegative bacteria, as well as unusual organisms such chlamydiae, mycoplasmas,
and rickettsiae, and protozoan parasites.

7. Why are we doing antibiotic sensitivity test?
– By doing antibiotic sensitivity test, we can observe which antibiotic that helps and be
most effective against specific bacteria type and fungal infection
8. What is the optimum temperature use to incubate the growth of the bacteria in
antibiotic sensitivity test?
– 35oC for about 26 to 18 hours

9. What media is used for antibiotic testing?
– Mueller Hinton Agar

10. What is the common type of bacteria tested during antibiotic susceptibility
– Gram positive = S. aureus
– Gram negative = P. aeruginosa

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