Student ID: 14067834
Course: Pharmacy
Module name: Pharmacotherapeutics II
Module code: 5LMS0016
Lecturer: Ms. Kaur- Bring
Date: 19th of March, 2017
Antimicrobial susceptibility testing of two strains (B3 and B7) of
Staphylococcus aureus to penicillin G
Staphylococcus aureus (S. aureus) is a Gram-positive bacterium that exists microscopically in
grapelike clusters. S. aureus is normally present in the nose and skin of healthy humans.
Most times it is not harmful however it might cause skin and soft-tissue infections as well as
invasive infections. Antimicrobial susceptibility testing is used to determine which antibiotic
can be used to treat diseases and to detect resistance to certain antibiotics. MIC (minimum
inhibitory concentration) is the lowest concentration of antimicrobial agent that completely
inhibits microbial growth while MBC (minimum bactericidal concentration) is the minimum
concentration of antimicrobial agent that kills ≥99.9% of bacteria in a test. The aim of this
study is to test the susceptibility of two different strains of S. aureus to penicillin G.
The MIC of penicillin G was determined against two different strains of S. aureus by using
broth dilution method and the MBC was determined by using agar plates. Seven test tubes
each containing ISO broth, S. aureus and different concentrations of penicillin G ranging
from 0.03-2 mg/L which were prepared and incubated overnight at 37oC, were used. Two
control test tubes were also prepared whereby one contained culture inoculated into drugfree ISO broth showing positive growth (C+) and the other contained only drug free ISO
broth showing negative growth (C-). The MIC was identified by observing the concentration
of S. aureus that the test tube lacked turbidity. The tubes in the MIC testing which did not
show visible growth (i.e did not show turbidity) were sub-cultured onto solid agar plates and
incubated overnight at 37oC. The MBC was determined by calculating the percentage kill of
the cultures that were inoculated onto solid agar plates.
Out of all the 9 test tubes observed, the MIC for penicillin G against Strain B3 of S. aureus
was 0.5 mg/L while for the MIC against strain B7 of S. aureus was 1mg/L.
For the B3 strain of S. aureus, the %kill of 0.5mg/L, 1.0mg/L and 2.0mg/L of Penicillin G were
99.79%, 99.99% and 99.99% respectively while for the B7 strain of S. aureus, the %kill of
1.0mg/L and 2.0mg/L of Penicillin G were both 99.99%. According to the %kill values for
strain B3, the lowest penicillin G concentration that resulted in %kill of ≥99.9% of S. aureus
was 1g/ml while for strain B7, it was also 1g/ml.
MIC is the lowest concentration of penicillin G that resulted in no visible growth of S.
aureus, which was demonstrated by lack of turbidity in the tube. The MIC for strain B3
compared to that of strain B7 is lower which indicates that strain B3 of S. aureus requires a
lower concentration of penicillin G to completely inhibit the growth of S. aureus. An
advantage of this method is that MBC can be determined using the same test tubes. The
MBC indicated that both strains of S. aureus require 1g/ml of penicillin G to kill ≥99.9% of
S. aureus. The MIC and MBC testing have helped to determine the concentration of
penicillin G that can be used to treat the different strains of S. aureus.
Keywords: S. aureus, Gram-positive, penicillin G, MIC, MBC.