2.7. Determination of antibacterial activity
2.7.1. Bacterial stains
Two Gram-positive (Staphylococcus aureus ATCC 25923 and
Streptococcus pyogenes ATCC19615), and two Gram-negative
(Escherchia coli ATCC 25922 and Klebsiella pneumonia ATCC
13883) bacteria were selected for antibacterial activity assay.
The cultures of bacteria were maintained in their appropriate
agar slants at 4 C throughout the study and used as stock
cultures.
2.7.2. Preparation of standard inoculums
The microorganisms were inoculated into Muller Hinton broth
(MHB) supplemented with 5% defibrinated sheep blood and
incubated at 37 C for 12–15 h. The turbidity of the resulting
suspension was diluted with MHB to match with 1 McFarland
turbidity standard. This level of turbidity is equivalent to
approximately 3.0 · 108 CFU/mL, equivalent to 0.5 Macfarland
standards.
2.7.3. Agar diffusion method
The protocols used in this study were based on guidelines of
CLSI, formerly known as NCCLS (CLSI, 2006) with slight
modification. Briefly, 200 lL fresh overnight cultures of the
indicator strains of bacteria (108 CFU/mL) were added onto
Muller Hinton agar (MHA) containing 5% defibrinated sheep
blood. The MHA was vigorously mixed and poured over Petri
plates with previously dried correspondent agar medium on
the surface of which the sterile tubes (7 mm diameters) were
placed. After solidification of the MHA, the tubes were removed
and the obtained wells were filled with 10 lL of the
M. piperita extracts and oil. In order to accelerate diffusion
of the essential oil into agar, plates were incubated at 4 C
for 1 h and were then incubated at 37 C. After 24–48 h of
incubation, the antibacterial activity was evaluated by measuring
the width of the zone of inhibition (clear) of growth against
the indicator organisms in comparison to a control of reference
standards. All tests were performed in triplicate.
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