Microbial inhibition assays
Detection of antimicrobial activity in the extracts and measurements
of minimum inhibitory (MIC) and minimum bactericidal
(MBC) concentrations against bacteria and yeast were accomplished
using a broth dilution technique performed in 96-well
microtiter plates according to methods described by Delaquis
et al.22 Stock solutions of each extract in water were adjusted to
pH 4, 5, 6 and 7 with 5 mol L−1 HCl or 10 mol L−1 NaOH. Suitably
diluted aliquots (100 μL) were transferred to the wells of microtiter
plates containing 100 μL TSBYE + 0.30% (w/v) agar adjusted to
the desired pH to achieve final extract concentrations of 0.02–2.50
mg mL−1 for methanol extracts and 0.08–10.00 mg mL−1 for
hot-water extracts. Inocula with approximately 1.0 × 104 CFU
mL−1 were then added to each well. The plates were incubated at
30 ◦C for 24 h with the lids on and the wells were examined for the
presence of growth. MIC was defined as the lowest concentration
that prevented visible growth. Aliquots (10 μL) from wells without
growth were transferred to TSAYE which was incubated for 24 h at
30 ◦C to recover viable bacterial cells. MBC was defined as the lowest
concentration for which there was no evidence of growth after
recovery. Three independent replicates performed with inoculum
prepared from fresh cultures were carried out for each MIC/MBC
determination. Screening of extracts for antifungal effects was
accomplished by the poisoned agarmethod. Extracts were added
to cooled (50 ◦C) PDA at the desired concentration before dispensing
into Petri plates.Mycelial plugs (approximately 2mm)removed
from active cultures with a sterile cork borer were added to the
center of the PDA plates. The plates were observed for evidence
of radial growth after 5 days incubation at room temperature.