Methods
Microbial strains
S. cerevisiae PYCC 4455, K. marxianus PYCC 3886T and C. tropicalis
PYCC 3097T were obtained from Biology Department – University
of Minho. Yeast were cultivated at 30 ◦C for 48 h and maintained on
YPDA medium [yeast extract 1% (w/v), peptone 2% (w/v), glucose
2% (w/v) and agar 2% (w/v)].
Hypholoma fasciculare (Huds.) P. Kumm. was isolated from
chestnut orchards at Braganc¸ a (Northeast Portugal) and molecularly
identified by the amplification and sequencing of the internal
transcribed spacer (ITS) region (Pereira et al. 2011). Fungal axenic
cultures were maintained on Potato Dextrose Agar (PDA, Difco)
medium or on modified Melin-Norkans (MMN) agar medium [NaCl
0.0025% (w/v); (NH4)2HPO4 0.025% (w/v); KH2PO4 0.05% (w/v);
FeCl3 0.005% (w/v); CaCl2 0.05% (w/v); MgSO4·7H2O 0.015% (w/v);
thiamine 0.01% (w/v); casamino acids 0.1% (w/v); malt extract 0.1%
(w/v); glucose 1% (w/v); agar 2% (w/v), at pH 6.6]. Cultures were
maintained at 25 ◦C, in the dark, being sub-cultured periodically.
Evaluation of fungal antimicrobial activity in standard conditions
An inoculum of H. fasciculare fungus was transferred to the centre
of a PDA plate and incubated at 25 ◦C, in the dark, for 6 days.
After incubation, the plates were overlaid with the sensitive indicator
strain. To prepare the sensitive microbial suspension, yeast
biomass was scraped from a 48 h YPDA culture plate and suspended
in 0.5 ml of NaCl 0.85% (w/v). The titre of yeast suspension was calculated
using a Neubauer chamber and the suspension was mixed
with melted-agar 0.8% (w/v) in order to obtain 106 CFU (colony
forming units)/ml. A volume of 8 ml of the mixture was then seeded
as a lawn onto the surface of the plates previously inoculated with
H. fasciculare. Incubation was performed at 25 ◦C for 48 h. Anti-yeast
activity was observed when the fungal inoculum was surrounded
by a clear zone of growth inhibition, and evaluated by determining
both the diameter of the fungal colony and the diameter of the
inhibition halo. Values were used to calculate the areas occupied by
the fungus and by the inhibition halo formed by its antimicrobial
activity. Fungal antimicrobial potential (AP) was arbitrarily defined
as the ratio between both diameter values (diameter of the inhibition
halo/diameter of the fungal colony). Data are presented as
the mean of 3–4 independent experiments, with 3 replicas. The
corresponding standard error values are displayed.
Antimicrobial assay optimization
Antimicrobial activity assay was optimized using different fungal
culture media (MMN or PDA). After optimization of the assay
medium, the yeast inoculum size to be used in the bioassay was