LAB that displayed strong antifungal activity on MRS agar were
selected for testing against P. commune on cottage cheese. Some
LAB with no antifungal activity were also selected as negative
controls. LAB isolates were grown anaerobically in MRS broth for
48 h at 30 C. After incubation, the optical density of each LAB in
broth was measured and recorded, and 2 ml was centrifuged at
16,873 g (Eppendorf Centrifuge, Model 5418) for 1 min to obtain
the bacterial cell pellet. The spent MRS broth was discarded and
bacterial cells were resuspended with peptone water (0.1% w/v) to
obtain a standard number of cells per ml using the OD600 nm as a
reference. Each LAB suspension was estimated to contain
1 109 CFU/ml and the exact concentration of each LAB suspension
was determined by plating dilutions on MRS agar and incubation.
Twelve grams of commercially obtained cottage cheese which
contained 11.3 g protein, 5.4 g fat and 2.6 g sugar per 100 g (Dairy
Farmers, Queensland, Australia) were weighed out on each petri
dish (9 cm in diameter; Labtek) and inoculated with 0.1 ml of LAB
suspension. The cottage cheese was mixed on the petri dish using a
sterile spreader for at least 1 min to ensure even distribution of
bacterial cells. Controlswere prepared by inoculating cottage cheese
with 0.1 ml of peptone water not containing LAB. The plates were
then incubated at room temperature (~24 C) for 2 days. P. commune
fungal inoculum was prepared according to the method described
earlier and 0.1 ml (1 106 spores/ml) was inoculated onto each
cottage cheese plate (final concentration was ~1 104 spores/g of
cottage cheese), including control plates without LAB, and mixed
again as above to ensure even distribution. Plates were incubated at
room temperature and examined every fewdays for mould growth:
() no mould growth; (þ) small mould spots; (þþ) moderate sized
mould spots or patches; (þþþ) mostly or completely covered by
mould. Each strain was tested at least twice and consistent results
were obtained. The data presented is from the replicate experiment
which showed the least antifungal activity.