Three LAB strains (AC-1, AC-5 and AC-6) were selected based
on the results of the freeze challenge and were considered as
potential inhibitors of the chosen target pathogenic microorganisms
(Table 1). These strains could potentially produce
bacteriocin or bacteriocin-like substances with antimicrobial activity.Target
pathogens were chosen on the basis of their notable
deleterious influence on the safety of food products. The presumed
LAB strains were cultured overnight in MRS broth at 37 °C.
Cells were then removed by centrifuging at 9000 g for 15 min
at 4 °C. To rule out any inhibition due to pH reduction caused
by organic acid production, the supernatant was adjusted to pH
6.0 with sterilized 2.5 mole/l NaOH. Later, the supernatant was
filtered through a syringe filter with a pore size of 0.22 m (Merck
Millipore) and maintained in sterile conditions. The screening
for antimicrobial activity was carried out according to Vaconcelos,
Gomes de Andrade Lima, and Torres Calazans (2010) with some
modifications. The cell-free supernatants were treated by addition
of a sterile solution of catalase (Sigma) (1 mg/ml) for 30 min
to eliminate any possible inhibitory action of hydrogen peroxide.
They were then added at the same volume to MH (Mueller–
Hinton) (Acumedia) broth inoculated with 10% (v/v) of a diluted
1:10 (v/v) overnight culture (37 °C) of the target pathogenic strains.
This extract was placed into a 96-well plate, and the inhibitory
growth test was measured after 3 and 6 h of growth at 37 °C using
a detection absorbance reader microplate spectrophotometer
model PowerWave XS at 630 nm (Biotek,Winooski,VT, USA).Tests
were performed in duplicate, and the percentage of growth
inhibition was calculated using the following equation: