Antagonistic activity of Lactobacillus fermentum against Aspergillus species obtained from pasteurized fruit syrup

Abstract

According to the WHO, a particular bacterium must possess specific characteristics to be considered a probiotic bacterium used in the food industry. Antagonistic activity against other microorganisms by producing antimicrobial substances is one such character. This study investigated the in vitro antagonistic activity of two naturally isolated Lactobacillus fermentum strains against spoilage of Aspergillus spp. isolated from pasteurized fruit syrup. Lactobacillus fermentum strains were isolated from spontaneously fermented buffalo milk. Molecular identification of L. fermentum (strain L-18 and T5d) and A. foetidus, A. flavus, A. oryzae and A. elegans was carried out by the authors previously. For the assay, each L. fermentum strain was streaked as one line in a modified MRS agar plate and incubated anaerobically at 37 °C for 48 h. A fungal agar block was placed on an incubated MRS agar plate, maintaining a distance of 2 cm from the L. fermentum streaked line and incubated aerobically at 28 °C for up to seven days. The area of the fungal mycelium in photograph images was transferred to square pixels using Adobe© photoshop C6 (13.0 - 64bit) software kit. The square pixels were then converted to square centimetres by using MS Excel 2019. The ability of antifungal activity by the two strains of L. fermentum was detected by comparing the growth area of each tested fungal mycelium corresponding to the same mycelium without the bacterium (positive control). Both L. fermentum strains showed positive antagonistic effects against all four tested Aspergillus species. Zero growth of A. foetidus indicated 100% control by both L. fermentum strains, while the growth of A. elegans was controlled close to 100%. Aspergillus flavus and A. oryzae were moderately controlled. The study identified two potential in-vitro antifungal L. fermentum isolates, exhibiting the benefit of consuming traditional buffalo curd and its potential to be used as a probiotic food source.