Effect of Carbon Source in Exopolysaccharide Production by Lactic Acid Bacteria Isolated from Nunu

ABSTRACT
This study investigates the production and antimicrobial potential of exopolysaccharides (EPS) from lactic acid bacteria (LAB) isolated from nunu, a traditional Nigerian fermented milk product. Six LAB isolates were obtained on MRS agar and characterized using morphological, cellular, and biochemical tests. Screening based on catalase and oxidase activity, shape, and sugar fermentation. Broth cultures of each of the LAB isolates were prepared, then the growth patterns were monitored using optical density at 560 nm after 24 hours, and cell-free supernatant was extracted from each of the LAB isolates by centrifugation. The extracted cell-free supernatant after centrifuging was tested for preliminary screening for the production of EPS antimicrobial activity using the agar well diffusion method against pathogenic bacteria, including Bacillus spp. and Staphylococcus spp. EPS production was carried out in modified MRS broth supplemented with three different carbon sources (glucose, sucrose, and dextrose) to evaluate the effect of carbon source on growth and EPS yield. Fourier-transform infrared (FTIR) spectroscopy was employed for structural characterization of the EPS, confirming the presence of functional groups consistent with polysaccharides. The morphological, cellular and biochemical tests identified LAB 1 (Lactobacillus fermentum), LAB 3 (Leuconostoc mesenteroides), and LAB 5 (Lactobacillus plantarum) as promising EPS producers. The cell-free supernatant extracted from LAB isolates exhibited notable inhibitory effects, particularly LAB 1, which produced the highest zones of inhibition (25 mm against Staphylococcus sp.). The highest EPS yield (150 mg/1 L) was obtained with sucrose, while glucose and dextrose yielded 100 mg/1 L and 130 mg/1 L, respectively. The functional groups detected in the EPS reveal peaks at 3213.0;48.591, 2085.4;94.586, 1636.3;51.357, 1559.9;65.259, 1438.8;52.714, 1246.8;76.336, 1071.6;26.339, 931.8;66.552, and 889.0;69.377. This study demonstrates that EPS from LAB isolated from nunu can serve as natural antimicrobial agents, offering promising applications in food preservation and biopolymer development.