Bioactive proteins against pathogenic and spoilage bacteria

Glycinin, basic subunit and β-conglycinin were isolated from soybean protein isolate and tested for their antimicrobial action against pathogenic and spoilage bacteria as compared to penicillin. The three fractions exhibited antibacterial activities equivalent to or higher than penicillin in the next order; basic subunit>glycinin> β-conglycinin with MIC of 50, 100 and 1000 μg/mL, respectively. The IC50% values of the basic subunit glycinin and β-conglycinin against Listeria monocytogenes were 15, 16 and 695 μg/mL, against Bacillus subtilis were 17, 20, and 612 μg/mL, and against Salmonella Enteritidis were 18, 21 and 526 μg/mL, respectively. Transmission electron microscopy images of L. monocytogenes and S. Enteritidis exhibited bigger sizes and separation of cell wall from cell membrane when treated with glycinin or basic subunit. Scanning electron microscopy of B. subtilis indicated signs of irregular wrinkled outer surface, fragmentation, adhesion and aggregation of damaged cells or cellular debris when treated with glycinin or the basic subunits but not with penicillin. The proliferation of L. monocytogenes, S. Enteritidis and Escherichia coli O157:H7 artificially inoculated into raw milk stored at 4 or 25 °C were particularly and significantly (P<0·05) reduced by glycinin subunit and nisin (0·5% w/v), but only slightly by β-conglycinin and moderately by lysozyme.