ANTIMICROBIAL ACTIVITY OF LACTIC CULTURES: PARTIAL PURIFICATION AND CHARACTERIZATION OF ANTIMICROBIAL COMPOUND(S) PRODUCED BY Streptococcus thermophilus

Abstract

ABSTRACTSkim milk fermented by various lactic cultures was tested for antimicrobial activity by agar diffusion technique, using Pseudomonas fluorescens, P. aeruginosa and Bacillus subtilis as test organisms. Lactobacillus bulgaricus, L. acidophilus and Streptococcus thermophilus strongly inhibited the growth of the test organisms. Other lactic cultures, singly or in combination, showed varying degrees of antimicrobial activity. A methanol‐acetone (M‐A) extract was prepared by lyophilizing fermented milk and extracting the powder sequentially with cold methanol and acetone. The methanol‐acetone (M‐A) extract strongly inhibited the growth of B subtilis, B. pumulis, P. aeruginosa, P. fluorescens, Flavobacterium capsulatum, Satmonella typhimurium, Escherichia coli, Shigella sp. and Streptococcus lactis. The antimicrobial activity of S. thermophilus was not reported in the literature previously. The M‐A extract contained ninhydrin‐positive materials and did not lose antimicrobial activity even when it was exposed to a temperature of 100°C for 10 min. Five milliliters of M‐A extract were applied to 20 × 2.2 cm Sephadex G‐10 column equilibrated with 0.05N HCl which later served as the eluant. Fractions of 15 ml were collected at a flow rate of 2.5 ml/mm Sephadex fractionation of M‐A extract showed that there were at least three active fractions. One of the fractions appeared in the void volume followed closely by the other fractions indicating that the latter two fractions have compounds of molecular weight less than 700 daltons. The purification achieved was about 400‐fold. The activity (recovery) of the compound(s) increased upon fractionation on Sephadex indicating the possible presence of inhibitor(s). the cultures of L. acidophilus. acidophilus was shown to possess very potent antimicrobial activity.The purpose of this investigation was to screen various strains of lactic cultures for antimicrobial activity and to partially purify antimicrobial compound(s) produced by Streptococcus thermophilus in culture.