Antimicrobial activity of native and esterified legume proteins against Gram-negative and Gram-positive bacteria

Three legume proteins (broad bean, soybean and chick pea proteins) were isolated and esterified with methanol in the presence of hydrochloric acid (50 mol acid/mole carboxylic group) for 10 h at 4 °C to give yields of 75%, 80% and 83%, respectively. The SDS–PAGE molecular weight distribution of proteins showed slight increments after esterification. Esterification raised the pIs (iso-electric points) of legume proteins from pH 4.5 for the native legume proteins, to pH 6 in the case of broad bean protein and pH 8 in the case of soybean and chick pea proteins. Applying methylated proteins at three different concentrations (0.1, 1.0 and 10 mg/ml) to Petri dishes containing nutrient agar infected with two pathogenic Gram-positive (Bacillus subtilis and Staphylococcus aureus) and two Gram-negative bacteria (Pseudomonas aeroginosa and Escherichia coli) gave rise to concentration-dependent inhibition zones. Applying native or methylated proteins at low or medium concentrations (0.1% and 0.5%) to 100 ml nutrient broth infected with 200 μl of either Gram-positive or Gram-negative bacteria resulted in lower media turbidity (OD600) values, when compared to control and referring to lower bacterial growth.