Enhanced Inhibition of Escherichia coli O157:H7 by Lysozyme and Chelators

Abstract

This study examined the effects of three chelating agents (EDTA, disodium pyrophosphate [DSPP], and pentasodium tripolyphosphate [PSTPP]) on the inhibition of the growth of Escherichia coli O157:H7 by lysozyme. The objective of this study was to identify replacement chelators that exhibit synergistic properties similar to those of EDTA. The inhibitory effects of EDTA at 300 to 1,500 μg/ml and of DSPP and PSTPP at 3,000 to 15,000 μg/ml in combination with lysozyme at 200 to 600 μg/ml for up to 48 h at pHs of 6.0, 7.0, and 8.0 on four strains of E. coli O157:H7 was studied with the use of a microbroth dilution assay. The addition of EDTA enhanced lysozyme's inhibitory effect on strains of E. coli O157:H7. EDTA at ≥300 μg/ml combined with lysozyme at 200 to 600 μg/ml was sufficient to inhibit the growth of the strains at pHs of 6.0 and 8.0. At pH 7.0, lysozyme at 200 to 600 μg/ml and EDTA concentrations of ≥1,000 μg/ml were effective in inhibiting three of the four strains. DSPP at pH 6.0 was inhibitory at ≥10,000 μg/ml when combined with lysozyme at 200 to 300 μg/ml. In contrast, PSTPP increased the inhibitory activity of lysozyme more effectively at pH 8.0. Lysozyme at 200 to 600 μg/ml was effective against two strains of E. coli O157:H7 when used in conjunction with PSTPP at ≥5,000 μg/ml. The remaining strains were inhibited by PSTPP at ≥10,000 μg/ml. Our results indicate that inhibition occurred with each lysozyme-chelator combination, but the concentrations of phosphates required to increase the antimicrobial spectrum of lysozyme against E. coli O157:H7 were higher than the EDTA concentrations required to achieve the same effect.