The effects of ethyleneglycoltetra-acetic acid on bactericidal activity of human serum against Escherichia coli

The effects of ethyleneglycoltetra-acetic acid (EGTA) and EGTA + magnesium (MgEGTA) on the viable counts of 10 strains of Escherichia coli O6 have been studied in normal human serum (NHS), heat-inactivated serum (HIS) and in culture media with and without the addition of a beta-lactam antibiotic. The addition of EGTA to NHS largely prevented bactericidal activity against serum-sensitive strains while, in contrast, it reduced the growth of a serum-resistant strain. These apparently paradoxical effects are due to the lower growth rate permitted by the reduced amount of available magnesium in the presence of EGTA. Experiments with equimolar concentrations of EGTA and magnesium indicated that whilst MgEGTA is a reagent allowing alternative complement-pathway activity, such activity must be determined by comparison with results in HIS + MgEGTA rather than in HIS alone, classical-pathway activity being taken as the difference between the results in NHS and in NHS + MgEGTA. By these criteria, prompt killing by serum was found to occur via the classical pathway while delayed serum bactericidal activity occurred by the alternative pathway in some strains and by the classical pathway in others.