Serum Dilution Test for Bactericidal Activity. I. Selection of a Physiologic Diluent

Abstract

with 50% pooled human serum (MHB-S/HS); and pooled human serum alone (HS). Protein binding of 13 antimicrobial agents was determined in MHB-S/5%A, MHB-S/ HS, and HS. Results obtained by macro- and micromethods for broth and serum dilution tests were compared with those obtained by the agar dilution test for carbenicillin, oxacillin, and gentamicin against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. Activity of oxacillin against S. aureus was decreased fourfold by the presence of human serum in concentrations of -25%. Physiologic concentrations of Ca++ and Mg++ decreased the susceptibility of P. aeruginosa to gentamicin fourfold. The performance of MHB-S/HS was similar to that of HS, whereas the performance of MHB, MHB-S, and MHB-S/5%A was not similar to that of HS. MHB-S/HS is a suitable diluent for use in standardization of the serum dilution test. The serum dilution test is a direct method of