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

Abstract

There is no standardized method for the serum dilution test. The broths commonly used to dilute the serum do not resemble it in protein, divalent cations, or performance. To select a physiologic, yet practical, diluent, we evaluated the following media: Mueller-Hinton broth (MHB); MHB supplemented with Ca⁺⁺ and Mg⁺⁺ (MHB-S); MHB-S combined with 5% albumin (MHB-S/5%A); MHB-S combined 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, anrl 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-SjHS is a suitable diluent for use in standardization of the serum dilution test.