Serum bactericidal rate as measure of antibiotic interactions

The serum bactericidal rate (SBR) assay was used to assess the antipseudomonal activity of gentamicin with or without piperacillin. Heat-inactivated donor serum was spiked with gentamicin, piperacillin, or a combination at concentrations representative of levels in serum and tested against five P. aeruginosa strains isolated from blood. The SBR assay was performed as follows: colony counts in test samples (control, gentamicin, piperacillin, and combination) were determined at 0, 2, 4, 6, and 8 h after inoculation of a test strain. Log10 CFU per milliliter was plotted versus time, and linear regression analysis was performed; the slope of the regression line was defined as the SBR. The SBRs of agents alone and in combination were compared statistically. The SBR of gentamicin was dependent on the serum concentration. The combination of gentamicin and piperacillin always resulted in a higher SBR than did either drug alone. However, this difference was not statistically significant for highly gentamicin-susceptible strains (MIC, less than or equal to 2 micrograms/ml) until the gentamicin concentration was reduced below the MIC. For a gentamicin-resistant strain (MIC, 8 micrograms/ml), the combination of gentamicin and piperacillin produced mean SBRs similar to that found with gentamicin-susceptible strains. These results provide evidence that the SBR assay may be a useful method of evaluating antibiotic interactions, since it can be done by using serum and since it compares the antibacterial activity of drugs statistically rather than requiring arbitrary criteria to define interactions.