Protein binding and antimicrobial activity of ceftriaxone: comparative assessments by gradient plate technique and...

In conjunction with initial bactericidal rate measurements, this study evaluated the impact of protein binding on the antimicrobial activity of ceftriaxone employing the gradient plate technique. Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923 were employed as test organisms. At various albumin concentrations (0 to < or = 16% w/v), the effects of albumin binding were estimated by the shift in concentration-responses (via initial bactericidal rate) and the distance of inhibition zone (DIZ) on gradient agar plates. Supplementation with 4% albumin reduced the antimicrobial activity of ceftriaxone against E. coli as suggested by the similar 1.5-fold shift in both initial bactericidal rate and DIZ when compared to those with no albumin. As with S. aureus, the inclusion of albumin increased the initial bactericidal rate but the DIZ assessments showed a reduction in antimicrobial activity. The reason for the difference observed on the initial bactericidal effect between the two species is unknown. However, the similar patterns of change in the DIZ data at increasing albumin concentrations from 0 to 4% for both organisms suggest consistency and reproducibility of the gradient plate technique. This technique appears to be an attractive alternative for the assessment of the effect of protein binding on antibiotics.