The bactericidal pharmacodynamics of antibiotics against Escherichia coli were analyzed by a concentrationkilling curve (CKC) approach, and the novel parameters median bactericidal concentration (BC 50 ) and bactericidal intensity (r) for bactericidal potency were proposed. By using the agar plate method, about 500 E. coli cells were inoculated onto Luria-Bertani plates containing a series of antibiotic concentrations, and after 24 h of incubation at 37°C, all the viable colonies were enumerated. This resulted in a sigmoidal CKC that could be perfectly fitted (R 2 > 0.9) with the function N ؍ N 0 /[1 ؉ e r(x ؊ BC50) ], where N is number of colonies surviving on each plate with an x series of concentrations of an antibiotic, and N 0 represents the meaningful inoculum size. Construction of the CKC method was based on the bactericidal effect of each antibiotic against the bacterial strain versus the concentration in two dimensions and may be a more valid, accurate, and reproducible method for estimating the bactericidal effect than the endpoint minimum bactericidal concentration (MBC) method. Mathematically, the CKC approach was point symmetrical toward its inflexion (BC 50 , N 0 /2); thus, 2BC 50 could replace MBC. The parameter BC 1 can be defined as BC 50 ؉ [ln(N 0 ؊ 1)/r], which is the drug concentration at which only one colony survived and which is the least critical value of MBC in the CKC. The variate r, which determined the tangent slope on inflexion when N 0 was limited, could estimate the bactericidal intensity of an antibiotic. This verified that the CKC approach may be useful in studies with other classes of antibiotics and has considerable value as a tool for the accurate and proper administration of antibiotics.Over the last century, antibiotics have enjoyed widespread application in the treatment of bacterial diseases (24). The correct estimation of bactericidal potency, an important parameter of pharmacology, is a critical issue for the safe and proper use of antibiotics (2, 29); and a number of alternative methods and standards, including those described by the National Committee for Clinical Laboratory Standards and the European Committee on Antimicrobial Susceptibility Testing (39), have been developed. All such methods are based on a similar principle, whereby bactericidal effectiveness, the MIC, or the minimum bactericidal concentration (MBC), or derivatives thereof, are determined by measurement of the diameter of the zone of growth inhibition (the disk diffusion method and E-test), culture turbidity (the broth dilution and microdilution methods), and colony formation in agar plate (agar dilution tests). All these assays are carried out following incubation of target inocula for an optimal time in liquid or agar media containing ranges of antibiotic concentrations (3,14,30). The presence either of resistant mutants or of variations in susceptibility results by diffusion and the superior growth of highly fit drug-resistant bacterial cells in antibiotic-containing broth, so the broth dilution...