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Antimicrobial Agents and Chemotherapy, October 2004, p. 3884-3891, Vol. 48, No. 10
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.10.3884-3891.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Novel Concentration-Killing Curve Method for Estimation of Bactericidal Potency of Antibiotics in an In Vitro Dynamic Model

Y. Q. Liu,{dagger} Y. Z. Zhang,* and P. J. Gao*

State Key Laboratory of Microbial Technology, Shandong University, Jinan, Shandong Province, China

Received 9 November 2003/ Returned for modification 10 November 2003/ Accepted 30 May 2004

The bactericidal pharmacodynamics of antibiotics against Escherichia coli were analyzed by a concentration-killing curve (CKC) approach, and the novel parameters median bactericidal concentration (BC50) 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 (R2 > 0.9) with the function N = N0/[1 + er(x – BC50)], where N is number of colonies surviving on each plate with an x series of concentrations of an antibiotic, and N0 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 (BC50, N0/2); thus, 2BC50 could replace MBC. The parameter BC1 can be defined as BC50 + [ln(N0 – 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 N0 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.

* Corresponding author. Mailing address: State Key Laboratory of Microbial Technology, Shandong University, Shanda South Road 27, Jinan 250100, China. Phone: (86) 531-8563756. Fax: (86) 531-8564326. E-mail for P. J. Gao: gaopj{at}sdu.edu.cn. E-mail for Y. Z. Chang: zhangy{at}sdu.edu.cn.

{dagger} Present address: Institute of Animal Science and Veterinary Medicine, Shandong Academy of Agricultural Science, Jinan 250100, China.

Antimicrobial Agents and Chemotherapy, October 2004, p. 3884-3891, Vol. 48, No. 10
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.10.3884-3891.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.





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