INADEQUANCY OF MODIFIED CHECKERBOARD TECHNIQUE TO DETECT ANTAGONISM BETWEEN NITROFURANTOIN AND NALIDIXIC ACID AGAINST ESCHERICHIA COLI

Document Type : Original Article

Author

Department of Microbiology and Immuselogy Facelty of Pharmacy-Zagazig University Zagazig Egypt

Abstract

The in vitro combined antibacterial effect of nitrofurantoin and nalidixic acid was assessed using seven strains of Escherichia coli. Checkerboard broth dilution methods for determining bacteriostatic or bactericidal effects as well as the agar diffusion method were used. The simplified checkerboard broth dilution method did not only fail to detect antagonism, but conversely demonstrated partial synergism. Agar diffusion method (paper strip test) and the viable count tests clearly demonstrated an antagonism between the two drugs. Such antagonism could apparently be attributed to partial protection of cells, by nitrofurantion-induced bacteriostasis, from the lethal effect of nalidixic acid. Failure of checkerboard method to detect this antagonism is probably due to its inability to assess bactericidal effect.