Inhibitory effects of citrates in the determination of trace amounts of penicillin.

In the microbiological assay for the determination of trace amounts of penicillin, it has been shown that sodium citrate and penicillin act synergistically against Sarcina lutea. The inhibitory action of citrate appears to involve a sequestering of divalent cations essential to the growth of the organism as shown by minimal inhibitory concentration and agar diffusion studies. The addition of calcium or magnesium to broth or agar at various concentrations overcomes this inhibition, and 0.02 M magnesium was found to be the most effective concentration. On agar supplemented with magnesium chloride, penicillin dose-response curves with and without citrate present were practically superimposable, and larger inhibition zones against a greater amount to total growth resulted in improved assay susceptibility and an interference-free method. Misleading results can be obtained when pharmaceutical products containing citrates are tested by conventional methods for penicillin cross-contamination.