Influence of methodology upon apparent concentrations of antibiotics in tissue

This study compares the diffusion in agar of antibiotics prepared in homogenates of tissue, tissue supernatant fluids, whole serum, and buffer solution. Penicillin, erythromycin, cephalothin, gentamicin, and colistin sulfate were studied. With the exception of erythromycin in supernatant fluid of human tonsil, zones of inhibition in agar were consistently smaller in homogenate and supernatant fractions of tissue than in serum and buffer. Thus, a significant error of underestimation occurred when a buffer standard curve was used for assaying antimicrobial activity in homogenates of tissue. This error ranged from 23 to 66% in 10% guinea pig liver homogenate and from 43 to 78% in 20% liver homogenate. A similar, but smaller error of under-estimation resulted with supernatant fractions of tissue except with erythromycin, for which a 25 to 56% error of overestimation occurred when activity in supernatant fraction of tonsil was read off a buffer standard curve. These results emphasize the importance of preparing the standard antibiotic curve with material having agar diffusion properties similar to those of the material being assayed.