Microbiological method for determining macrolides in animal feeds in the presence of other drugs by thin-layer chromatography detection

A method was developed to separate, detect, and quantitate erythromycin (ERY) and tylosin (TYL) in animal feeds in the presence of 11 other drugs: 3 nitrofurans, 2 tetracycline antibiotics, 3 sulfonamides, 2 coccidiostats, and 1 antibacterial growth promoter, ERY and TYL were separated from coexisting drugs, detected by thin-layer chromatography, and quantitated microbiologically by an agar diffusion method, Analysis of 125 experimental animal feed samples fortified at 5 levels (7.5-400 ppm) with ERY and TYL and at 1 level (50 ppm) with the rest of the drugs gave limits of quantitation of 2 and 5 ppm, recoveries of 90.3 and 92.4%, and relative standard deviations of 4.3-7.3% and 3.6-6.1%, respectively.