Improved agar diffusion method for detecting residual antimicrobial agents

The improved agar diffusion method for determination of residual antimicrob ial agents was investigated, and the sensitivities of various combinations of test organisms and assay media were determined using 7 organisms, 5 medi a, and 31 antimicrobial agents. Bacillus stearothermophilus and synthetic a ssay medium (SAM) showed the greatest sensitivity for screening penicillins (penicillin G and ampicillin). The combination of Bacillus subtilis and mi nimum medium (MM) was the most sensitive for tetracyclines (oxytetracycline and chlortetracycline), B. stearothermophilus and SAM or Micrococcus luteu s and Mueller-Hinton agar (MHA) for detecting tylosin and erythromycin, B. subtilis and MHA for aminoglycosides (streptomycin, kanamycin, gentamicin, and dihydrostreptomycin), B. stearothermophilus and SAM for polyethers (sal inomycin and lasalocid), and B. subtilis and MM or Clostridium perfringens and GAM for polypeptides (thiopeptin, enramycin, virginiamycin, and bacitra cin). However, gram-negative bacterium Escherichia coli ATCC 27166 and MM w ere better for screening for colistin and polymixin-B. For detecting the sy nthetic drugs tested, the best combination was B. subtilis and MM for sulfo namides, E. coli 27166 and MM for quinolones (oxolinic acid and nalidixic a cid), B. subtilis and MM for furans (furazolidone), and the bioluminescent bacterium Photobacterium phosphoreum and luminescence assay medium for chlo ramphenicol and oxolinic acid. The results showed that the use of four assa y plates, B. stearothermophilus and SAM, B. subtilis and MM, M. luteus and MHA, and E. coli 27166 and MM, was superior to the currently available tech niques for screening for residual antimicrobial agents in edible animal tis sues.