A Assay Formats

The basic microbial inhibition assay format involves a standard culture of a test organism, usually Bacillus stearothermophilus, Bacillus subtilis, Bacillus cereus, Micrococcus luteus, Escherichia coli, Bacillus megatherium, or Strepto-793 

Microbial inhibition tests are extremely sensitive for -lactam antibiotics, primarily penicillin, but mostly are more than 100-fold less sensitive for other commonly used antibacterials such as macrolides, sulfonamides, tetracyclines, or chloramphenicol (4, 5). Therefore, inhibition tests usually classify residues as belonging to the -lactam group. Antibiotics other than -lactams and sulfonamides can be detected by use of the enzyme penicillinase and aminobenzoic acid, respectively (1, 6). 

Investigations into performance characteristics have shown that various factors affect the efficiency of the microbiological assays tlieir relative influence depends upon the kind of antibacterials assayed and, especially, tlie test organism (7). Agar composition and pH, type of test strain, incubation temperature, depth of the agar, and the manner of incubation are all parameters of critical importance (7-9). 

More versatile than the growth-inhibition assays and potentially applicable to determining the presence of different antibiotic residues in different matrices are the microbial receptor CHARM I and II test assays (19, 20). The Charm I test, developed exclusively for -lactams in milk, constitutes the first rapid test recognized by The Association of Official Analytical Chemists (AOAC) with a test time of 15 min (19). The speed and sensitivity of this test permitted testing of milk tankers before they unloaded at the processing plant (21). In 1984-1985, the CHARM I test was further developed to test for antibiotics beyond -lactams to include tetracyclines, sulfonamides, aminoglycosides, chloramphenicol, novobiocin, and macrolides. The extended version has been referred to as CHARM II test. 

The CHARM II test is a general screening and identification test for members of different groups of antibacterial residues in milk in a rapid 15 min procedure (22-24). The test has been also applied to the analysis of other animal products, but a simple 30 min extraction with an aqueous buffer is usually required for tissue and eggs. 

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