Bioassay procedure for the evaluation of erythromycin activity in aquaculture environments

A new bioassay procedure was developed for the detection of erythromycin in aquaculture samples using a strain of a Srenotraphomonas as an indicator o rganism. Conventional disk-plate and well-plate radial diffusion assay proc edures were developed, as well as a third procedure using the same indicato r organism in Luria-Bertani (LB) broth, supplemented with the indicator dye Brilliant Black (40 mu g/mL) in a multi-well microtiter plate. For both th e disk-plate and well-plate radial diffusion assays, the response reflected in the size (width) of the growth inhibition zone, which was linear over t he tested concentration range of 0.05 to 2.0 mu g erythromycin/mL. The limi t of quantitation of the bioassay was at 0.05 mu g erythromycin/mL. Among t he three methods of assay tested with Stenotrophomonas sp., the semiquantit ative dye reduction method is easy to read and is not diffusion dependent. This method allows for processing of more samples and more replication on a single titer plate. This new indicator organism is specific for erythromyc in when tested in the presence of other antibacterial agents, i.e., oxytetr acycline (Terramycin((R))) and/or Romet-30((R)). This new bioassay procedur e is suitable for quantitation of low concentrations of erythromycin in aqu aculture water and sediment samples.