Accelerated transformation and deactivation of erythromycin in superheated water. 2. Transformation reactions and bioassays

The work presented in this second part of a two-part series on the accelerated transformation of erythromycin in superheated water focuses on the chemical nature and resultant antimicrobial implications of the overall reaction observed. Analyses of reactor effluents employing infrared spectroscopy and liquid chromatography/mass spectrometry indicated that the initial step in the decomposition pathway is one of dehydration. Subsequent hydrolysis in the presence of an acetate buffer appeared to result in the loss of the sugar cladinose. Chemical transformation under superheated conditions was tied to the loss of antibiotic function by an agar dilution test. The sensitivity of this test was verified by determination of the minimum inhibitory concentration (MIC) of erythromycin corresponding to each of two test microorganisms. MIC values for the selected strains of Escherichia coli and Bacillus subtilis were 35 and 0.5 mg/L, respectively. To relate the loss of antimicrobial activity to increased reaction temperature and thus to increased extent of parent compound transformation to microbially benign intermediates, bioassays using E coli as test microorganism were performed on flow reactor effluents resulting from reaction of initial erythromycin concentrations of 75-150 mg/L.