Inhibition of chlortetracycline activity by magnesium ions

Up to 50% of the antimicrobial activity of chlortetracycline (CTC) can be inhibited by the presence of MgSO4 in microbiological turbidimetric assays. From 50 to 200 mM MgSO4 inhibited CTC activity, and the relationship between CTC inhibition and MgSO4 concentration is linear. The possible mechanism of this inhibition and its potential implication are discussed.