monocarboxylic acid polyether antibiotic monensin A A spiroketal, monensin A is the major component of monensin, a mixture of antibiotic substances produced by <em>Streptomyces cinnamonensis</em>. An antiprotozoal, it is used as the sodium salt as a feed additive for the prevention of coccidiosis in poultry and as a growth promoter in cattle. cyclic hemiketal spiroketal