Sieburth, John McNeill (Virginia Polytechnic Institute, Blacksburg, Va.). Antibiotic properties of acrylic acid, a factor in the gastro-intestinal antibiosis of polar marine animals. J. Bacteriol. 82: 72–79. 1961.—Observations were made on acrylic acid to study some of the antibiotic, chemical, and physical properties of this volatile acid which occurs at a concentration of 8% (dry weight) in the marine alga Phaeocystis pouchetii . The sodium salt inhibited both gram-positive and gram-negative bacteria by filter paper disc assays at concentrations of 0.012–12.0 mg/ml. Activity was enhanced by acid reactions approximating those of the avian gut. In an attempt to explain the absence of typical strains of Escherichia coli and the suppression of the atypical coliform microflora in the anterior gastrointestinal segments of pygoscelid penguins in areas where the phytoplankton was dominated by P. pouchetii , chick trials were conducted with sodium acrylate. Acrylate supplementation of chicken feed at levels as low as one-fifth (0.01%) of those estimated to be ingested by penguins under natural conditions, suppressed the E. coli population and permitted its partial replacement by Aerobacter aerogenes . This latter phenomenon was explained by an increase in acrylate resistance by the A. aerogenes population. Acrylate feed levels between 0.01 and 1.0% caused an apparent increase in the growth rate of chicks, whereas 10% levels caused anorexia and death.