Bacillus subtilis Spores labelled with Radiophosphorus

Summary: Bacillus subtilis was grown in surface culture on media containing about 0·6 mC./ml. of 32P as phosphate, and harvested after 2 days. The spore suspension was heated at 60° for 1½ hr. and washed by centrifuging and resuspending in water six times. The fresh suspension contained about 2 × 109 spores/ml., of which 20 % were viable. The radioactivity was about 0°05 mC./ml. of which more than 95 % was carried by the spores. The radioactivity was sufficient for 103 spores to be readily detected and assessed by conventional Geiger-Müller counting methods. These highly radioactive suspensions are particularly useful in studying the respiratory retention of bacterial aerosols.