SUMMARY: A solidified medium (NP medium) was devised which supported luxuriant growth of type B. It consists of peptone, extracts of yeast and liver, blood, glucose and certain other compounds essential as nutrients and/or for poising oxidation-reduction potential. The peptone could be replaced by a commercial preparation of acid-hydrolysed casein or a mixture of 18 amino acids, but the presence of cysteine and either ascorbic acid, thioglycollate (mercapto-acetate) or dithiothreitol (DTT) was essential for consistent rapid and luxuriant surface growth. Blood was not essential in media supplemented with cysteine and DTT, although in its presence the number of colonies which developed was maximal and the formation of zones of haemolysis facilitated the location of smaller colonies. When supplemented with cysteine+DTT several other solidified media, none of which contained yeast—or liver—extract, supported good growth. By incorporating antitoxin into certain clear blood-free plating media containing cysteine+DTT it was possible to show that only some bacilli of a culture produced colonies which developed opaque haloes of toxin antitoxin flocculum. A liquid version of the NP medium was also devised.


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