A study of some effects of subculture on two BCG strains

A marked improvement in the ability to consistently distinguish between colonies cultured from BCG strain 1331 (Copenhagen), employed for the routine production of Danish freeze-dried BCG vaccine, and those cultured from BCG strain 1077 (Glaxo), employed for the routine production of British freeze-dried BCG vaccine, was obtained in our laboratory by modifying the constituents of a Dubos-type solid-medium. Several batches of British BCG vaccine had been specially prepared by the manufacturer, using strain 1331 as seed culture, and samples from these batches were cultured on this medium in our laboratory. Two of the batches produced, in place of the spreading colonies normally cultured from strain 1331, a large majority of nonspreading colonies, normally characteristic of strain 1077. To study this very rapid change, modifications of the system of serial transfers employed routinely by the manufacturer were set up in our laboratory, and the relative proportions of the two colony forms cultivated at each stage of transfer were noted. The rapid changes that had occurred during manufacture were reproduced under our experimental conditions, and it was observed that transfer through deep culture in liquid Dubos medium tended to shift the balance between the numbers of spreading and non-spreading colonies in favour of the latter, whilst the reverse was true of the transfers through deep culture in production medium. The ability to monitor changes occurring in BCG strains during serial transfer should facilitate the provision of measures to prevent such changes taking place.