The microstructure of colonies of the Connaught BCG strain.

It has previously been shown that there is a certain correlation between the biological properties of BCG strains-properties on which their immunogenicity and allergenicity depend-and the macroscopic appearance of the growth of these colonies on solid and liquid media. To investigate this phenomenon in greater detail, the author examined the microstructure of colonies of the Connaught BCG strain grown on both solid and liquid media. Colonies were fixed in agar, embedded in paraffin, sectioned and stained by the Ziehl-Neelsen technique.A striking finding was the alternation of acid-fast and non-acid-fast zones in colonies grown on bovine-serum agar or Ogawa egg medium; the strata nearest the surface of the solid media were usually more acid-fast than were the deeper strata. Colonies grown in Sula's liquid medium, on the other hand, showed no such stratification and were equally acid-fast at all points. These differences may be the result of genetic factors or of the different nutritional conditions provided by solid and liquid media.