Strains of Bordetella pertussis in which all the organisms contain agglutinogens 1 and 3 or 1, 2 and 4 are easy to identify as serotypes 1,0,3,0 and 1,2,0,4 respectively; and similarly, stable strains of serotype 1,0,3,4 are occasionally found. During repeated subcultures, passage in vivo, and lyophilization and preservation for many years, these serotypes do not change. Mixing 1,0,3,0 and 1,2,0,4 serotypes and culturing them together in vivo and in vitro produces cultures from which organisms of the same two serotypes can be isolated.
In contrast, strains which type as 1,2,3,4 are often a heterogeneous group. We have attempted to classify these as 'stable', 'variable' and 'mixed' cultures. Some strains comprise organisms all of which contain the four agglutinogens and are as easy to type as the strains described above. These we have called 'stable' 1,2,3,4 strains. Other 1,2,3,4 strains are made up of colonies possessing all four agglutinogens, as shown by agglutinin production, but in amounts varying from day to day so that direct typing is inconsistent. These we have called 'variable' 1,2,3,4 strains. The last category, 'mixed', is made up of organisms most of which give rise to stable 1,2,3,4 cultures; a few of the component organisms, however, have one or two of the four agglutinogens missing. The importance of the 'variable' cultures is emphasized for work on apparent change of serotype, e.g. during infection.