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Gene probes derived from the insertion sequence IS986, which have previously been shown to differentiate isolates of Mycobacterium tuberculosis for epidemiological analysis, are also capable of distinguishing two groups of BCG vaccine strains. Most BCG strains have a single copy of IS986, at the same chromosomal site, while the Brazilian, Japanese and USSR strains have an additional copy at a different, common location. These results correlate with the results of previous antigenic analysis and may reflect a different clonal origin of the two groups of BCG strains.

Four collaborating BCG production laboratories had each prepared vaccine from four different BCG strains. In this laboratory the morphology of colonies cultured from samples of the 16 vaccine lots thus available was examined, and it was found that changes monitored in earlier small-scale experimental reconstructions had also occurred during some of the full-scale production procedures, in accordance with prediction. In particular, a minority population carried by the Danish BCG strain had replaced the original majority when this strain had been employed as seed for the production of vaccine by the British procedure. Similarly, a minority carried by the Japanese strain had replaced the original majority when this strain was introduced as seed into the production procedures in the United Kingdom and France. Serial subcultures made in this laboratory showed, in accordance with previous experience, that the changes that had occurred in the Japanese strain could be completely reversed by serial subculture as a surface pellicle on Sauton medium.

Changes in the Danish BCG strain under certain regimens of subculture have been shown in preceding studies to be associated with selection of a minority population. Three Czechoslovakian BCG strains, all originally derived from the Danish strain but thereafter and in distinction from it maintained on potato media, have now been investigated. Changes in the immunizing potency of two of these strains have been attributed by other workers to employment of the richer potato media in place of Sauton medium as used for maintenance of the parent Danish strain. However, results from the present study suggest rather that selection of a pre-existing minority genotype or of a new mutant occurred. This proposal is supported by the finding that the third strain has maintained characteristics similar to those of the Danish parent despite many previous transfers on potato media.Another BCG strain investigated was the Japanese which, like the three Czechoslovakian strains, had been previously maintained on potato media. This strain has been shown in the present study to resemble the Danish strain in supporting a minority population yielding non-spreading colonies. Czechoslovakian vaccine prepared with seed culture supplied from Tokyo has retained characteristics similar to those of the Japanese parent. Although a majority population yielding spreading colonies appears so far to have been retained in both centres, it is considered that selection of the minority could still occur in the course of routine transfer.

Samples of Bacille Calmette-Guérin (BCG) vaccines from four collaborating production laboratories, each of which had prepared vaccine from four different daughter strains of BCG, had previously been monitored for changes in colony morphology and the present study was undertaken to determine whether the changes observed were reflected in the patterns of protein secretion and lipid content. In the samples examined there was evidence for a correlation between colony morphology and the presence or absence of mycoside B. As the components of BCG that determine virulence and protective immunity are unknown, care must be taken to ensure constancy of the strains during the manufacture of vaccines.

Under appropriate conditions of growth colonies showing fine wrinkling (rugosity) of their surface and characteristic of certain BCG strains can be distinguished from colonies with a smoother non-rugose morphology that are characteristic of some other BCG strains. This study has been concerned with the influence of nutritional and other media constituents on the evolution of these colonies. In a preliminary investigation nutritional components such as enzymic digest of casein, asparagine and salts were systematically eliminated from an agar medium, separately and together, and in the presence or absence of glycerol. From this and supplementary experiments, it appeared that the nutritional element with most effect on the size and morphology of colonies is the carbohydrate. Small inocula of BCG will normally grow on simple agar media only in the presence of enrichments such as blood or albumin, or if charcoal has been added. However, in a second investigation it was found that individual colonies would develop from such inocula placed 2 cm or more away from a concentrated inoculum that had been seeded onto the medium approximately 7-14 days earlier. It appears that a diffusible agent is produced from multiplying bacilli within a colony which counteracts toxic factors in the medium and may also assist nutrition within the colony. To examine the effects of carbohydrate on the evolution of colonies, in a third investigation glycerol was added in increasing quantities to an agar medium enriched with increasing quantities of bovine albumin and/or 5% of blood, and suspensions of French (Pasteur) and British (Glaxo) BCG vaccines were seeded onto it. Individual colonies cultured from these two strains have a rugose and non-rugose morphology respectively, and to highlight the effects in question, the evolution over a prolonged period of concentrated 'drop-colonies' seeded onto the medium from each strain was studied. There was a very marked difference between the two strains in the evolution of such 'drop-colonies', and it appeared that the lateral spread of fine rugosity from those of the Pasteur strain represented an enhanced ability of small numbers of bacilli to take up the nutrient.

To follow an earlier study of the effects on BCG of deep subculture in liquid media, transfers on the surface of Löwenstein-Jensen and Sauton's media have been made. When a vaccine preparation that yields on culture 99+% of spreading-type colonies, with a small minority of non-spreading forms, is transferred on Löwenstein-Jensen medium, the minority population can be readily selected out. Growth of BCG on this medium is not homogeneous, and the selection of either the minority or majority populations appears fortuitous, whereas it was previously demonstrated that deep subculture in Dubos medium tends to favour selection of a minority yielding non-spreading colonies. When such a minority population has been favoured during transfer on Löwenstein-Jensen medium, subsequent subculture in Dubos medium usually accentuates the effect. Subculture as a surface pellicle on Sauton's medium has a powerful effect in reducing the relative size of a minority population yielding non-spreading colonies, and thereafter maintaining 99+% of spreading forms. The relationship between transfers on Löwenstein-Jensen and Sauton's media and those on potato-Sauton medium is briefly discussed.

BCG originated from a virulent bovine strain of the tubercle bacillus after prolonged serial subculture on a potato medium. Since attenuation was achieved, the BCG strain has been distributed to a large number of centres where BCG vaccine is produced. Many of these production laboratories have maintained their BCG lines by continuing serial transfers, but have employed a variety of media for this purpose, and have produced BCG vaccine by a variety of techniques. Distinct differences have developed between some of the daughter strains of BCG, but the mechanism through which these changes have occurred has not been clear. In recent years methods have been developed which have enabled changes taking place within some BCG strains during experimental serial subculture to be monitored. In this survey the relationship of the changes observed to the different techniques employed for the maintenance of BCG lines and for the preparation of vaccine is considered. It is suggested that selection of minority populations within BCG strains noted during experimental studies may provide an analogy with the mechanism through which the original attenuation of the virulent bovine strain was brought about. The relevance of small-scale laboratory investigations to full-scale production procedures is also discussed, and finally some additional measures that might be taken to minimise changes in BCG strains are proposed.

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.