Studies on Variability of B. diphtheriae

Typical cultures of virulent diphtheria bacilli grown in broth containing 10% specific antibacterial serum gave origin to distinct and rather stable variant types differing markedly from the parent strain in colony form, lacking in fermentative properties and in virulence and toxigenicity. Likewise, continued cultivation of typical cultures (at 37°C.) in broth containing 0.1% phenol, of 0.5% lithium chloride or 2.5% pancreatin as well as cultivation in K-medium (Kendall) diluted 1 :3 with physiologic salt solution and cultivation in plain broth at 45°C., gave rise to the development of similar variant types.

In some instances transformation from the original characteristic colony type to the variant type occurred suddenly while in other instances certain variations or intermediate changes in colony form with loss of virulence were noted before the appearance of the extreme variant type. The intermediate types probably corresponded to those noted by other observers of bacterial dissociation in B. diphtheriae cultures.

The variant or ‘V’ colonies are smaller, less opaque and grow less rapidly on various culture media. In morphology the ‘V’ organisms are pleomorphic, exhibiting small coccoid or occasionally somewhat elongated solid staining types. Broth cultures develop a uniform turbidity which is slow in appearing. The ‘V’ cultures are practically devoid of fermentative ability. Guinea pigs are not affected by inoculation with living ‘V’ cultures or with broth filtrates, indicating loss in virulence and in toxigenicity incident to the forced transformation.

These properties, which characterize the ‘V’ type culture have remained constant in cultures maintained over periods of 6 months to 2 years indicating considerable stability of the variant type.

Antisera containing agglutinins for the mother-type organisms were devoid of agglutinating effect upon some of the ‘V’ strains while other ‘V’ strains were agglutinated, indicating that agglutinogen common to the parent organism might be variably transmitted to the variant.