SUMMARY: Several type C and D strains of , which had been converted to the toxigenic state by phages, were serially transferred through cooked meat medium with and without specific anti-phage serum. Most of the converted strains lost their toxigenicity even during transfer without antiserum, and the non-toxigenic variants that appeared were resistant to lysis and conversion by the original phage. However, in some combinations of phage and host bacteria toxigenicity was stable after ten transfers, though it showed a transient decrease, and the non-toxigenic variants that arose remained sensitive to lysis and conversion. When converted strains were transferred in medium containing anti-phage serum, toxigenicity was lost more rapidly than in the absence of serum and the non-toxigenic variants that appeared remained sensitive to lysis and conversion by the parent phage.

Filtrates of the supernatants of culture fluids of strains transferred without anti-phage serum converted non-toxigenic strains to toxigenicity at varying rates. However, a non-converting phage was demonstrated in one of these filtrates. This phage was identical to the original converting phage in its morphology, host range and antigenicity. Indicator strains treated with this phage acquired resistance to lysis by the parent phage.

The results suggest that re-infection and conversion to toxigenicity occurred in combinations showing stable toxigenicity after ten transfers. However, in those combinations that lost toxigenicity, re-infection with a non-converting mutant of the original phage may have occurred with the result that non-toxigenic variants became resistant to the converting phage. This appears to be one of the causes of the loss of toxigenicity which is common in some type C and D strains of


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