Characterization of bacteriophages from fox-containing, non-toxigenic isolates of Corynebacterium diphtheriae

Non-toxigenic strains of Corynebacterium diphtheriae continue to cause disease within immunized populations. A subset of these corynebacteria carry the diphtheria toxin gene but in a cryptic form. To determine whether such strains might contribute to the re-emergence of functional toxin genes, the phages and fox mutations within three clone types were examined. fox-containing, β-related phages were isolated from two of the strain types. The third isolate appeared to harbour a defective prophage. One of the tox^- phages encoded truncated, yet enzymatically-active, forms of diphtheria toxin, suggesting that it had sustained a point mutation within the latter half of its toxin gene. In contrast, the other mutant phage did not elicit the production of either a cross-reacting material or an ADP-ribosylating activity. Complementation tests employing a series of double lysogens confirmed that the mutations responsible for the non-toxigenic phenotype of all of the phages were cis dominant. Given these findings, it is reasonable to hypothesize that tox^- genes can arise within human populations by either homologous recombination between two distinct tox^- phages or spontaneous reversion within a single mutant allele.