SUMMARY: strain 136-R4 is lactose-negative, mannitol-negative (Lac-Mann-) and is sensitive to bacteriophage T7. Carrier clones of strain R4 (contaminated with T7) were found to maintain their association with phage T7 through as many as fifty consecutive single-colony isolations (from an ancestral colony which had survived lysis by phage T7). All carrier cultures (so-called pseudo-lysogenic strains) were found to be lactose-positive and mannitol-positive (Lac Mann). Passage of Lac Mann bacteria through media containing antiserum directed against phage T7 resulted in a change back to Lac Mann and in the complete elimination of phage T7.

Biochemical, genetic and immunochemical evidence indicates that the change from Lac Mann to Lac Mann is the result of a phage-controlled alteration in the phenotype of This dysentery bacillus is cryptic with respect to the expression of Lac Mann and the crypticity is attributable to surface structures (which are probably not a part of the Y or permease system). Under appropriate conditions decryptification may be brought about by phage-associated endolysin. Similar phenomena were observed in carrier strains of certain other members of the Enterobacteriaceae.


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