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Derivation of a Proteus mirabilis converting phage for ampicillin resistance.

SUMMARY: A lysate of phage 5006M was prepared on Proteus mirabilis
pm5006 which harboured the plasmid P-lac to which the ampicillin resistance gene of R plasm id RP4 had been transposed. This lysate transduced the ampicillin resistance marker to pm5006 at a frequency of 5 x 10−10 per plaque-forming unit adsorbed. Five independently derived transductants were induced with ultraviolet light and yielded lysates which transduced ampicillin resistance at frequencies greater than unity. The lysates contained a majority of particles which formed small, slightly hazy plaques on pm5006. The other plaque-type was larger with a turbid centre and resembled the wild-type plaque. On replication of the phage titration plates to ampicillin agar, the bacterial growth in all the small plaques grew on the agar (plaque-replica transductants) whereas only that of a small and variable proportion of the large plaques did so. Low dilution titration plates (with many plaques) gave more replica transductants than could be accounted for by plaque counts, whereas with higher dilution plates the two tallied perfectly. The marker was unstable during lytic growth and, in a further cycle of growth of phage from small plaques, the proportion of wild-type plaque-forming particles increased and was accompanied by a reduction in the proportion of the large wild-type plaques which replicated to ampicillin agar. The marker translocated from the phage to another R plasmid, and also inserted into the chromosome of pm804 and a newly discovered host pm98, rendering some transductants of these strains auxotrophic.