Jourml of General Microbiology (I 977),99. 127-1 38 Printed in Great Britain Derivation of a Pruteus mirabilis Converting Phage for Ampicillin Resistance

A lysate of phage 5006M was prepared on Proteus mirabilis ~ ~ 5 0 0 6 which harboured the plasmid P-lac to which the ampicillin resistance gene of R plasmid RP4 had been transposed. This lysate transduced the ampicillin resistance marker to ~ ~ 5 0 0 6 at a frequency of 5 x 10-l ~ 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 ~ ~ 5 0 0 6. 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 propor-tion 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 un-stable 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 ~ ~ 8 0 4 and a newly dis-covered host ~ ~ 9 8, rendering some transductants of these strains auxotrophic