Hybrid between temperate phage P22 and virulent phage MB78

Bacteriophage MB78, a virulent phage of Salmonella typhimurium can not grow in rifampicin resistant mutant of the host. However, the temperate phage P22 which grows normally in this host helps MB78 to grow in its non-permissive host. P22 can not itself multiply under the condition of mixed infection and the burst size of MB78 is very much reduced. The burst sizes of both are reduced even when the permissive host LT2 is mixedly infected with P22 and MB78. When rifampicin resistant mutants are mixedly infected, only P22-specific mRNAs are produced in the early stage. This is followed by transcriptions from both P22 and MB78 genomes. Subsequently, only MB78-specific messengers are detected in the infected host. Hybrids between the two phages have been isolated from the mixedly infected cells. These hybrids which contain only 15-20% of P22 genome can grow in the rifampicin resistant mutants of the host. The analysis of hetero-duplex between MB78 and hybrid phages revealed the presence of at least three mismatch regions. Experiments are in progress to identify the parts of P22 genome present in the hybrid phage.