Accessory replicons of species of Salmonella and Shigella

Shigella and Salmonella strains isolated from clinical samples were examined. Out of 42 Shigella strains tested, 17 (40%) were found to be colicinogenic and another 3 were lysogenic. All three lysogens yielded a phage antigenically homologous to coliphage P2. Out of 30 strains tested, only 1 was found to be resistant to both neomycin and sulfamethoxazole. Out of 48 strains of Salmonella tested for drug resistance, only 2 showed multiple drug resistance. In contrast to Shigella isolates, the Salmonella isolates were infrequently (approximately 5%) bacteriocinogenic. The frequency of lysogeny in Salmonella strains was found to be 6% when tested on Salmonella typhimurium LT2, but by using a set of five indicators belonging to species Salmonella potsdam, Salmonella mbadanka, Salmonella dublin, Salmonella london, and Salmonella wandsworth, 50% of the strains were shown to be lysogenic. Salmonella phages related to P22 were recoverable from Salmonella saintpaul, Salmonella indiana, and Salmonella heidelberg. Some isolates of S. typhimurium yielded a temperature-sensitive and P22-heterologous phage which was found to be a more efficient transducer of bacterial genetic markers than P22. EcoRI-generated fragments of the DNA of some phages permitted the establishment of a clonal descent for some of the wild-type lysogenic bacterial strains. This last observation points out the potential usefulness of prophages as epidemiological markers.