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At least one factor that causes low fertility of Salmonella typhimurium LT2 strains in crosses with Escherichia coli K-12 Hfr's can be inhibited by growing the female strains in supplemented minimal salts medium rather than in nutrient broth and by incubating the female strains at 50 C immediately before mating with the Hfr. These pretreatments can enhance the recovery of prototrophic recombinants for markers injected early by the Hfr by a factor of as much as 10(4). The heat treatment is effective only on the female in intergeneric crosses and gradually loses (within 50 min) its effectiveness after return of heat-treated cells to 37 C. It is concluded that the restriction system of the female is heat-sensitive. Since markers injected late by the male enter females in which the heat-impaired restriction system has recovered, few recombinants for late markers are found. The presence of the leading end of an E. coli Hfr in an S. typhimurium-E. coli hybrid enhances by up to sevenfold the frequency of lac(+) recombinants in subsequent crosses with an E. coli Hfr if the E. coli segment is integrated into the chromosome of the hybrid; the effect is less marked if the E. coli segment is not integrated.

Fertility of Salmonella typhimurium Crosses with Escherichia coli