Effect of mutS and recD mutations on Salmonella virulence

Hybrid derivatives of closely related bacteria may be used to dissect strai n-specific functions that contribute to virulence within a host. However, m ismatches between DNA sequences are a potent barrier to recombination. Reci pients with mutS and recD mutations overcome this barrier, allowing constru ction of genetic hybrids. To determine whether Salmonella hybrids construct ed in a mutS recD host can be used to study virulence, we assayed the effec t of mutS and recD mutations on the virulence of Salmonella typhimurium 140 28s in mice. Mutants defective in either mutS or recD do not affect the tim e course or the 50% lethal dose (LL50) of the infection. In contrast, the i nactivation of both mutS and recD results in a synthetic phenotype which su bstantially increases the time required to cause a lethal infection without changing the LD50. This phenotype results from an inability of mutS recD d ouble mutants to rapidly adapt to purine-limiting conditions present within macrophages, Although the disease progression is slower, S. typhimurium mu tS recD mutants retain the ability to cause lethal infections, and, thus, h ybrids constructed in mutS recD hosts may permit the analysis of virulence factors in a surrogate animal model.