ABSTRACT: Staphylococcus aureus is a human pathogen, and S. aureus bacteremia can cause serious problems in humans. To identify the genes required for bacterial growth in calf serum (CS), a library of S. aureus mutants with randomly inserted transposons were analyzed for growth in CS, and the aspartate semialdehyde dehydrogenase (asd)-inactivated mutant exhibited significantly reduced growth in CS compared with the wild type (WT). The mutant also exhibited significantly reduced growth in medium, mimicking the concentrations of amino acids and glucose in CS. Asd is an essential enzyme for the biosynthesis of lysine, methionine, and threonine from aspartate. We constructed inactivated mutants of the genes for lysine (lysA), methionine (metE), and threonine (thrC) biosynthesis and found that the inactivated mutants of lysA and thrC exhibited significantly lower growth in CS than the WT, but the growth of the metE mutant was similar to that of the WT. The reduced growth of the asd mutant was recovered by addition of 100 ?g/ml lysine and threonine in CS. These results suggest that S. aureus requires lysine and threonine biosynthesis to grow in CS. On the other hand, the asd-, lysA-, metE-, and thrC-inactivated mutants exhibited significantly reduced growth in mouse serum compared with the WT. In mouse bacteremia experiments, the asd-, lysA-, metE-, and thrC-inactivated mutants exhibited attenuated virulence compared with WT infection. In conclusion, our results suggest that the biosynthesis of de novo aspartate family amino acids, especially lysine and threonine, is important for staphylococcal bloodstream infection.Studying the growth of bacteria in blood is important for understanding its pathogenicity in the host. Staphylococcus aureus sometimes causes bacteremia or sepsis. However, the factors responsible for S. aureus growth in the blood are not well understood. In this study, using a library of 2,914 transposon-insertional mutants in the S. aureus MW2 strain, we identified the factors responsible for bacterial growth in CS. We found that inactivation of the lysine and threonine biosynthesis genes led to deficient growth in CS. However, the inactivation of these genes did not affect S. aureus growth in general medium. Because the concentration of amino acids in CS is low compared to that in general bacterial medium, our results suggest that lysine and threonine biosynthesis is important for the growth of S. aureus in CS. Our findings provide new insights for S. aureus adaptation in the host and for understanding the pathogenesis of bacteremia.
