Mutational and transcriptional analyses of the Staphylococcus aureus low-affinity proline transporter OpuD during in vitro growth and infection of murine tissues

Staphylococcus aureus continues to be a major health problem. This species' requirement for proline and proline transport from the extracellular environment is not well understood. Here, we identify a S. aureus low-affinity proline transport gene (opuD) with homology to the OpuD protein of Bacillus subtilis. Mutation of the opuD gene caused a significant decline in proline uptake under low-affinity conditions as compared with wild type, but the opuD mutant strain showed no significant attenuation in a murine abscess model of infection. The S. aureus opuD gene was transcriptionally activated during growth in moderate osmolarity media with high levels of proline or glycine betaine independent of SigB. In murine abscesses, the opuD gene was activated at a later time point, whereas the opuD expression dropped over the course of an 18-h period within murine urinary tracts. Transcriptional regulation of opuD in S. aureus appears to be coordinated within this species when grown in moderate to high NaCl environments, but the level of extracellular proline had a marked effect on expression of this proline transport gene. The differential regulation of proline transport genes in S. aureus may be an adaptation for life in a variety of environments, including survival within the human body.