Streptococcus pneumoniae has a potential ABC-type transporter (Pot) for extracellular polyamines. Polyamine transport protein D (PotD) is a membrane-associated, surface protein that putatively binds polyamines such as putrescine and spermidine. In this study we used quantitative PCR (qPCR) to analyze potD mRNA expression under physiologically relevant stress conditions in vitro, during in vivo infection, and in the presence of polyamines and choline. Expression of potD mRNA was elevated 2- and 4-fold when cells were grown at either 34 or 42 degrees C, respectively, in a choline restricted environment. Expression increased by 5- and 11-fold in response to oxidative stress in either low or high choline environments, respectively. Putrescine led to an increase in potD mRNA transcription, while choline and spermidine resulted in decreased gene expression. Transcription of potD in pneumococci harvested from blood of systemically infected mice was 43-fold higher compared to in vitro transcription levels. Flow cytometry analysis using PotD antiserum confirmed increased PotD expression on the pneumococcal surface. These results indicate that polyamines and polyamine transport systems potentially play an important role in Streptococcus pneumoniae pathogenesis, and may be important for bacterial response to temperature shock, oxidative stress, choline limitation and in vivo growth.