| Summary: | <p>Abstract</p> <p>Background</p> <p>We previously identified <it>ebpR</it>, encoding a potential member of the AtxA/Mga transcriptional regulator family, and showed that it is important for transcriptional activation of the <it>Enterococcus faecalis </it><b>e</b>ndocarditis and <b>b</b>iofilm associated <b>p</b>ilus operon, <it>ebpABC</it>. Although <it>ebpR </it>is not absolutely essential for <it>ebpABC </it>expression (100-fold reduction), its deletion led to phenotypes similar to those of an <it>ebpABC </it>mutant such as absence of pili at the cell surface and, consequently, reduced biofilm formation. A non-piliated <it>ebpABC </it>mutant has been shown to be attenuated in a rat model of endocarditis and in a murine urinary tract infection model, indicating an important participation of the <it>ebpR-ebpABC </it>locus in virulence. However, there is no report relating to the environmental conditions that affect expression of the <it>ebpR-ebpABC </it>locus.</p> <p>Results</p> <p>In this study, we examined the effect of CO<sub>2</sub>/HCO<sub>3</sub><sup>-</sup>, pH, and the Fsr system on the <it>ebpR-ebpABC </it>locus expression. The presence of 5% CO<sub>2</sub>/0.1 M HCO<sub>3</sub><sup>- </sup>increased <it>ebpR-ebpABC </it>expression, while the Fsr system was confirmed to be a weak repressor of this locus. The mechanism by which the Fsr system repressed the <it>ebpR-ebpABC </it>locus expression appears independent of the effects of CO<sub>2</sub><sup>- </sup>bicarbonate. Furthermore, by using an <it>ebpA</it>::<it>lacZ </it>fusion as a reporter, we showed that addition of 0.1 M sodium bicarbonate to TSBG (buffered at pH 7.5), but not the presence of 5% CO<sub>2</sub>, induced <it>ebpA </it>expression in TSBG broth. In addition, using microarray analysis, we found 73 genes affected by the presence of sodium bicarbonate (abs(fold) > 2, <it>P </it>< 0.05), the majority of which belong to the PTS system and ABC transporter families. Finally, pilus production correlated with <it>ebpA </it>mRNA levels under the conditions tested.</p> <p>Conclusions</p> <p>This study reports that the <it>ebp </it>locus expression is enhanced by the presence of bicarbonate with a consequential increase in the number of cells producing pili. Although the molecular basis of the bicarbonate effect remains unclear, the pathway is independent of the Fsr system. In conclusion, <it>E. faecalis </it>joins the growing family of pathogens that regulates virulence gene expression in response to bicarbonate and/or CO<sub>2</sub>.</p>
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