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Background   Borrelia burgdorferi  has limited biosynthetic capabilities and must scavenge N-acetylglucosamine (GlcNAc), an essential component of the microbial cell wall, from the surrounding environment. Spirochetes cultured in the absence of free GlcNAc exhibit biphasic growth; however, addition of chitobiose (a dimer of GlcNAc) substitutes for free GlcNAc resulting in a single exponential phase. We evaluated the effect of RpoS and RpoN, the only alternative sigma factors in  B. burgdorferi , on biphasic growth and chitobiose utilization in the absence of free GlcNAc. In addition, we investigated the source of GlcNAc in the second exponential phase.    Results  By comparing the growth of wild-type cells to insertional mutants for  rpoS  and  rpoN  we determined that RpoS, but not RpoN, partially regulates both biphasic growth and chitobiose utilization. The  rpoS  mutant, cultured in the absence of free GlcNAc, exhibited a significant delay in the ability to initiate a second exponential phase compared to the wild type and  rpoS  complemented mutant. Expression analysis of  chbC , which encodes the membrane-spanning protein of the chitobiose phosphotransferase system, suggests the delay is due to the inability of the  rpoS  mutant to up regulate  chbC . Furthermore, supplementing GlcNAc starved cultures with high concentrations (75 or 150 μM) of chitobiose resulted in biphasic growth of the  rpoS  mutant compared to a single exponential phase for the wild type and  rpoS  complemented mutant. In contrast, growth of the  rpoN  mutant under all conditions was similar to the wild type. 5' Rapid amplification of cDNA ends (5' RACE) revealed the transcriptional start site for  chbC  to be 42 bp upstream of the translational start site. Analysis of the  chbC  promoter region revealed homology to previously described RpoD and RpoS  B. burgdorferi  promoters. We also determined that yeastolate, a component of the growth medium (BSK-II), is not essential for second exponential phase growth.    Conclusion  Together these results suggest that RpoD and RpoS, but not RpoN, regulate biphasic growth and chitobiose utilization in  B. burgdorferi  by regulating the expression of the chitobiose transporter ( chbC ). The data also demonstrate that the second exponential phase observed in wild-type cells in the absence of free GlcNAc is not due to free chitobiose or GlcNAc oligomers present in the medium.

Chitobiose utilization in Borrelia burgdorferi is dually regulated by RpoD and RpoS