Reference genes for quantitative, reverse-transcription PCR in Bacillus cereus group strains throughout the bacterial life cycle.

Quantitative reverse-transcription PCR (RT-qPCR) has become a major tool to better understand the biology and pathogenesis of bacteria. One prerequisite of valid RT-qPCR data is their proper normalization to stably expressed reference genes. To identify and evaluate reference genes suitable for normalization of gene expression data in Bacillus cereus group strains, mRNA levels of eleven candidate reference genes (rpsU, nifU, udp (UDP-N-acetylglucosamine 2-epimerase), BT9727_5154/BC_5475, BT9727_4034/BC_4293, BT9727_4549/BC_4813, pspA, gatB_Yqey (gatB_Yqey domain containing protein), helicase (SWF/SNF family protein), adk and pta) and a target gene (BT9727_3305/BC3547+BC3546) were quantified by RT-qPCR at different time points throughout the entire life cycle of the wild-type B. cereus ATCC 14579 and Bacillus thuringiensis subsp. konkukian 97-27, a phylogenetically closely related strain to Bacillus anthracis. The programs geNorm and Normfinder were used to calculate expression stabilities and identified the genes gatB_Yqey, rpsU and udp as the most stably expressed reference genes. Compared to this combination or the sets of reference genes as recommended by geNorm or Normfinder, normalization using a traditional housekeeping gene (adk) alone resulted in significantly different gene expression results and in a significant overestimation of the target gene transcription. Normalization of the data to the reference gene gatB_Yqey alone showed no or only small differences to the reference gene combinations indicating that gatB_Yqey may be used as a single reference gene when investigating rather large changes in mRNA transcription. Otherwise, a combination of the stably expressed reference genes is recommended. In conclusion, the present study underlines the importance of normalization to stably expressed reference genes and presents valid endogenous controls suitable for normalization of transcriptional data throughout the life cycle of B. cereus group strains.