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Over the past decade, metagenomics has become the preferred method for exploring complex microbiota such as human gut microbiota. However, several bias affecting the results of microbiota composition, such as those due to DNA extraction, have been reported. These bias have been confirmed with the development of culturomics technique. In the present study, we report the contamination of a gnotobiotic mice unit with a bacterium first detected by gram staining. Scanning electron microscopy and transmission electron microscopy permitted to detect a bacterium with a thick cell wall. However, in parallel, the first attempt to identify and culture this bacterium by gene amplification and metagenomics of universal 16S rRNA failed. Finally, the isolation in culture of a fastidious bacterium not detected by using universal PCR was successfully achieved by using a BCYE agar plate with CO 2  atmosphere at 30 °C. We performed genome sequencing of this bacterium using a strong extraction procedure. The genomic comparison allowed us to classify this bacterium as  Klenkia terrae . And finally, it was also detected in the stool and kibble that caused the contamination by using specific qPCR against this bacterium. The elucidation of this contamination provides additional evidence that DNA extraction could be a bias for the study of the microbiota. Currently, most studies that strive to analyze and compare the gut microbiota are based on metagenomics. In a gnotobiotic mice unit contaminated with the fastidious Actinobacteria  Klenkia terrae , standard culture, 16S rRNA gene amplification and metagenomics failed to identify the micro-organism observed in stools by gram-staining. Only a procedure based on culturomics allowed us to identify this bacterium and to elucidate the mode of contamination of the gnotobiotic mice unit through diet.

Klenkia terrae resistant to DNA extraction in germ-free mice stools illustrates the extraction pitfall faced by metagenomics