Lactate is mainly fermented to butyrate by human intestinal microfloras but inter-individual variation is evident C. Bourriaud 1 , R.J. Robins 2 , L. Martin 3 , F. Kozlowski 1 , E. Tenailleau 2 , C. Cherbut 1 and C. Michel 1 1 Human Nutrition Research Centre, UFDNH – INRA, Nantes cedex 03, 2 Universite´ de Nantes, LAIEM – CNRS UMR6006, Nantes cedex 03, and 3 Human Nutrition Research Centre, UNE – ENV, Nantes cedex 03, France 2004/0806: received 9 July 2004, revised 1 October 2004 and accepted 2 October 2004 ABSTRACT C. BOURRIAUD, R.J. ROBINS, L. MARTIN, F. KOZLOWSKI, E. TENAILLEAU, C. CHERBUT AND C. MICHEL. 2005. Aim: To assess the role of lactate as a precursor for butyrate biosynthesis in human colonic microflora. Methods and Results: Three human faecal microfloras were incubated in vitro with media supplemented with 30 mmol l )1 unenriched or 13 C-enriched lactate. Lactate metabolism and short-chain fatty acid (SCFA) production were quantified. Lactate conversion to butyrate was investigated by gas chromatography-mass spectrometry and the pathways involved were identified by 13 C nuclear magnetic resonance spectroscopy. All human faecal microfloras rapidly and completely fermented lactate, yielding approx. 19 mmol l )1 total SCFAs. However, the SCFA composition varied markedly between microfloras. Butyrate was the main end-product for two microfloras but not for the third (60 and 61% vs 27% of the net concentration of SCFA produced respectively). The latter was typified by its ability to produce propionate as a major product (37%), and valerate (3%). 13 C-Labelling showed that butyrate was produced through the acetyl-CoA pathway and that the three microfloras possessed significant differences in their metabolic pathways for lactate consumption. Conclusions: In contrast to the ruminal microflora, the human intestinal microflora can utilize both D- and L-lactate as precursors for butyrate synthesis. Inter-individual variation is found. Significance and Impact of the Study: This study suggests that the butyrogenic capability of colonic prebiotics could be related to lactate availability. These findings will direct the development of selection strategies for the isolation of new butyrate-producing bacteria among the lactate-utilizing bacteria present in the human intestinal microfloras. Keywords: butyrate synthesis, colonic fermentation, human intestinal microflora, lactate metabolism, NMR spectroscopy. INTRODUCTION Butyrate is a short-chain fatty acid (SCFA) produced by the intestinal bacteria during fermentation of nondigestible carbohydrates (NDC). Based on its physiological properties, butyrate is perceived as a key factor in controlling colonic mucosal homeostasis (Scheppach et al. 2001). Therefore, favouring its production may be strategic for preventing colonic pathologies such as colorectal cancer or inflamma- tory bowel disease. Controlling butyrate production in the colon is difficult, however, due to a limited knowledge of the intestinal bacteria that are responsible for butyrate synthesis (Pryde et al. 2002). Butyrate-producing bacteria (BPB) are assumed to belong mainly to the genera Clostridium, Eubacterium and Fusobacterium (e.g. Clostridium butyricum, Eubacterium limo- sum) (Holdeman et al. 1977; Salyers 1995). From both the enumeration of these species in human faeces (Finegold et al. 1977, 1983; Moore and Moore 1995) and from an estimation of total BPB by most-probable number after cultivation of human faecal microflora on glucose-containing medium (Macfarlane and Gibson 1994), BPB in the large Correspondence to: Catherine Michel, UFDNH – INRA, Rue de la Ge ´raudie `re, BP 71627, F-44316 Nantes cedex 03, France (e-mail: michel@nantes.inra.fr). ª 2005 INRA Journal of Applied Microbiology 2005, 99, 201–212 doi:10.1111/j.1365-2672.2005.02605.x