Late blowing is one of the most relevant fermentation defects observed in hard and semi-hard cheeses. Contamination of milk with Clostridium tyrobutyricum spores occurs during milking. C. tyrobutyricum then germinates and outgrows consuming lactate and producing butyrate, carbon dioxide and molecular hydrogen. Little is known about the genes responsible for its metabolism in cheese, in particular the enzymes enabling conversion of lactate into pyruvate. In the present study, a differential expression analysis was conducted on C. tyrobutyricum grown on glucose compared to L-lactate medium mimicking cheese composition. Our data show an overall large transcriptomic response, highlighted by 21% of the genes being differentially transcribed (p<0.01; log2 fold change > 2). Amongst the most affected genes we found an operon that comprises five coding sequences related to the conversion of L-lactate into pyruvate. Its expression level was more than 200-fold higher in L-lactate than in glucose medium. Furthermore, we observed that out of six lactate dehydrogenase coding genes one gene predicted to encode a FAD-dependent D-lactate dehydrogenase was significantly higher expressed in the L-lactate medium. This implies that formation of D-lactate should occur. In fact, we observed the racemization of L-lactate in L-lactate medium and in 3-months old cheese inoculated with C. tyrobutyricum. The transcriptomic study provides evidence that the conversion pathways of L-lactate to pyruvate and D-lactate are important for cell growth and energy formation of C. tyrobutyricum in the absence of glucose.
