Citrate metabolism in Enterococcus faecium reveals a high genetic diversity

QUINTANA, I; MARTINO, G; GALLINA, G; ESTEBAN, L; ESPARIZ, M; BLANCATO, VS; MAGNI, C

San Miguel de Tucuman

Simposio; IV Simposio Internacional de Bacterias Lácticas (SIBAL); 2013

Lactic Acid Bacteria (LAB) are recognized as safe microorganisms capable of improving dairy products qualities. When the starter microorganism Lactococcus lactis is employed for rapid acidification of the milk, increases the concentration of important aroma compounds such as diacetyl. Enterococcus faecium is one of the most common non starter LAB present in commercial and artisanal dairy products and contributes to the development of organoleptic features. The capacity to ferment citrate by E. faecium was previously studied but little is known about the genetic organization of the genes involved in this pathway. This metabolism depends on the presence of citrate transporters and the citrate lyase complex, which split citrate into oxaloacetate and acetate. The oxaloacetate is further metabolized to pyruvate and finally to diacetyl and acetoin. We have started our studies with a comparative genomic analysis which lead us to 239 strains of E. faecium previously sequenced. Three different genetic organization were identified. 76 % of the strains found belonged to cit cluster, in which the genes codifying the citrate lyase complex are absent. 19 % of these strains show a genetic Organization corresponding to the citI cluster. In this cluster there are two divergent operons; one which condifies for the transcriptional Activator CitI in one direction and the other operon with genes codifying the soluble oxaloacetate decarboxylase (citM), the citrate transporter (2HCT family) and the citrate lyase complex (citDEF) in the other direction. We also found 5 % of the strains with the citO cluster which consists as well of two divergent operons but in this case with the genes for the citrate transporter (CitHM family) and the transcriptional activator CitO in one direction and the citrate lyase complex (citDEF) associated to membrane oxaloacetate decarboxylase genes (oadHABD) in the other. Finally, the bioinformatic analysis on the genes involved in the C4 route was performed. The presence of a putative alsSD operon was identified encoding for the alpha acetolactate synthetase and alpha acetolactate decarboxylase gene in the 56% of the strain whereas acetoin dehydrogenase (butBA operon) was not identified.