QUANTIFICATION OF CITRATE LYASE BY ENZYME-LINKED-IMMUNOSORBENT-ASSAY FOR DETERMINING THE POPULATION OF LACTOCOCCUS-LACTIS SUBSP LACTIS BIOVAR DIACETYLACTIS IN PURE AND MIXED CULTURES

Citrate lyase production by Lactococcus lactis subsp. lactis biovar di acetylactis DRC2 was quantified by an enzyme-linked immunosorbent assa y (ELISA). The citrate lyase reached a concentration equivalent to 41 +/- 4 mu g/ml purified citrate lyase in pure culture. When the strain DRC2, grown in mixed culture with L. lactis subsp. cremoris AM2, repre sented around 70% (DC culture) or 30% (CD culture) of the total initia l population, the level of citrate lyase decreased to 21 +/- 7 mu g/ml and 4.5 +/- 1.5 mu g/ml respectively. The maximum bacterial concentra tion of strain DRC2 in pure culture reached 2.6 x 10(9) cfu/ml and dec reased to 1.5 (+/- 0.2) x 10(9) cfu/ml and 0.5 (+/- 0.3) x 10(9) cfu/m l in DC and CD mixed cultures respectively. In mixed cultures, the pro portion of the strain DRC2 was 8.5 +/- 5.0% lower at the end of the fe rmentation than immediately after inoculation, thus showing that this strain was clearly inhibited. However, the maximum rate of citrate con sumption was the same during pure DRC2 culture and CD mixed culture (2 .5 +/- 0.3 mmol/h) and slightly higher in DC culture (3.07 mmol/h). Th e maximum rate of acidification was 0.37 +/- 0.04 pH unit/h regardless of the culture. A good correlation was obtained between the populatio n of the strain DRC2 and the citrate lyase concentration determined by ELISA but no relationship was found between citrate consumption and c itrate lyase synthesis. Therefore an ELISA test of this kind can be us ed to monitor the growth of L. lactis subsp. lactis biovar diacetylact is in mixed cultures.