What induces respiration in lactic acid bacteria? Characterization of respiratory metabolism of Lactococcus lactis in bioreactors for production of starter cultures with improved acidification capacity

Commercial freeze-dried lactic acid bacteria starter cultures for cheese making are produced mainly via anaerobic batch fermentations. Recently, it has been shown that some Lactococcus lactis species are able to sustain respiration under aerobic conditions when hemin is added to the growth medium, since it completes the electron transport chain for respiration, which is otherwise defective. Respiration is energetically beneficial and under respiratory conditions, higher biomass yield is obtained together with a changed by-product pattern, compared to fermentation [1]. So far it has not been studied how the different culture conditions and thereby different metabolism affect the starter culture performance. In this project, we investigate respiratory culture conditions, and the effect on the milk acidification capacity of the culture. Since Lactococcus lactis is a fastidious microorganism, a rich chemically defined medium was developed to support the nutrient requirements, and was applied for bioreactor cultivations with quantitative approaches. The product profile and on-line gas analysis revealed that with hemin addition at the start of the process, cells switch to respiratory metabolism only in the second phase of growth, after an initial mixed-acid fermentative phase. To characterize the observed respiratory switch, a multivariate study was performed: a set of bioreactor batch experiments were carried out with different initial sugar concentrations under anaerobic, aerobic, and respiratory conditions. The results indicate that hemin addition together with some yet not defined threshold must be met in order to induce the respiratory metabolic state of the culture.

[1] Lechardeur D et al Curr Opin Biotechnol 2011,22(2):143‐149