Identification of a major facilitator superfamily protein that is beneficial to L-lactic acid production by Bacillus coagulans at low pH

Background Product inhibition is one of the major problems in lactic acid (LA) fermentation.

Our previous study revealed that Bacillus coagulans 2–6 was an efficient producer of high-optical-purity L-LA.

Its mutant strain B. coagulans Na-2 has better resistance to sodium lactate stress but the resistance mechanism has not been understood.

Results In this study, the whole-genome sequencing of B. coagulans Na-2 was performed and one mutant gene mfs coding for the major facilitator superfamily (MFS) protein was revealed by comparative genome analysis.

Ten mutation sites were identified between the wild (MFS-2-6) and mutant (MFS-Na-2) proteins, among which T127A and N154T were predicted locating in the center of the transmembrane transport channel.

The MFS-2-6 and MFS-Na-2 were expressed separately in a genetically operable strain, B. coagulans DSM1, using the genes’ native promoter.

The expression of the two MFS proteins had no effect and a negative effect on L-LA production when the pH was controlled at 6.0 and 7.0 by sodium hydroxide, respectively.

However, 4.2 and 4.6-fold of L-LA concentrations were obtained at pH 5.0 by the strains expressing MFS-2-6 and MFS-Na-2 than that by the control strain, respectively.

The intracellular pH values of the strains expressing MFS-2-6 and MFS-Na-2 were approximately 0.69 and 0.45 higher than that of the control strain during pH-controlled fermentation at 5.0.

Results suggest that the expression of MFS-2-6 and MFS-Na-2 were both conducive to L-LA production at low pH, while the better performance of the latter was probably due to the more appropriate intracellular pH during the whole fermentation process.

Conclusions The MFS protein identified here can improve the ability of B. coagulans to resist acidic environments and produce more L-LA at low pH.

The MFS protein has an application potential in environment-friendly L-LA production.