Enhancing lactic acid bacteria resistance to acid to improve their functionality as novel catalysts for the acidic production of lactic acid.

Abstract

Biomanufacturing strategies exploit microbial metabolism to produce bio-products sustainably at an industrial scale. Lactic acid (LA), a valuable bio-chemical, has recently gained attention due to its important role in manufacturing renewable plastics such as polylactic acid (PLA). Currently, most LA is produced via biological approaches primarily using lactic acid bacteria (LAB) even though the process has some caveats. LAB are sensitive to the pH drop that occurs during a normal fermentation and therefore bases have to be added to the process to regulate the acidity. However, this contingency measure accounts for ≈ 50% of the total costs of LA production due to the complex downstream recovery of bases.

This study investigated a promising strategy to reduce the cost of base recovery in LA production by utilising experimentally evolved acid tolerant LAB strains capable of effectively producing LA in acidic conditions. Experiments showed that in liquid fermentations of 20 gL-1 of glucose, the experimentally evolved Weissella paramesenteroides was able to achieve LA yields within the maximum glucose to LA conversion rate (> 88%) at a pH of 3.8. These results illustrated the potential of experimental evolution to produce acid adapted W. paramesenteroides strains that could yield significantly more LA in an acidic pH than their unevolved parental strain. However, due to a limited acid tolerance, these promising results could not be replicated in higher initial glucose concentrations, (100 gL-1) or in solid-state fermentation (SSF) of sugar-rich plant biomass (≈ 100 gL-1 of fermentable sugars).

Further optimisations of LAB acid tolerance through experimental evolution could allow to convert higher concentrations of sugars (100 gL-1) to LA at a low pH. This technology could unlock the potential of base-free LA fermentations and effectively decrease the cost of LA (≤ 0.8 $kg-1), which is paramount for the commercialisation of sustainable LA derivatives.