Maximal specific growth rates of the three lactic acid bacteria and their ldh deletions

L. lactis, E. faecalis and S. pyogenes are referred to as LAB because of the fact that in the presence of glucose lactate is produced as the main fermentation product . This metabolic pathway is relatively inefficient, since only 2 ATP are generated from one glucose molecule. All three LAB possess the genetic make up for mixed acid fermentation, a more effective way of fermentation generating 3 ATP per molecule of glucose. All three genomes reveal (at least) two genes encoding a lactate dehydrogenase (LDH), both one L-lactate dehydrogenase (L-LDH) and one D-lactate dehydrogenase (D-LDH). L . lactis also possesses a third lactate dehydrogenase indicated as LDHX. In both L . lactis and E. faecalis it was shown that the L-LDH is responsible for over 95% of total lactate synthesis [6, 8]. In all three LAB’s the ldh gene (encoding the L-LDH) was removed and studied with respect to its physiological parameters [6, 8]. The resulting ldh deletion strains were analyzed in a batch growth setup in chemically defined medium supplied with glucose (CDM-LAB) or rich THY medium at pH 6.5 and pH 7.5. All three the ldh deletion strains did not show a significant difference with respect to growth rate as compared to the wild-type in CDM-LAB, except for wild-type E. faecalis grown at pH 6.5 and for wild-type S. pyogenes at pH 7.5 compared to their ldh deletion mutants. In THY medium all three wild-type LAB at both pH’s showed higher growth rates than their isogenic ldh deletions, although not to the extent observed previously for L. lactis in MRS medium [8]. No pH dependence of the maximal growth rate was observed for wild-type L. lactis and E. faecalis. S. pyogenes however showed a lower specific growth rate at pH 7.5 in rich medium. All ldh deletions grew 10-20% slower than the wild-type strains, except for S. pyogenes at pH 7.5 where deletion of ldh did not result in a significant decrease in growth. In late stationary phase all three ldh deletion strains grew to a higher optical density and activity of LDH remained below 10% of total fermentation activity for all the three ldh deletion strains. To verify whether a similar μmax also signified that in a mixed culture deletion of ldh does not represent an evolutionary disadvantage, the S. pyogenes M49 wild-type and its ldh deletion were co-cultivated anaerobically in THY medium (pH 7.5). A 52%/48% distribution of wt and mutant bacteria was shown after 18 h of co-cultivation of both strains in THY medium and subsequent plating of serial dilutions on THY agar plates with and without spectinomycin. This indicates that the lack of an L-LDH represented no significant disadvantage to the organism in the tested conditions.