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 . 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.
FAIRDOMHub ID: https://seek.sysmo-db.org/assays/107
Assay type: Experimental Assay
Technology type: Technology type
Created: 11th Jun 2010 at 11:07
Last updated: 11th May 2016 at 17:19