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Lactic Acid Bacteria

Lactic Acid Bacteria. Metabolism. Common Energy Metabolism in LAB. Glycolysis (Embden—Meyerhof pathway) --homolactic fermentation The 6-phosphogluconate/phosphoketolase pathway--heterolactic fermentation Significance: fermentation end products relevant to industrial applications. Milk.

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Lactic Acid Bacteria

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  1. Lactic Acid Bacteria Metabolism

  2. Common Energy Metabolism in LAB • Glycolysis (Embden—Meyerhof pathway) --homolactic fermentation • The 6-phosphogluconate/phosphoketolase pathway--heterolactic fermentation • Significance: fermentation end products relevant to industrial applications

  3. Milk • Lactose • major fermentable sugar, 40–50 g/l • The glucose moiety of lactose is used faster than galactose moiety by lactococci • Proteins • Fat • At the end of the growth phase, less than 0.5% of the lactose is used by lactococci • The fermentation product of the lactococci is L(+)-lactic acid

  4. Lactose utilization in LAB • Transport of lactose into cell • Hydrolysis of lactose • Metabolism of the monosaccharides • Efflux of lactic acid and protons from the cell • Unstable

  5. Sugar Transport by LAB • Several different systems are used by LAB to transport carbohydrates • Depend on species and specific sugar • Phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS) • By most mesophilic, homofermentative LAB • Such as lactococci and pediococci for lactose and glucose transport

  6. Sugar Transport by LAB • Symport or ATP-dependent systems • Other LAB • Precursor-product exchange

  7. Transport hydrolysis systems • The PEP-PTS system • Lactose phosphorylated during transport • Multicomponent group translocation system • Two cytoplasmic proteins: Enz I and HPr • Two lactose-specific components: the membrane-located LacE and the soluble phosphocarrier LacF (or Enz IIlac and Enz IIIlac)

  8. Transport hydrolysis systems • Lactose 6-phosphate hydrolyzed by phospho-beta-galactosidase • Exclusively found in G+ • Staphylococcus aureus, L. lactis, Lb. casei, pediococcus spp.

  9. LACTOSE PEP-PTS SYSTEM membrane Medium E-I PEP P-HPr out in pyruvate P-EI HPr LACTOSE P-EIII-lac EII-Lac E-III-lac Lactose-P P-beta-Galactosidase Galactose-6P Glucose

  10. Pathways for Galactose and Lactose Catabolism Galactose Lactose Galactose PEP-PTS Permease PEP-PTS Lactose-P Galactose Galactose-6P P-beta-Gal Gal-1P Glucose Tagatose-6P Glu-1P Glucose-6P Glyceraldehyde-3P +DHAP Tagatose 1,6-diP Glycolysis

  11. Lactose translocated unmodified Disaccharide hydrolysed by beta–galactosidase (lacz) Primary-involve a sugar transport ATPase Agrobacterium radiobacter, Strep. mutans Secondary-couples with ions or other solutes L. lactis ATCC 7962 (proton), E.coli (LacY) Primary and secondary transport systems

  12. Secondary transport systems • Secondary-couples with ions or other solutes • L. lactis ATCC 7962 (proton-coupled), E.coli (LacY) • LacS in Strep. thermophilus • Proton symport or lactose-galactose antiporter

  13. lactose galactose lactose galactose Bata-Gal S. thermophilus Lb. bulgaricus Lb. acidophilus Lb. lactis- don’t have the ability to ferment galactose glucose glycolysis

  14. LACTOSE Beta-Gal LACTOSE Gal Glu Gal-1-P Glu-6-P Glu-1-P Glycolysis Lb. helveticus

  15. Symport and ABC Transport Systems in LAB • Driven by ion gradients, and ATP-binding cassette (ABC) systems • Often a bacterium can use a PTS for one sugar and a symport or ABC system for another sugar • Consist a membrane permease with binding sites for both the substrate and a coupling ion, such as H+ or sodium ion (Fig. 2-15)

  16. Symport and ABC Transport Systems in LAB • Such as transport of lactose • Lb. brevis, Lb. delbrueskii, Lb. acidophilus • Galactose • L. lactis • Raffinose • P. pentosaceus • Melibiose • L. lactis • Xylose • Lb. pentosus

  17. Precursor-product Exchange Systems • Widely distributed in LAB • Used to transport fermentation substrates, amino acids, and organic acids • Eletroneutral or electrogenic (Fig. 2-4)

  18. Proton pump • Acid tolerant • Inside: pH ~5.3 • Outside: pH~4.2

  19. Summary • Glucose fermentation • Homo- heterolactic fermentation • Lactose utilization trait unstable • Strain dependent diversified pathways • Transport, hydrolysis • Select for proper starters for specific application

  20. Protein Metabolism • Cannot assimilate inorganic nitrogen • Rely on amino acids and small peptides • Limited in milk • Depend on proteolysis of casein • Essential for m/o growth • Contribute to flavor and texture development

  21. Cell Wall Cell Membrane Cytoplasm Milk Amino Acid Transport System Amino Acids Amino Acids Di- and Tri- Peptidases ? Di/Tri Peptide Transport System Di/Tri peptides Di/Tri Peptides ? SmallerOligo- peptides Peptidases ? Oligopeptide Transport System Oligopeptides Large Oligo- peptides Proteinase Casein

  22. The Proteolytic System • The cell envelop-associated serine proteianse • Peptide transport system • Oligopeptide transport system • Di-, tri-peptide transport system • Intracellular peptidases

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