Lactic Acid Bacteria

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Lactic Acid Bacteria

• Metabolism

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Common Energy Metabolism in LAB

• Glycolysis (EmbdenMeyerhof pathway) --homolactic
  fermentation
• The 6-phosphogluconate/phosphoketolase
  pathway--heterolactic fermentation
• Significance fermentation end products relevant
  to industrial applications

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Milk

• Lactose
• major fermentable sugar, 4050 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

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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

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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

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Sugar Transport by LAB

• Symport or ATP-dependent systems
• Other LAB
• Precursor-product exchange

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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)

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Transport hydrolysis systems

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

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LACTOSE PEP-PTS SYSTEM
membrane
Medium
E-I
PEP
P-HPr
out
in
HPr
P-EI
pyruvate
LACTOSE
P-EIII-lac
EII-Lac
E-III-lac
Lactose-P
P-beta-Galactosidase
Galactose-6P
Glucose
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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
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Primary and secondary transport systems

• Lactose translocated unmodified
• Disaccharide hydrolysed by betagalactosidase
  (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)

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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

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lactose
galactose
lactose
galactose
Bata-Gal
S. thermophilus Lb. bulgaricus Lb.
acidophilus Lb. lactis- dont have the ability
to ferment galactose
glucose
glycolysis
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LACTOSE
Beta-Gal
LACTOSE
Gal
Glu
Gal-1-P
Glu-6-P
Glu-1-P
Glycolysis
Lb. helveticus
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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)

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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

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Precursor-product Exchange Systems

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

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Proton pump

• Acid tolerant
• Inside pH 5.3
• Outside pH4.2

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Summary

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

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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

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

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