3) Lactic Acid Bacteria Flashcards
what are 3 production methods of LAB?
- from break down of simple CHO (glucose, sucrose, galactose) by lactic acid bacteria (LAB)
- from growth in the mouth. The acid they produce causes tooth decay and cavities
- from muscles during exertion (usually L-lactate)
what are the acidic properties of LAB from?
due to H atoms separating from the rest of the molecule by ionization
what can be used for LAB fermentation?
almost any C5 or C6 sugar
what are the 2 classes of LAB? what do they each produce? what pathways do they use?
homofermentation: 1 glucose –> 2 lactate (glycolytic pathway)
heterofermentative: 1 glucose –> 1 lactate + 1 CO2 + 1 ethanol + 1 ATP (6-PGPK pathway)
what are general characteristics of LAB?
- Gram-positive bacteria
- produces lactic acid during the fermentation of CHOs
- non-spore forming
- can be both rods and cocci
- commonly found on healthy mucosal surfaces of humans and animals (mouth, intestine, vagina)
- very adapted to environments
rich in nutrients and energy sources with limited biosynthetic capability - has strategies to compete with other organisms (especially by producing acid and being acid tolerant)
what strategies do LAB have to compete with other organisms?
producing acid
being acid tolerant
is Bifidobacterum a LAB?
what is it often used for?
it appears to be a LAB, but is phylogenetically unrelated
it has a unique mode of sugar
added to yogurt
what are examples of the importance of LAB?
- involved in dairy, vegetable and cereal fermentations
- most popular and widely available human and animal probiotics
- used to decrease occurrence of disease in fish farms
- major part of the human microbiome (mouth, gut, vagina)
what are 4 most common strains of LAB?
streptococcus
tetrad-forming
coccoid
lactobacillus
what is the largest genera of LAB?
lactobacillus
what are characteristics of lactobacillus?
very heterogeneous; has species with a variety of phenotypic, biochemical and physiological properties
most acid tolerant of the LABs; thus will be the final successors of many lactic acid fermentations
associated with the oral cavity, GI tract, and vaginas of humans and animals
which is the most acid tolerant LAB? what is the result of this?
lactobacillus
will be the final successors of many LAB fermentations
what are 3 groups of lactobacillus? describe each.
- Obligately homofermentative: sugars are only fermented by glycolysis (group 1 lactobacili and other species from other genera)
- Facultatively heterofermentative: uses glycolysis for hexose fermentation, but some sugars have a heterolactic fermentation rxn
- Obligately heterofermentative: only the 6-PG/PK pathway is available. Key enzymes of glycolysis are missing
what is PTS? describe it.
phosphotransferase system
- function: translocate sugar across membrane with simultaneous phosphorylation (by phosphoenolyruvate - PEP as the phosphoryl door)
describe how LAB ferments lactose
- lactose is converted to glucose and galactose-6-phosphate by beta-galactosidase
- products enter another pathway
describe how LAB ferments maltose
- maltose is cleaved into glucose and B-glucose-1-phosphate
- glucose is used in glycolysis
- B-glucose-1-phosphate is a precursor to cell wall synthesis
describe how LAB ferments sucrose
- sucrose is cleaved into glucose and fructose
2. enters major pathways (glucose is used in glycolysis; fructose is terminal electron accepter)
why is it debated if LAB are aerotolerant anaerobes or facultative aerobes
LAB can use O2 as a terminal electron accept, but can’t protect themselves against the toxic effects of O2
describe the relatioship of LABs with O2
some substrates can only be used by LAB if O2 is available
when grown anaerobically, what other compounds can be used by LAB as terminal electron acceptors?
Citrate: can be cleaved into acetate and oxaloacetate
Glycerol
Fructose
why do LAB have limited biosynthetic capability?
b/c they are very adapted to environments rich in nutrients and energy
thus, they don’t need to synthesize AA from N sources. They obtain AA from environment
how does a phage infection affect LAB fermentations?
what industry is this especially bad for?
- permanent threat
- especially problematic in dairy industries
- complete failure of the starter culture can occur “dead-vat”
describe the result of phage attacks on LAB
very low LAB + flavour compound production + decreased proteolysis
to prevent economic losses due to phage infection, what do phage control plans involve?
- characterization of phage population involved (including genomic analysis)
- analysis of natural and intelligent bacterial systems of phage defense
- identification of phage counter-defense mechanisms
different phages infect ____ viruses
different
hence, if the processing plant is susceptible for phage infection to one bacteria, you can use different bacteria
what are 4 naturally occur phage defense mechanisms?
- absorption inhibition
- blocking DNA penetration
- restriction enzymes/modification systems
- abortive infection mechanisms (bacterial cell traps the phages from emerging)
what are 4 artificial phage resistance mechanisms?
- Antisense RNA strategies: clone in antisense RNA which binds to phage DNA and stops viral replication
- Cloned ORI: clone the ORI for the phage into the bacterial genome. Competes with the phage one and slows viral growth
- Clone in a phage repressor: phage have a repressor for cell lysis, which can be expressed by the bacteria, which traps the phage
- Phase Triggered Death: bacterial suicide genes placed under control of a phage inducible promoter
what are mycotoxins?
what is this an issue for?
- wide spread contaminants of food and feed
- contains many potent carcinogens
- not an issue in most fresh foods, but can be if questionable grains or nuts are eaten by farm animals, resulting in contamination of animal products
what are two groups of dairy product starters?
mesophilic cultures
thermophilic cultures
describe mesophilic and thermophilic starter cultures
- mesophilic
- grows in temp of 10-48C; optimum is at 38C
- composed of acid-forming lactococci (flavour producers) which are used in production of many cheeses, fermented milk products and ripened cream butter - Thermophilic cultures:
- optimum growth at 48-58C
- used for yogurt and cheeses with high cooking temp (eg. Emmental or Gruyere)
what are ways that LAB fermentation influences the final organoleptic qualities of cheese?
- fermenting sugars –> pH decrease –> clotting
- reduction of growth of adventitious microflora
- protein hydrolysis –> affects texture and taste of cheese
- synthesis of flavour compounds
- synthesis of texturizing agents
- production of inhibitory components
describe the proteolytic activity of LAB in dairy
- since LAB gets AAs from surroundings, they have proteinases and peptidases
- all milk proteins (including whey proteins) can be hydrolyzed at start of the fermentation rxn
- hydrolysis of milk proteins, which causes bitter tastes in cheese
what are 2 categories of flavour compounds produced by LAB in dairy production?
- produced by fermenting milk: lactic acid, acetic acid, acetaldehyde, diacetyl and acetoin
- produced during cheese maturation: LAB have less of a role here
what inhibitory substances does LAB grow?
hydrogen peroxide
diacetyl
bacteriocins
how is the growth of non-LAB inhibited by the presence of LAB in dairy fermentations?
due to inhibitory components of LAB
- very few bacteria can grow at the pH that LAB produce
- LAB also produces inhibitory substances (eg. hydrogen peroxide, diacetyl, bacteriocins)
what is the function of vegetable fermentations?
to preserve plant material
what are benefits of veg fermentation with LAB?
- high degree of hygenic safety from pathogenic bacteria
- products are “clean label”
- interesting flavours
- less energy input that other methods of preservation
- storage without refrigeration
how are most fermented veg products produced?
by spontaneous fermentation, without starter cultures; typically involves succession of microbial populations
describe the lactic acid fermentation occurring in sauerkraut
what is the main MO involved. What is the mechanism?
Ln. mesenteroides:
- produces lactic acid + acetic acids + CO2.
- Causes pH to decrease which limits activity of undesirable MO and enzymes that might soften the shredded cabbage
- CO2 produced replaces air which creates an anerobic atmosphere. This is imp to prevent oxidation of ascorbic acid and avoid darkening color of the cabbage
