LAB Flashcards
Lactic acid is also known as:
milk acid
2-hydroxypropanoic acid
What organism makes lactic acid, and what substrate is used?
lactic acid bacteria
breaks down SIMPLE CARBS: glucose, sucrose, galactose
Where can lactic acid be found in the human body? (2)
produced by LAB in mouth (causes cavities)
L-lactate produced by muscles during exertion
True/false: lactic acid is the most common organic acid produced in fermentations
true
What sugars can be used for lactic acid fermentations?
almost any C5 or C6 sugar (sucrose, glucose, raw sugar, etc)
The 2 classes of LAB:
- homofermentative
2. heterofermentative
What is the main difference between the 2 classes of LAB?
homofermentative makes 2 MOLES of lactic acid per mole glucose
heterofermentative makes 1 MOLE lactic acid + CO2 + byproduct (ethanol, acetic acid, etc)
Describe the cell morphology and characteristic of LAB:
GRAM POSTIVE
Rods or cocci
non-spore forming
ferment simple sugars into lactic acid
name 5 common LAB types:
Enterococcus Leuconostoc Lactobacillus Lactococcus Streptococcus
Why are LAB so useful in the food industry? (3)
fermentation process
prevent pathogen/spoilage by outcompeting other microbes (and acidity)
Use as probiotic
True/False: LAB are very good at surviving in low nutrient environments.
FALSE: adapted to nutrient/sugar rich environments; limited biosynthetic activity (cannot make many compounds)
True/False: LAB are acid tolerant
True
What is the streptococcus species used in food, and for what purpose?
S. thermophilus
used in yogurt; or in co-culture for cheese
Is enterococcus useful for food production?
No; not commonly used, and some species PATHOGENIC
what species of lactococcus are used by the food industry?
L. lactis
used in dairy technology
What are the tetrad forming species of LAB? What is the significance of each in food products?
Aerococcus (greening/off color of meat): BAD
Pediococcus: (beer spoilage): BAD; (silage or sausage inoculant): GOOD
Tetragenococcus: (used in soy sauce): GOOD
What is a particular feature of Tetragenococcus?
very salt tolerant (up to 18%)
What are the coccoid LAB? What are their associated uses?
Leuconostoc: spontaneous veg. fermentations
Oenococcus
Weisella: used in silage; found in meat
What is the largest genus in LAB? What is their defining feature?
lactobacillus
most acid tolerant (final survivors of fermentation)
What are some LAB species that thrive in many habitats?
brevis, casei, plantarum
What are the 3 groups of lactobacillus?
Group 1: obligately homofermentative
Group 2: facultative heterofermentative
Group 3: obligately heterofermentative
What groups of Lactobacillus will produce CO2?
Groups 2 and 3
True/False: a ferment done with Group 1 lactobacillus will become acidic at a faster rate than using Group 3
True
What is the effect of changing the growing conditions for Lactobacillus?
different chemical environment (O2, diff. substrate) -> change metabolism to produce different products -> affect flavor and other qualities
Why is a sugar/nutrient rich environment necessary to support LAB?
they cannot synthesize their own macromolecules; must obtain from environment
What mechanism allows for efficient uptake of sugar in LAB?
Incoming sugars are PHOSPHORYLATED using 1 ATP
prevents concentration gradient so cell can keep uptaking sugars
the 2 primary hexose fermentation pathways in LAB:
heterofermentative
homofermentative
Describe the homofermentative metabolic process:
GLYCOLYTIC PATHWAY (only) glucose -> 2 lactate, 2 ATP
Describe the heterofermentative metabolic process:
6-PG/PK PATHWAY
glucose -> 1 lactate, 1 ATP, CO2, ethanol
Do all sugars undergo glycolysis in LAB?
NO; some converted to acetic acids, organic acids, amino acids, etc
What step is required prior to glycolysis in metabolism? What other function does this have?
forming high energy phosphate bond (phosphorylate)
prevents concentration gradient
What is a metabolic system used to phosphorylate sugars?
PTS: phosphotranferase system: translocates across membrane, adds P
P donor is phosphoenolpyruvate
true/false: LAB cannot ferment disaccarides
False
How are lactose, sucrose, and maltose metabolized by LAB?
broken down by B-galactosidase -> monosaccarides
glucose enter pathway
other component used in cell wall synthesis, as electron acceptor, or secreted
True/False: maltose can be cleaved into 2 glucoses, both of which can be metabolized for energy
False; produces 1 glucose; other half is B-glucose-1-P
can use in cell wall synth
Why can’t heterofermentative LAB use glycolysis?
key enzymes missing
How might using a pentose affect the metabolism of some LAB?
cause facultative heterofermentative LAB to do heterolactic ferment
Most genera of LAB are ____ (metabolism type)
heterofermentative
What evidence argues that LAB are aerotolerant, and not facultative aerobes?
does not possess protective mechanisms against oxidation from O2 like most aerobic organisms
What is the difference between an aerotolerant anaerobe vs a facultative aerobe?
aerotolerant: can withstand presence of air, does not utilize it or benefit from it
facultative aerobe: will use air if it is present (grows better)
How can LAB use O2, and when are aerobic conditions necessary?
terminal electron acceptor
required for certain substrates; Lb. brevis can only ferment glucose aerobically
what is an effect of oxygen exposure on a LAB fermentation?
oxidize lactic acid -> acetic acid and CO2
What compounds act as terminal electron acceptors in LAB if oxygen is not present?
This is especially common in:
citrate (cleave to oxaloacetate)
glycerol
fructose
heterofermentative (produce a lot of these byproducts)
How does the composition of cabbage make it very suitable for LAB fermentation?
high in sucrose, glucose, fructose
use glucose as energy, fructose as TEA
How do LAB obtain their amino acids?
proteolytic enzymes - break down proteins in nutrient rich environment (ie: milk)
What nutrients do LAB require?
vitamins
nucleotides or precursors
sugars
amino acids
What is the main concern in commercial LAB ferments? What occurs?
Phage infection kills LAB (usually target specific taxa) -> decimates population -> complete failure of ferment (dead vat)
What do control plans of phage infection involve? (3)
characterize phage population
analyze natural bacteria defenses
identify phage counter mechanisms
What can be done if phage infection occurs?
Ferment is discarded
Can attempt to decontaminate; usually not feasible
usually switch to new strain that is resistant
What are some natural phage resistance mechanisms? (4)
- absorption inhibition
- block DNA penetration
- restriction enzymes/modification systems
- abortive infection mechanisms (prevent phages from emerging from cell)
What are some artificial phage resistance mechanisms? (4)
- Antisense RNA: clone in/add to phage DNA -> stop replication
- Cloned ORI: (origin of replication) into bacteria: compete with phage ORI -> slower phage rep
- Clone in phage repressor: stop cell from lysing (trap phage)
- Phage triggered death: put suicide genes next to phage induced promoter: phage enter -> cell dies
How could LAB be used to rectify spoiled grain?
if spoilage is mycotoxin (highly carcinogenic), could use L. rhamnoses to remove
In the dairy industry, how are LAB classified?
according to best growing temperature:
MESOPHILIC (10-48)
THERMOPHILIC (48-58)
A fermentation process using lactococci to make cheese is best at what temperature?
10-48 (best at 38C)
mesophilic
What activities occur during cheese production? (6)
sugars decrease pH decrease -> clotting, prevent other microbes protein hydrolysis -> texture/flavor flavor compound synthesis texture agent synthesis produce inhibitory components
what is responsible for the bitter taste in cheese?
proteolysis
what are the 2 categories of flavor compounds in cheese? Which involve LAB?
- produced during milk ferment (INVOLVES LAB)
2. produced during ripening
What flavor compounds do LAB contribute to cheese? (5)
lactic acid, acetic acid, acetaldehyde, diacetyl, acetoin
What are some inhibitory compounds LAB produce?
acid (low pH)
H2O2
diacetyl
bacteriocins
What is the importance of adding salt to cabbage for LAB fermentation?
breaks down cabbage to release sugars to feed LAB
What is the initial LAB present in sauerkraut, and what important role does it play?
Ln. mesenteroides: produce lactic and acetic acid, CO2 -> rapidly lower pH (prevent microbial growth and oxidation)
