Lecture 2: Lactic Acid Bacteria Flashcards
How is lactic acid produced in food fermentations?
from the breakdown of simple carbohydrates such as glucose, surcrose or galactose by LAB
T or F: LAB can grow in the mouth and the acid they produce are responsible for tooth decay and cavities
True
What are the two classes or lactic acid bacteria?
Homofermentative and heterofermentative
What is the difference between homofermentative and heterofermentative LAB
homofermentative bacteria that can produce two moles of lactate from one mole of glucose while heterofermentative can produce one mole of lactate from one mole of glucose, and produce carbon dioxide and acetic acid or ethanol as biproducts
What is homo-fermentative hetero-fermentative differential agar? Explain the purpose of this agar.
Fructose is the fermentable carbohydrate in the medium. bromo cresol green is the pH indicator.
heterofermentative lactic acid induce lesser acidification and thus vary in the colour formation by indicator in the medium
homofermentative bacteria cultivated on this medium form bluish green-colony on agar while heteto bacteria form white colonies on agar surface.
What is lactic acid bacteria?
- group of gram positive bacteria that produce lactic acid during the fermentation of carbohydrates.
- non sporeforming, rods&cocci
T or F: bifidobacterium is often considered LAB.
True even though it is phylogenetically unrelated and has a unique mode of sugar fermentation.
LAB are very adapted to environments that are rich in nutrients and energy sources but have limited biosynthetic capability. what is meant by low biosynthetic ability?
everyone wants to live where there is a lot of nutrient but in those environments there are a lot competition. What they do is they produce lactic acid, so they change environment by killing competitors.
Considered tetrad forming LAB
aerococcus, pediococcus and tetragenoccous
used in the food industry but is responsible for the greening of meat products
Aerococcus
spoilage organism in the beer industry responsible for the buttery taste in beer
P damnosus
used for starter culture in sausage making and silage inoculants
P acidilactici and P pentosaceus
extremely salt tolerant (>18% NaCl) and important in high salt containing foods like soysauce
Tertagenococcus
important in spontaneous vegetable fermentations like sauerkraut
Leuconostocs
associated with meat and proliferate at low temps
Weisella
The most acid tolerant of LAB and will therefore be the final successors of many lactic acid fermentations?
Lactobacillus
Summarize LAB metabolism
- characterized by efficient carbohydrate fermentation ending in lactic acid.
- LAB are able to change their metabolism to adapt to various conditions –> leading to production
- Lab obtain nutrients from their environment
- Lab phosphorylate all incoming sugars to avoid conc gradients.
T or F: Lab are able to synthesize many of their essential macromolecules and must obtain them from their environments
False, they are unable
What are the two primary hexose fermentation pathways used to classify LAB genera?
heterofermentative and homofermentative
What is homofermentative LAB metabolism?
glucose is converted into 2 lactate and 2 ATP using the glycolytic pathway.
what is heterofermentative LAB metabolism?
glucose is converted into 1 lactate and an ethanol and one co2 molecule and single ATP using the 6-PG/PK-pathway
what is required for sugar activation prior to glycolysis in LAB metabolism
a high energy phosphate bond (phosphorylation)
what is the main function of PTS
the main function of the sugar phosphenolpyruvate is to translocate sugar across the membrane with simultaneous phosphorylation
How do LAB ferment disaccharides?
- lactose can be cleaved by beta gal into glucose and galactose-6-phosphate and then enter one of several major metabolic pathways.
- maltose fermentation starts when maltose is cleaved into glucose and beta glucose-1-phosphate. the glucose is used in glycolysis and the beta glucose 1 phosphate is a precursor for cell wall synthesis.
- sucrose is cleaved into glucose and fructose which can then enter major pathways (glucose can be used in glycolysis, and fructose can be terminal electron acceptor)
what are the three categories of LAB metabolism?
- obligately homofermentative; meaning that sugars are only fermented by glycolysis
- obligately heterofermentative; meaning that only the 6-PG/PK pathway is available fr fermentation and the difference is that key enzymes of the glycolysis pathway are missing
- facultatively heterofermtentative; use glycolysis for hexose fermentation but some sugars induce a heterolactic fermentation rxn to take place
explain the debate between LAB being classified as aerotolerant or as facultative aerobes?
so LAB use oxygen as terminal electron acceptor but are not able to protect themselves against the toxic effects of oxygen the way genuine aerobic organisms would. LAB do however have a very active and dynamic relationship with oxygen. When grown anaerobically several organic compounds can be used by LAB as terminal electron acceptors.
why must LAB obtain performed amino acids from their environments
they have limited ability to synthesize amino acids from their environment
what can issues happen when a fermentation relies on active LAB that were added as a starter culture?
bacteriophage contamination which can result in a dead vat, inactivation of starter cultures.
This can also result in unacceptably low LAB and flavour compounds produced along with decrease proteolysis.
Due to economic losses dairy fermentation factory have phage control plans that include?
- characterization of phage population involved (genomic analysis), 2. analysis of natureal and intelligent bacterial systems of phage defense. identification of phage counter defense mechanisms.
what are the types of naturally occuring phage defense mechanisms?
- absorption inhibition
- blocking DNA
- restriction of enzymes/modification
- abortive infection mechanisms (bacterial cell traps the phages from emerging)
what are the artificial phage resistance mechanisms?
- antisense RNA strategies
- cloned ORI
- Clone is phage repressor
- phage triggered death
what are antisense RNA strategies?
clone in antisense RNA which binds to phage DNA and stops viral replication
what is the strategy in cloned ORI?
the ORI of the phage is cloned into the bacterial genome, it competes with the phage one and slows viral growth
what is clone in the phage repressor?
phages have a repressor for cell lysis. this can be constitutively expressed by bacteria, which traps the phage.
what is phage triggered death?
bacterial suicide genes are placed under the control of a phage inducible
have been shown to have a greatest ability to remove mycotoxins from food
Lactobacillus rhanosus
the starter cultures of LAB used in dairy products can be divided into what 2 categories?
thermophillic and mesophillic
define mesophillic starter cultures?
grow 10-48 deg C, with optimum 38C.
composed of acid forming lactococci are often flavor producers and are used in the production of many cheese varieties, fermented milk products and ripened cream butter.
define thermophillic starter cultures?
have optimum growth temp btwn 48-58 deg C. used for yogurt and for cheese varieties with high cooking temps
responsible for the bitter taste in cheese
hydrolysis of milk proteins by proteinases and peptidases of LAB
the flavor compounds produced by LAB in cheese can be divided into what 2 categories?
compounds produced by fermenting milk and compounds produced during cheese maturation
what other compounds or inhibitory substances can be produced by LAB;
hydrogen peroxide, diacettly and bacteriocin
what type of fermentation is used to produce vegetable products?
spontaneous
Benefits of vegetable fermentation with LAB include
High degree of hygienic safety from pathogenic bacteria
Products are still “clean label”
Interesting and appealing flavours
Less energy input than other methods of preservation
Explain the process of sauerkraut fermentation?
sugars in cabbage = 3-9%
cabbage is shredded and salt is mixed in to form immediate brine.
container is sealed and pressed with a weight to maintain anaerobic conditions.
The a
what LAB initiate sauerkraut fermentation? followed by? then?
L mesenteroides followed by Lb.brevi and s, P pentosaceus and finally Lb plantarum
what is the function of Ln mesenteroides?
produce lactic acid, acetic acid and xo2 which rapidly lower the pH and thus limit the activity of undesirable microorganisms and nzymes that might soften the shredded cabbage.
Why is the replacement of air by Co2 and anaerobic environment important in sauerkraut fermentation?
prevent oxidation of ascorbic acid and avoid darkening of natural color cut cabbage
