Lactic Acid Bacteria Flashcards
what is lactic acid?
the resulting product from the breakdown of simple carbohydrates such as glucose, sucrose, or galactose by LAB
where is lactic acid produced?
in your muscles during exertion, and in mouths (cause of tooth decay and cavities)
what are the two classes of lactic acid fermentation?
- homofermentative
2. heterofermentative
what are the products of homofermentative fermentation?
two moles of lactate from one mole of glucose
what are the products of heterofermentative fermentation?
one mole of lactate from one mole of glucose, and carbon dioxide and acetic acid or ethanol as byproducts
What is in HHD agar? (homofermentative heterofermentative differential agar)
- casein hydrolysate
- papaic digest
- yeast extract
- fructose- fermentable carbohydrate in the medium
what is the pH indicator of HHD agar?
bromo cresol green
difference between homo-and hetero- fermentative agar LAB?
homofermentative LAB produce only lactic acid. heterofermentative LAB produce CO2, lactic acid, acetic acid, and ethanol and mannitol
difference between homo and heterofermentative colonies?
homo show green colonies from production of lactic acid, and heterofermentative show blue/white colonies. less induction of acidity
what is LAB?
LAB are a group of gram-positive bacteria that produce lactic acid during the fermentation of carbohydrates
where are LAB found?
mucosal surfaces of animals and humans (mouth, intestine, and vagina)
important LAB species
enterococcus, lactobacillus, lactococcus, leuconostac, and streptococcus
bifidobacterium?
LAB but phylogenetically unrelated and has a unique mode of sugar fermentation
what fermentations are involved in LAB?
dairy, vegetable, and cereal fermentations
what is the one streptococcus species associated with food?
S. thermophilus- in co-culture with lactobacillus delbruckii spp. and bulgaricus
what species of Lactococcus is most commonly involved in dairy products?
Lactococcus lactis
what species of LAB are a known food borne pathogen?
Enterococcus
what species of LAB are tetrad-forming LAB
- Aerococcus
- Pediococcus
- Tetragenococcus
what does P. damnosus result in?
spoilage, in the beer industry and causes a buttery taste in beer
what is a characteristic of Tetragenococcus?
they are extremely salt-tolerant and are important in high-salt containing food like soy-sauce
which species of LAB are coccoids?
- Leuconostoc
- Oenococcus
- Weisella
what are leuconostocs important for?
spontaneous vegetable fermentations
what food is Weissella species associated with?
meat
what is the largest genera (genus) of LAB?
Lactobacillus
why are lactobacillus considered the final successors in LAB fermentation?
they are the most acid-tolerant of LAB
where are lactobacillus found?
oral cavity, GI tract, and vaginas of humans and animals
what are the three groups of LAB taxonomy?
Group I: obligately homofermentative
Group II: Facultatively heterofermentative
Group III: obligately heterofermentative
what fermentation does LAB perform?
carbohydrate fermentation
what is the outcome of LAB fermentation?
lactic acid production
why is LAB’s metabolism unique?
they can change their metabolism depending on various conditions
T or F: LAB are unable to synthesize many of their essential macromolecules and must obtain them from their environment
True!
why do LAB phosphorylate all in-coming sugars?
to avoid concentration gradients
what are the two primary hexose fermentation pathways in LAB genera?
- Glycolytic pathway
2. Pentose Phosphoketolase (6-PG/PK) pathway
which pathway does homofermentative use?
glycolytic
which pathway does heterofermentative use?
pentose phosphoketolase
what are the hexoses that LAB ferment?
glucose, galactose, fructose, and mannose
what is the process of LAB metabolism once the sugar is translocated across the bacterial membrane?
Sugar is translocated across the bacteria’s membrane using the phosphotransferase system (PTS) which gives the sugar to phosphoenolpyruvate (PEP) as the phosphoryl donor. PEP then converts the sugar into pyruvate, which then produces lactate
what disaccharides can LAB ferment?
lactose, maltose, sucrose
what is maltose fermentation?
maltose cleaved into glucose and B-glucose-1-phosphate
how is glucose utilized LAB fermentation?
used in the glycolytic pathway
what is the function of B-glucose-1-phosphate?
acts as a pre-cursor to cell wall synthesis
how is the disaccharide sucrose utilized in LAB?
cleaved into glucose and fructose which can then enter the major pathways. glucose used in glycolysis, fructose can be the terminal electron acceptor
how is lactose utilized in LAB fermentation?
cleaved into B-galactose and galactose-6-phosphate which can then enter one of several major metabolic pathways
what are the three categories of LAB metabolism?
- obligately homofermentative
- obligately heterofermentative
- facultatively heterofermentative
what is LAB’s relationship with oxygen?
LAB can use oxygen as a terminal electron acceptor, but cannot withstand the toxic effects of oxygen due to being anaerobic organisms
what organic compounds can LAB use as terminal electron acceptors?
- citrate - can be cleaved into acetate and oxaloacetate (oxaloacetate can also be used as a TEA in several major pathways)
- Glycerol - TEA in anaerobic glucose fermentation
- Fructose - electron acceptor for heterofermentative LAB
whats the biggest problem in using active LAB added as a starter culture?
bacteriophage infection. Phage infection is a permanent threat in LAB fermentation commercial processes
what is a “dead-vat”
complete failure of the starter culture
what are the phage control plans implemented by dairy fermentation factories?
- characterization of the phage population involved (genomic analysis)
- analysis of “natural” and “intelligent” bacterial systems of phage defense
- identification of phage counter-defense mechanism
what are the four naturally occurring phage defense mechanisms?
- absorption inhibition
- blocking DNA penetration
- restriction enzymes/modification systems
- abortive infection mechanisms (bacterial cell traps the phages from emerging)
what are the four artificial phage-resistance mechanisms?
- antisense RNA strategies - clone in antisense RNA which binds to phage DNA and stops viral replication
- cloned ORI - slows viral growth
- clone in a phage repressor - expressing the phage’s repressor for cell lysis in the bacteria which traps the phage
- phage triggered death - bacterial suicide genes are placed under the control of a phage inducible promoter
mycotoxins
wide-spread contaminants of food and feed and contain some of the most potent carcinogens
which species of LAB are the best at removing mycotoxins from foods?
Lactobacillus rhamnosus
what are the two types of LAB starters/ two “philic” groups of LAB?
- mesophilic - cultures grow in temps of 10-48C with the optimum temperature being 38C
- thermophilic - cultures have their optimal growth temperature between 48C and 58C
what does LAB contribute to in cheese production
- proteolytic activity - obtaining amino acids from their surroundings
- aroma formation - produce flavor compounds by fermenting the milk and producing compounds during cheese fermentation
- inhibitory components - produce inhibitory substances such as hydrogen peroxide, diacetyl, and bacteriocins. LAB also increase pH so that very few bacteria can grow
what does LAB contribute to in vegetable fermentation?
- provides high hygienic safety from pathogenic bacteria
- interesting and appealing flavors?
- less energy input than other methods of preservation
- storage without refrigeration
what kind of fermentation are most fermented vegetable products produced by?
spontaneous fermentation, no starter cultures
what is the process of sauerkraut production?
cabbage is shredded and salt mixed in, forming a brine solution
what species of LAB initiates lactic acid fermentation?
Ln. mesenteroides, followed by Lb. brevis and finally Lb. plantarum
what species of LAB produces lactic acids and acetic acids?
Ln. mesenteroides, production of lactic acids and acetic acids along with CO2 rapidly lowers pH
how is the anaerobic environment est. in sauerkraut fermentation?
CO2 replaces air. also prevents the oxidation of ascorbic acid to avoid darkening the natural color of the cut cabbage
