Lecture 2: Lactic Acid Bacteria

Question 1

How is lactic acid produced in food fermentations?

Answer 1

from the breakdown of simple carbohydrates such as glucose, surcrose or galactose by LAB

Question 2

T or F: LAB can grow in the mouth and the acid they produce are responsible for tooth decay and cavities

Answer 2

True

Question 3

What are the two classes or lactic acid bacteria?

Answer 3

Homofermentative and heterofermentative

Question 4

What is the difference between homofermentative and heterofermentative LAB

Answer 4

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

Question 5

What is homo-fermentative hetero-fermentative differential agar? Explain the purpose of this agar.

Answer 5

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.

Question 6

What is lactic acid bacteria?

Answer 6

• group of gram positive bacteria that produce lactic acid during the fermentation of carbohydrates.
• non sporeforming, rods&cocci

Question 7

T or F: bifidobacterium is often considered LAB.

Answer 7

True even though it is phylogenetically unrelated and has a unique mode of sugar fermentation.

Question 8

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?

Answer 8

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.

Question 9

Considered tetrad forming LAB

Answer 9

aerococcus, pediococcus and tetragenoccous

Question 10

used in the food industry but is responsible for the greening of meat products

Answer 10

Aerococcus

Question 11

spoilage organism in the beer industry responsible for the buttery taste in beer

Answer 11

P damnosus

Question 12

used for starter culture in sausage making and silage inoculants

Answer 12

P acidilactici and P pentosaceus

Question 13

extremely salt tolerant (>18% NaCl) and important in high salt containing foods like soysauce

Answer 13

Tertagenococcus

Question 14

important in spontaneous vegetable fermentations like sauerkraut

Answer 14

Leuconostocs

Question 15

associated with meat and proliferate at low temps

Answer 15

Weisella

Question 16

The most acid tolerant of LAB and will therefore be the final successors of many lactic acid fermentations?

Answer 16

Lactobacillus

Question 17

Summarize LAB metabolism

Answer 17

1. characterized by efficient carbohydrate fermentation ending in lactic acid.
2. LAB are able to change their metabolism to adapt to various conditions –> leading to production
3. Lab obtain nutrients from their environment
4. Lab phosphorylate all incoming sugars to avoid conc gradients.

Question 18

T or F: Lab are able to synthesize many of their essential macromolecules and must obtain them from their environments

Answer 18

False, they are unable

Question 19

What are the two primary hexose fermentation pathways used to classify LAB genera?

Answer 19

heterofermentative and homofermentative

Question 20

What is homofermentative LAB metabolism?

Answer 20

glucose is converted into 2 lactate and 2 ATP using the glycolytic pathway.

Question 21

what is heterofermentative LAB metabolism?

Answer 21

glucose is converted into 1 lactate and an ethanol and one co2 molecule and single ATP using the 6-PG/PK-pathway

Question 22

what is required for sugar activation prior to glycolysis in LAB metabolism

Answer 22

a high energy phosphate bond (phosphorylation)

Question 23

what is the main function of PTS

Answer 23

the main function of the sugar phosphenolpyruvate is to translocate sugar across the membrane with simultaneous phosphorylation

Question 24

How do LAB ferment disaccharides?

Answer 24

1. lactose can be cleaved by beta gal into glucose and galactose-6-phosphate and then enter one of several major metabolic pathways.
2. 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.
3. 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)

Question 25

what are the three categories of LAB metabolism?

Answer 25

1. obligately homofermentative; meaning that sugars are only fermented by glycolysis
2. 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
3. facultatively heterofermtentative; use glycolysis for hexose fermentation but some sugars induce a heterolactic fermentation rxn to take place

Question 26

explain the debate between LAB being classified as aerotolerant or as facultative aerobes?

Answer 26

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.

Question 27

why must LAB obtain performed amino acids from their environments

Answer 27

they have limited ability to synthesize amino acids from their environment

Question 28

what can issues happen when a fermentation relies on active LAB that were added as a starter culture?

Answer 28

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.

Question 29

Due to economic losses dairy fermentation factory have phage control plans that include?

Answer 29

1. characterization of phage population involved (genomic analysis), 2. analysis of natureal and intelligent bacterial systems of phage defense. identification of phage counter defense mechanisms.

Question 30

what are the types of naturally occuring phage defense mechanisms?

Answer 30

1. absorption inhibition
2. blocking DNA
3. restriction of enzymes/modification
4. abortive infection mechanisms (bacterial cell traps the phages from emerging)

Question 31

what are the artificial phage resistance mechanisms?

Answer 31

1. antisense RNA strategies
2. cloned ORI
3. Clone is phage repressor
4. phage triggered death

Question 32

what are antisense RNA strategies?

Answer 32

clone in antisense RNA which binds to phage DNA and stops viral replication

Question 33

what is the strategy in cloned ORI?

Answer 33

the ORI of the phage is cloned into the bacterial genome, it competes with the phage one and slows viral growth

Question 34

what is clone in the phage repressor?

Answer 34

phages have a repressor for cell lysis. this can be constitutively expressed by bacteria, which traps the phage.

Question 35

what is phage triggered death?

Answer 35

bacterial suicide genes are placed under the control of a phage inducible

Question 36

have been shown to have a greatest ability to remove mycotoxins from food

Answer 36

Lactobacillus rhanosus

Question 37

the starter cultures of LAB used in dairy products can be divided into what 2 categories?

Answer 37

thermophillic and mesophillic

Question 38

define mesophillic starter cultures?

Answer 38

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.

Question 39

define thermophillic starter cultures?

Answer 39

have optimum growth temp btwn 48-58 deg C. used for yogurt and for cheese varieties with high cooking temps

Question 40

responsible for the bitter taste in cheese

Answer 40

hydrolysis of milk proteins by proteinases and peptidases of LAB

Question 41

the flavor compounds produced by LAB in cheese can be divided into what 2 categories?

Answer 41

compounds produced by fermenting milk and compounds produced during cheese maturation

Question 42

what other compounds or inhibitory substances can be produced by LAB;

Answer 42

hydrogen peroxide, diacettly and bacteriocin

Question 43

what type of fermentation is used to produce vegetable products?

Answer 43

spontaneous

Question 44

Benefits of vegetable fermentation with LAB include

Answer 44

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

Question 45

Explain the process of sauerkraut fermentation?

Answer 45

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

Question 46

what LAB initiate sauerkraut fermentation? followed by? then?

Answer 46

L mesenteroides followed by Lb.brevi and s, P pentosaceus and finally Lb plantarum

Question 47

what is the function of Ln mesenteroides?

Answer 47

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.

Question 48

Why is the replacement of air by Co2 and anaerobic environment important in sauerkraut fermentation?

Answer 48

prevent oxidation of ascorbic acid and avoid darkening of natural color cut cabbage