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Food Microbiology > Lactic acid bacteria > Flashcards

Lactic acid bacteria Flashcards

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1
Q

What is the formula of lactic acid

What are the other names

A

CH3CH(OH)COOH

Hydroxypropanoic, milk acid

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2
Q

How lactic acid is produced in nature?

A
  • In food fermentations, lactic acid is produced from the break-down of simple carbohydrates such as glucose, sucrose, or galactose by lactic acid bacteria (LAB).
  • LAB also grow in your mouths and the acid they produce is responsible for tooth decay and cavities.
  • Lactic acid (actually L-lactate) also is produced in your muscles during exertion
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3
Q

Why lactic acid has acidic properties

A

The hydrogen atoms can separate from the rest of the molecule, by ionization, and this gives lactic acid its acidic properties

CH3CH(OH)COOH ⇌ CH3CH(OH)COO− + H+

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4
Q

What sugars can be used for lactic acid fermentation

A

•Almost any C5 or C6 sugar can be used for lactic acid fermentation (pure sucrose, glucose from starch, or raw sugar)

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5
Q

What classes of lactic acid bacteria are there and how they are different?

A

•Lactic acid bacteria can be divided into two classes:

–Homofermentative bacteria can produce two moles of lactate from one mole of glucose:

C6H12O6 → 2CH3COCO2− + 4H+ + H2O

–Heterofermentative species can produce one mole of lactate from one mole of glucose, and produce carbon dioxide and acetic acid or ethanol as biproducts:

C6H12O6 → CH3COCO2− + CO2 + CH3CO2H + 3H+

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6
Q

How does Homo-fermentative Hetero-fermentative Differential Agar works? What are the constituetns?

A
  • Medium constituents like casein enzymic hydrolysate, papaic digest of soybean meal and yeast extract supply all the necessary nutrients for the growth of lactic bacteria. Fructose is the fermentable carbohydrate in the medium. Bromo cresol green is the pH indicator. Heterofermentative lactic acid bacteria produce CO2, lactic acid, acetic acid, ethanol and mannitol. Homofermentative lactic acid bacteria produce only lactic acid.
  • Homofermentative lactic acid bacteria produce lactic acid from fructose and is indicated by yellow colour formation.
  • Heterofermentative lactic acid bacteria induce lesser acidification and thus vary in the colour formation by the indicator in the medium.
  • Homofermentative bacteria cultivated on this medium form bluish-green colony on agar while heterofermentative bacteria form white colonies on agar surface
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7
Q

Are LAB gram positive or negative?

A

positive

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8
Q

Do LAB form spores and what shape

A

•These bacteria are non-sporeforming and can be both rods and cocci

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9
Q

Where LAB is found in human?

A

•They are commonly found on (healthy) mucosal surfaces of both humans and animals (mouth, intestine, vagina)

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10
Q

What are 5 genera of LAB that we should know

A

Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Streptococcus

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11
Q

Are bifidobacterium related to LAB

A

•Bifidobacterium is often considered a LAB, however it is phylogenetically unrelated, and has a unique mode of sugar fermentation

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12
Q

How LAB survive in the environment? What is their advantage?

A
  • LAB are very adapted to environments that are rich in nutrients and energy sources (and have limited biosynthetic capability)
  • LAB have several strategies to compete with other organisms (an important one being acid production and acid tolerance)
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13
Q

How LAB are used? Are they widespread?

A
  • LAB are involved in dairy, vegetable, and cereal fermentations
  • LAB are the most popular and most widely available human and animal probiotics
  • They are used to decrease the occurrence of disease in fish farms
  • LAB are a major part of the human microbiome (mouth, gut, vagina)
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14
Q

•The genera Enterococcus, Lactococcus, Streptococcus, and Vagococcus were earlier included in the ____

A

Streptococcus genus

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15
Q

Only one Streptococcus species is associated with food__, with what organism they are working in pair?

A

• S. thermophilus, which is used to manufacture yogurt, usually in co-culture with Lactobacillus delbruckii subsp. bulgaricus, and certain cheeses.

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16
Q

What genus is associated with dairy products? and one species that is commonly used in dairy technology

A

•Lactococcus is intimately associated with dairy products, but only one species, Lactococcus lactis is commonly used in diary technology

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17
Q

How Vagococcus and Enterococcus are related to food?

A

•Vagococcus and Enterococcus are not commonly used in food, and some species of Enterococcus are known foodborne pathogens (E. faecalis)

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18
Q

What are tetrad-forming LAB

What are the species that do that?

Why this class is important?

A

The tetrad occurs in a subgroup of the cocci where the bacterium divides in two planes to form a square of four bacteria called a tetrad.

Aerococcus, Pediococcus, and Tetragenococcus

•Tetragenococcus are extremely salt tolerant (>18% NaCl) and are important in high-salt containing food like soy-sauce

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19
Q

Why aerococcus is popular

A

•Aerococcus is not used in the food industry but is responsible for greening of cooked meat products

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20
Q

Why P.damnosus is popular?

A

•P. damnosus is a spoilage organism in the beer industry and is responsible for the buttery taste in beer

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21
Q

Why P.acidilactici and P.pentosaceus are popular?

A

•P. acidilactici and P. pentosaceus are both used as starter for sausage making and silage inoculants

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22
Q

Leuconostoc, Oenococcus, and Weisella- how you can categorize them? Why they are important?

A
  • Leuconostoc, Oenococcus, and Weisella are each coccoid LAB
  • Leuconostocs are important for spontaneous vegetable fermentations like sauerkraut
  • Weissella species are associated with meat and can proliferate at low temperatures
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23
Q

What is the largest genera of LAB

A

Lactobacillus

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24
Q

What are three groups of Lactobacillus? what is the difference between them?

A
25
Q

Why Lactobacilli are “popular”

A
  • Lactobacilli are the most acid-tolerant of the LAB, and will therefore be the final successors of many lactic acid fermentations.
  • They are associated with the oral cavity, gastrointestinal tract, and vaginas of humans and animals.
26
Q

Name some species of lactobacillus and where they are found (5)

A

•Some Lb. brevis, Lb. casei, and Lb. plantarum can be found in many habitats while some are more specialized:

–Lb. sanfransiscensis mostly found in spontaneous sourdough bread reactions

–Lb. delbrueckii (previously Lb. bulgaricus) is mostly yogurt associated

27
Q

Make a summary of LAB metabolism

A

Summary:

  • LAB metabolism is characterized by efficient carbohydrate fermentation ending in lactic acid production
  • LAB are able to change their metabolism to adapt to various conditions (leading to the production of different end products)
  • LAB are unable to synthesize many of their essential macromolecules and must obtain them from their environments
  • LAB phosphorylate all in-coming sugars to avoid concentration gradients
  • There are two primary hexose fermentation pathways that are used to classify LAB genera:

–Heterofermentative

–Homofermentative

28
Q

What are the metabolic pathways used by LAB for both homo and hetero fermentative

A
29
Q

Can glycolysis lead to heterofermentation? Does the theory of hetero and homo fermentation work in practice?

A

An important exception is that glycolysis may lead to heterolactic fermentation (producing alternative end products) under certain conditions

Although the above values work out in theory they are seldom observed in practice since the sugars are also converted into acetic acids, organic acids, and amino acids

30
Q

How LAB keep getting the sugar from outside without energy expenses? Special system, enzyme, etc.

A
  • A high-energy phosphate bond is required for sugar activation, before glycolysis
  • Some species use the phosphoenolpyruvate : sugar phosphotransferase system (PTS), in which phosphoenolpyruvate (PEP) is the phosphoryl donor
  • The main function of the PTS is to translocate sugar across the membrane with simultaneous phosphorylation
  • Since there are two different molecular species outside (sugar) and inside (sugar phosphate) the membrane the translocation process does not involve any concentration gradients
31
Q

What sugars LAB can ferment? Some bacteria have special requirements in terms of sugar, mention them

A

•LAB are also able to ferment disaccharides (such as lactose, maltose, sucrose)

–Some LAB (S. thermophilus and Lb. delbrueckii) only metabolize the glucose moiety and galactose is excreted into the medium

32
Q

How lactose is metabolized in LAB? Enzyme, products..

A

•Lactose can be cleaved by B-galactosidase into glucose and galactose-6-phosphate and then enter one of several major metabolic pathways

33
Q

How maltose is metabolized by LAB

A

•Maltose fermentation starts when maltose is cleaved into glucose and B-glucose-1-phosphate. The glucose is used in glycolysis, and the B-glucose-1-phosphate is a precursor to cell wall synthesis

34
Q

How sucrose is metabolized by LAB

A

•Sucrose is cleaved into glucose and fructose which can then enter major pathways (glucose can be used in glycolysis, and fructose can be the terminal electron acceptor)

35
Q

What metabolism obligately homofermentative LAB use, who in specifics do it

A

1.meaning that sugars are only fermented by glycolysis (group I Lactobacilli and some other species from other genera)

36
Q

What pathway obligately heterofermentative use? What species

A

1.meaning that only the 6-PG/PK pathway is available for fermentation (group III Lactobacilli, Oenococci, and Weissellas) the difference is that key enzymes of the glycolysis pathway are missing

37
Q

Facutatively Heterofermentative: what pathway it is using and what species

A

1.use glycolysis for hexose fermentation, but some sugars induce a heterolactic fermentation reaction to take place (pentoses) (group II Lactobacilli, Enterococcus, Lactococcus, Pediococcus, Streptococcus, Tetragenococcus, and Vagococcus)

38
Q

Why there is a debate if LAB should be classified as aerotolerant anaerobes, or as facultative aerobes

A

–LAB can use oxygen as a terminal electron acceptor, but are not able to protect themselves against the toxic effects of oxygen the way genuine aerobic organisms would

Facultative anaerobes can use oxygen, but also have anaerobic (i.e. not requiring oxygen) methods of energy production.

Aerotolerant organisms can survive in the presence of oxygen, but they are anaerobic because they do not use it as a terminal electron acceptor.

39
Q

–Certain substrates can only be used by LAB if oxygen is available

Example:

A
  • Lb. brevis is unable to ferment glucose anaerobically, but will ferment it aerobically
  • Lactic acid can be further oxidized to acetate and CO2 in the presence of oxygen
40
Q

When grown anaerobically several organic compounds can be used by LAB as terminal electron acceptors (especially ___ LAB) …..

A

•(especially heterofermentative LAB):

–Citrate can be cleaved into acetate and oxaloacetate, and oxaloacetate can be a terminal electron acceptor in several pathways

–Glycerol can be terminal electron acceptor in anaerobic glucose fermentation

–Fructose can be an electron acceptor for heterofermentative LAB

41
Q

Why using fructose as a terminal electron acceptor makes sense for LAB

A

•Some of these metabolisms make a lot of sense when you think about it. In the fermentation of plant materials (e.g. sauerkraut or kimchi) the main sugars are glucose, fructose, and sucrose. Glucose can be used as the energy source and fructose can be used as the terminal electron acceptor

42
Q

Can LAB generate their own nutrients? What they need from external environment except for sugars?

A
  • LAB are very adapted to environments that are rich in nutrients and energy sources, and therefore have limited biosynthetic capability
  • LAB have a very limited ability to synthesize amino acids from nitrogen sources, and are therefore dependent on obtaining preformed amino acids from their environment

–All dairy lactobacilli have proteolytic activity, which have been shown to be required for rapid growth in milk

  • LAB require nucleotides or nucleotide precursors
  • LAB require several external vitamins
43
Q

What is a permanent scares in commercial applications of LAB

A
  • Phage infection is a permanent threat in LAB fermentation commercial processes
  • This is a particular problem in the dairy industry
44
Q

Why phage are so scaring for LAB cultures in the industry

A
  • Phage attacks on LAB can result in unacceptably low LAB and flavor compounds produced along with decreased proteolysis
  • Complete failure of the starter culture can occur and result in a “dead-vat”
45
Q

How dairy fermentation factories control for phages?

A
  1. Characterization of the phage population involved (including genomic analysis)
  2. Analysis of “natural” and “intelligent” bacterial systems of phage defense
  3. Identification of phage counter-defense mechanisms
46
Q

What are naturally occurring phage defense mechanisms? (4)

A
  1. Absorption Inhibition (shown in figure)
  2. Blocking DNA Penetration
  3. Restriction Enzymes/ Modification Systems
  4. Abortive Infection Mechanisms (bacterial cell traps the phages from emerging)
47
Q

4 artficial phage-resisting mechanisms and their principle

A

1. Antisense RNA Strategies

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

2. Cloned ORI

Clone the ORI for the phage into the bacterial genome, it competes with the phage one and slows viral growth

3. Clone in a Phage Repressor

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

4. Phage Triggered Death

Bacterial suicide genes are placed under the control of a phage inducible promoter

48
Q

What relation is there between LAB and mycotoxins? what mycotoxins are dangerous?

A
  • Mycotoxins are wide-spread contaminants of food and feed (aflatoxins, ochratoxin A, zearalenone) and contain some of the most potent carcinogens known
  • Mycotoxins are not an issue in most fresh foods, but if questionable grains or nuts are eaten they can be an issue
  • If questionable products are consumed by farm animals, contamination of animal products can also occur
  • Lactobacillus rhamnosus has been shown to have the greatest ability to remove mycotoxins from foods
49
Q

The difference between mesophilic and thermophilic LAB cultures in dairy industry

A

–Mesophilic cultures grow in temperatures of 10–48C, with the optimum around 38 C. Mesophilic starter cultures, 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.

–Thermophilic starter cultures have their optimum growth temperature between 48C and 58C. Thermophilic starters are used for yogurt and for cheese varieties with high cooking temperatures (e.g. Emmental or Gruyere).

50
Q

How LAB is important for cheese production (6)

A

•LAB fermentation strongly influences the final organoleptic qualities of chesses by:

–fermenting sugars, leading to a pH decrease important in the clotting phenomenon

–reduction or prevention of the growth of adventitious micro-flora

–Protein hydrolysis which causes the texture and, partially, taste of cheese

–Synthesis of flavor compounds

–Synthesis of texturing agents, which may influence the consistency of the product

–Production of inhibitory components

51
Q

How LAB exhibits proteolytic activity in cheese? and what it leads to

A
  • LAB must get amino acids from their surroundings, therefore most have several proteinases and peptidases which provide the bacteria with free amino acids
  • All milk proteins, including whey proteins, are available for hydrolysis at the start of the fermentation reaction
  • The hydrolysis of milk proteins is responsible for bitter tastes in cheese
52
Q

Two classes of flavors produced in cheese are

A

–Compounds produced by fermenting milk (lactic acid, acetic acid, acetaldehyde, diacetyl, and acetoin)

–Compounds produced during cheese maturation (LAB have less of a role here)

53
Q

How LAB exhibit inhibitory properties on other organisms

A
  • Very few bacteria can grow at the pH that LAB produce
  • LAB also produce inhibitory substances including: hydrogen peroxide, diacetyl, and bacteriocins
54
Q

Examples of LAB fermentation on vegetables

A
  • The most common Western products are sauerkraut, cucumbers, and olives
  • In Korea, kimchi is a traditional fermented vegetable mix
55
Q

Benefits of vegetable fermentation by LAB

A
  • 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
  • Storage without refrigeration
56
Q

How fermentation is usually started with vegetables?

A

•Most fermented vegetable products are produced by spontaneous fermentation, without starter cultures and this typically involves succession of microbial populations

57
Q

How much sugars are in cabbage before fermenting?

A

•The concentration of fermentable sugars in cabbage is between 3 and 9% in fresh matter

58
Q

Desrribe the principle of cabbage fermentation by LAB (bacteria should be mentioned, their conditions, etc.)

A
  • Before processing the cabbage is shredded and salt is mixed in, brine begins to form immediately
  • The containers are sealed and pressed with a weight to maintain anaerobic conditions
  • The lactic acid fermentation is initiated by Ln. mesenteroides and followed by Lb. brevis, P. pentosaceus , and finally by Lb. plantarum
  • Ln. mesenteroides produces lactic and acetic acids and CO2, which rapidly lower the pH, thus limiting the activity of undesirable microorganisms and enzymes that might soften the shredded cabbage
  • The CO2 replaces air and creates an anaerobic atmosphere, which is important to prevent the oxidation of ascorbic acid and to avoid darkening the natural color of the cut cabbage

Food Microbiology (19 decks)

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