8. Yeast

Question 1

How is yeast reproduced?

Answer 1

The fungi reproduces vegetative (asexually) bu budding or fission.

Or sexual via spores.

Question 2

What is ascospores and teliospores.

Answer 2

When yeast reproduces sexually .

The most common is acsospores produced by Ascomycetes, reproductive structure known as the ascus containing ascospores

Basidiomycetes – teliospores or basidia: club-shaped reproduction organ called basidium. Imperfect yeast: spores ans sexual reproduction lacking or not observed.

Question 3

What are two types of light microscopy perfect for yest?

Answer 3

Differential interference contrast (DIC) microscopy, also known as Normarski interference contrast (NIC): optical microscopy illumination technique used to enhance the contrast in unstained, transparent samples . Creation of artificial shadows.

Phase contrast microscopy: creation of brightness differences based on (otherwise invisible) light phase shifts.

Question 4

Describe S. cerevisiae.

Answer 4

It’s the main organism in wine/beer/cider production.

High fermentation capacity and EtOH tolerance.
Pleasant taste and aroma.
Ferment sugar to EtOH in present of O2

Baker’s yeast
Produces CO2 from sugar at high rate.
Taste and aroma component.

Use to produce proteins and biochemicals.
Genetically modified, safe, advanced fermentation technology well developed.

Bioethanol
Nutritional yeast

Question 5

What is haploid and diploid?

Answer 5

S. cerevisiae can grow has a haploid (half of the chromosome, 1n), diploid (2n) and also polyploid(xn)

Question 6

How is haploid divided?

Answer 6

Haploid cells – either of two possible mating types (sex) a and alfa

Question 7

What is a zygote?

Answer 7

Two haploid cells can mate and form a zygote

Question 8

When can haploid cells initiate meiosis?

Answer 8

A diploid zygote, starts to divide, grows from the so called junction. In starvation (normally nitrogen), the diploid cells can initiate meiosis and form an ascus containing four haploid ascospores

Question 9

What is conjugation?

Answer 9

When two haploid cells (a and alfa) becomes a mating pair and create a budding zygote (diploid cell)

1. Cell adhesion (cells touching)
2. Cell fusion (requires Fus1 and Fus2 proteins)
3. Nuclear fusion

Question 10

What is the a-factor and alfa-factor?

Answer 10

Haploid cells produce mating peptides (pheromones): a-factor and alfa-factor – signal to mating partner which respond by preparing for mating. a specific genes produce a factor and alfa factor receptor. Alfa-specific genes vice versa

Question 11

Describe yeast sporulation.

Answer 11

A vegatative cell (2n) -meiosis-> to a tetrad (4x1n), containing spores surrounded be ascus.

Nitrogen starvation + poor carbon source -> S.cerevisiae undergoes meiosis.

Haploid nuclei produced in meiosis packaged into spores. Spore formation requires unusual cell division event – daughter cells formed within the cytoplasm of the mother cell. Two different cellular structures formed de novo: - novel membrane compartment within the cell cytoplasm -> spore plasma membrane. -

Extensive spore wall – protects the spore from environmental insults.

Life cycle pic

Question 12

What are some common non-saccharomyces yeast in wine?

What is good and bad with them?

Answer 12

Common non-saccharomyces yeast in a winery are: Brettanomyces, Zygosaccharomyces, Candida, Pichia, Hanseniaspora, Metchnikowia, and Schizosaccharomyces.

All of these yeast ar non-pathogenc to humans.

However they can make stinky wine. Brettanomyces – most talked about spoilage yeast in wine. Brettanomyces, commonly referred to as “brett” – aroma describe as “barnyard”, “sweaty saddle” or “burnt plastic”

Question 13

Describe Zygosaccharomyces.

What is it?
Why are food easily affected by it?
How does it affect food?

Answer 13

Zygosaccharomyces – long history as a spoilage yeast within the food industry. Important to spoilage of sweet and dry wines and many food product e.g. fruit juices, soft drinks, juice concentrates, sugar syrup, candied fruit, jams and preserves, cream, eggs, honey, tomato sauce.

Mainly because it is TOLERANT to many of the common food preservation methods.

High sugar tolerance (50-60%), high ethanol tolerance (up to 18%), high acetic acid tolerance (2.0-2.5%) Very high sorbic and benzoic acid tolerance (up to 800-1000 mg/L) Very high molecular SO2 tolerance (greater than 3 mg/L) and high xerotolerance (resistance to drough).

In addition to causing undesirable properties (off-flavours, hazing), the vigorous alcoholic fermentation may lead to explosion of canned and bottled foods and beverages.

Question 14

Describe Debaryomyces handenii.

What is it?
Why are food easily affected by it?
How does it affect food?

Answer 14

Spoilage yeast that performs well in high [NaCl], that prevents growth of most microorganisms.

Not always bad, can also be beneficial.

Growth at 4M NaCl in comparison S. cerevisiae stops at 1.7M – osmotolerant.

Marine yeast, but not only marine.

Many types of food – fruit juice, meat, salted foods, wine, beer .. part of cheese flora.

Question 15

Describe sour dough yeast.

Answer 15

Sourdough contains a mix of lactic acid bacteria and yeast.

Common yeast are: Kazachstania exigua, Saccharomyces cerevisiae, Candida humilis, Candida milleri, C. humilis, S. exiguus Issatchenkia orientalis (Picchia kudriavsevii)

Question 16

What is obligate aerobes and name some yeast species that is that.

Answer 16

One type of yeasts carbon metabolism-

Obligate aerobes – do not use glucose in the absence of oxygen.

All species in Rhodotorula, Rhodosporidium, Lipomyces, Saccharomycopsis, Cryptococcus and Sporobolomyces ◦ Some species within Torulopsis, Pichia, Debaromyces, Hansenula.

Question 17

What is Faculative anaerobes and name some yeast species that is that.

Describe fermentative yeast and respiratory yeast.

Answer 17

One type of yeasts carbon metabolism.

utilise glucose under both aerobic and anaerobic conditions.

A. Fermentative yeast – fermentation predominates <10% of glucose respired. Saccharomyces, Schizosaccharomyces, Brettanomyces, some species of Torulopsis. High rate of glucose catabolism: 100-300 nmol glucose per mg dry wt per min. Low respiration rate: 5-50 nmol O2 per mg per min. Crabtree positive – diauxic growth on glucose.

B. Respiratory yeast – respiration predominates under aerobic conditions <30% of glucose fermented. The majority of yeast species e.g. candida, hansenula, kluveromyces and most pichia. Low rate of glucose catabolism: 10-40 nmol glucose per mg dry wt per min. High respiratrion rate 150-200 nmol O2 per mg per min. Candida utilis, Hansenula nonfermentans, Brettanomyces intermedius and some species of Torulopsis purely Crabtree negative. High affinity glucose transport system.

Question 18

Describe S. cerevisiae catabolism. How much ATP is produced?

Answer 18

First some facts: Very high glycolytic flux in S. cerevisiae ~50 % of dissolved proteins are glycolytic.

Respiration energetically more favourable than fermentation. i.e. for a certain cell yield, (biomass/substrate) less glucose is required than for fermentation.

2 ATP per fermented glucose compared to ~38 per respired. The amount of ATP formed normally well correlated with the amount of biomass formed.

Question 19

Describe glucose repression.

Answer 19

Glucose represses the utilization of other sugars e.g. sucrose, maltose, galactose.
Happen if sugar concentration exceed a certain level (low; approx. one or few mM)
Expression of many proteins repressed.

Three main groups:
1, gluconeogenetic proteins.
2, mitochondrial enzymes involved in Krebs (TCA) cycle and respiration.
3, Proteins involved in the uptake of non-glucose carbon/energy source such as galactose and maltose.
(… sensing, signal transduction and transcriptional regulation)

Instead, high speed on glycolysis and fermentation, also at high O2. Hence, the yeast chooses an energetically unfavourable catabolism – energy and carbon dissipates as ethanol (and heat), and only 2 ATP per glucose, resulting in

LOW cell yield. -> Why? S. cerevisiae competes by quickly fermenting available glucose to ethanol. Competing microorganism disfavoured by ethanol, yeast use ethanol as carbon source only later.

Question 20

Describe the Medical/industrial use of the yeast cell wall (5).

Answer 20

1) The yeast cell wall is a non-specific stimulator of the immune system of both humans and animals. When ingested orally, yeast beta-1,3-glucan can stimulate the immune system (macrophages) and help to overcome bacterial infections.
2) Mannan-oligo-saccharide (MOS), has been demonstrated to prevent diarrhea in weaning pigs and poultry. MOS binds to pathogenic bacteria (see adhesion lecture) in the gut and then carries them through and out of the intestinal tract.
3) MOS also has prebiotic activity and can serve as a nutrient source for the growth of beneficial bacteria in the colon.
4) Yeast cell wall is also applied in the wine industry: its ability to bind undesirable components allows it to prevent and cure stuck fermentations.
5) It also binds some mycotoxins – the potential of yeast to detoxify fermented foods has been suggested and studied

Question 21

What does the yeast cell wall consist of?

Answer 21

Mannoprotein, Beta-glucan, beta-glucan + chitin, mannoprotein, membrane.

Question 22

Describe yeast flocculation.

Answer 22

Important process in the production of beer.

Causes the yeast to sediment at the end of the fermentation. .

Thus, the yeast can be harvested from the bottom and used for the next fermentation, while the beer may be matured without the need of a centrifugal step.

Ideally brewing yeast does not flocculate at the beginning of the fermentation, only after all nutrient have been used up.

If not: improper fermentation, need for centrifugation.

Question 23

How does the process of flocculation goes?

Answer 23

Complicated process. Requires the presence of at least two types of molecules on the yeast cell surface:

(1) mannans (carbohydrate chains), produced by the gene products of the MNN genes.

(2) flocculins (sugar binding proteins), the gene products of the FLO genes, activated only after depletion of nutrient.
The flocculins bind to mannans on the surface leading to the formation of flocs.

Been proposed that cells in a floc cooperate to survive stress. Stress protection – cells in the exterior shield cells in the interior from e.g. UV stress, oxidative stress.

Question 24

Why does yeast create a capsule?

Answer 24

Functions of the capsule: capsules determine survival and propagation primarily during water and nutrient stress. Improved desiccation tolerance. Assimilation of nutrients.

Question 25

What are the major functions of peroxisomes?

Answer 25

Major functions: lipid metabolism: beta-oxidation of fatty acid. Reactions involving the metabolism of H2O2 for which the peroxisome is named. H2O2 produced during Beta-oxidation – catalase detoxifies. H2O2  ½ O2 + H2O. Catabolism of a variety of substrate that are not oxidized by mitochondria, e.g. methanol.

Question 26

Describe Candida albicans – Candidiasis.

Answer 26

In immunocompromised people (e.g. AIDS) C. albicans can change from harmless to aggressive pathogen.

Powerful ability to adapt to various tissue environments.

Candidasis shows in a number of ways such as relatively simple skin infections to systemic life-threatening states.

Surface structures in the cell wall has crucial impact – direct contact with host cells, adhesion plus protection against host defence.

Other virulence factors: dimorphism (yeast, mycelia), germ tube formation, disturbing interactions with host immune system, production of hydrolytic enzymes (toxins) and other metabolites.

Question 27

What is a oleaginous (“oily”) yeast?

Answer 27

Cultured right way these yeasts accumulate large amounts of oil plus carotenoids for protection. Becomes energy rich. Fatty acid composition much like olive oil.

E.g. Rhodotorula glutinis.

Metabolism?

Question 28

What are some (4) use for oily yeast?

Answer 28

Potential use:
Feed for larval stage in aquaculture.

Oil for human consumption,

other feed,
fuel -biodiesel.

Question 29

Name some yeast inhibiting substances.

Answer 29

Cycloheximide –
soil often contains cycloheximide-producing Streptomyces griseus. Extra sensitive yeast – ex. Certain pichia. Can resist more – ex protecha. Totally resistant – ex. Candida.

Killer toxin-
specific protein, killer factor – produced by some yeast. Kills killer-sensitive yeast. Widely spread in many genera. Regarded to not affect bacteria. At least one report says that is untrue.

Fatty acids –
Certain plants contain high concentration of C6 -C12 (fairly short) – these are fungistatic many are inhibited but e.g. Candida ingens has strong lipolytic activity and oxidizes (degrades) these fatty acids.

Question 30

How is yeast associated with plants and fruits?

Answer 30

Contain plenty of yeast.

S. cerevisiae on grapes.

Cactus is a common yeast habitat.

On leaves (often dominated by other microorganism)
E.g. Rhodotorula, Cryptococcus commonly found. Some were found to synthesize lipophilic antibacterial compounds.

Question 31

How is yeast associated with insects and soil?

Answer 31

Insects: Most important vector for spreading yeasts, e.g. to a grape. Beetles seem to be a fantastic source of yeast. Many beetles have “their own” species of yeast in their intestine. Many beetles remains to be discovered, and hence also yeast. Bees. Potential for finding industrially useful species.

Soil: yeast are found in soils, particularly in nutrient-rich moist habitats. Non fermenting, basidiomycetes more common e.g. Cryptococcus. Capsule- and biofilm-forming.

Question 32

Describe aquatic yeast.

Answer 32

Widespread in seas and fresh water systems, but much lower number than bacteria.

Bacteria ca 10^5/ml sea water (can be much more) Yeast perhaps 10-100 cells/ml.

Depends on nutrients.

Clean sea: Rhodotorula, Cryptococcus, debaryomyces.

Useful properties for a marine microorganisms: Ability to use many carbon/energy sources.

Halotolerant/osmotolerant. Effcient respiratory growth. Strong growth below 15 degr C.

Developed ability to survive starvation. Associated with aquatic plants and animals.

Certain yeast has been shown to colonize fish intestine. Probiotics in aquaculture (fish farms).