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LF223: Microbial Pathogens > Lecture 3. Anaerobiosis > Flashcards

Lecture 3. Anaerobiosis Flashcards

1
Q

What does Clostridium mean?

A

Bunch of organisms that can be pathogenic or non-pathogenic

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2
Q

Why is oxygen important?

A

Oxygen is a universal component of cells and is always provided in large amounts by H₂O. However, prokaryotes display a wide range of responses to molecular oxygen O₂

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3
Q

What are obligate aerobes?

A

Require O₂ for growth; they use O₂ as a final electron acceptor in aerobic respiration

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4
Q

What are obligate anaerobes (occasionally called aerophobes)?

A

Do not need or use O₂ as a nutrient. In fact, O₂ is a toxic substance, which either kills or inhibits their growth. Obligately anaerobic prokaryotes may live by fermentation, anaerobic respiration, bacterial photosynthesis, or the process of methanogenesis

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5
Q

What are facultative anaerobes (or facultative aerobes)?

A

Organisms that can switch between aerobic and anaerobic types of metabolism. Under anaerobic conditions (no O₂) they grow by fermentation or anaerobic respiration, but in the presence of O₂ they switch to aerobic respiration

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6
Q

What are aerotolerant anaerobes?

A

Bacteria with an exclusively anaerobic (fermentative) type of metabolism but they are insensitive to the presence of O₂. They live by fermentation alone whether or not O₂ is present in their environment.

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7
Q

What are microaerophiles?

A

Grow in aerobic conditions, but only when O₂ levels are low (below 0.2 atm)

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8
Q

What determines an organisms response to oxygen in its environment?

A

Depends upon the occurrence and distribution of various enzymes which react with O₂ and various oxygen radicals that are invariably generated by cells in the presence of O₂

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9
Q

What do all cells contain?

A

All cells contain enzymes capable of reacting with O₂ e.g. oxidation of flavoproteins by O₂ invariably results in the formation of H₂O₂ (peroxide) as one major product and small quantities of an even more toxic free radical, superoxide or O₂⁻

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10
Q

When do chlorophyll and other pigments in cells react with O₂ and what does this generate?

A

In the presence of light
Generates a singlet oxygen, another radical form of oxygen which is a potent oxidising agent in biological systems

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11
Q

What prevents lethal accumulation of superoxide within aerobes and anaerobes?

A

The enzyme superoxide dismutase

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12
Q

What enzyme do nearly all organisms contain and what is this enzyme’s function?

A

Catalase. decomposes peroxide

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13
Q

How do certain aerotolerant bacteria (such as the lactic acid bacteria that do not possess catalase) decompose peroxide?

A

By means of peroxidase enzymes which derive electrons from NADH₂ to reduce peroxide to H₂O

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14
Q

Because obligate anaerobes lack superoxide dismutase, catalase and/or peroxidase, what do obligate anaerobes undergo?

A

Lethal oxidations by various oxygen radicals when they are exposed to O₂

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15
Q

What are carotenoid pigments?

A

Found in all photosynthetic (and some non-photosynthetic) organisms
Protect organisms from lethal oxidations of singlet oxygen through a physical reaction that lowers the singlet oxygen radical to its non-toxic “ground” (triplet) state
Carotenoids are said to “quench” singlet oxygen radicals

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16
Q

What is the chemical equation of superoxide dismutase activity?

A

[O₂⁻] + [O₂⁻] + 2H⁺ → O₂ + H₂O₂

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17
Q

What is the chemical equation of catalase activity?

A

O₂ + H₂O₂ → 2H₂O + O₂

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18
Q

What is the chemical equation of peroxidase activity?

A

O₂ + H₂O₂ → 2H₂O
NADH + H⁺ → NAD⁺

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19
Q

What do most Clostridia lack?

A

Respiratory chain cytochromes, catalase, peroxidases and superoxide dismutase

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20
Q

How do Clostridia obtain ATP?

A

Only by substrate-level phosphorylation (in which high energy phosphate bonds from organic intermediates are transferred to ADP)

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21
Q

What do a number of clostridia ferment?

A

Ferment sugars, producing butyric acid (and also acetone and butanol)
Others ferment amino acids

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22
Q

What does substrate level fermentation generate in Clostridium?

A

The energy they need to survive

23
Q

What are the properties of Clostridium botulinum?

A

Gram-positive, spore producing
Commonly found in soil smaples and aquatic sediments

24
Q

What is C. botulinum the causative agent of?

A

Botulinum food poising

25
Q

How potent is botulinum toxin?

A

Extremely potent (1mg of pure toxin can kill 1 million guinea pigs)
Clostridia can form spores = many processed foods are processed based on the need to protect consumers from botulinum outbreaks in food

26
Q

How do botulinum outbreaks in food occur?

A

Poorly canned foods create an anaerobic environment. Unkilled spores germinate and produce toxin

27
Q

How many exotoxin types are produced by the different strains of Clostridium?

A

7 (A, B, C₁, C₂, D, E, F, G)

28
Q

What botulinum exotoxin types are encoded by lysogenic bacteriophage?

A

C and D

29
Q

What does botulism result from?

A

From the ingestion of bacterially produced neurotoxins.

30
Q

What protein exotoxin types are the most toxic to humans and how are they often released?

A

Types A, B, E and F
Often released in an inactive form; proteolytic cleavage activates them

31
Q

What is the most potent exotoxin known to man?

A

Botulinum type A (10 ng can kill a normal adult)

32
Q

How do the botulinum toxins work?

A

These toxins block the release of the neurotransmitter acetylcholine resulting in double vision, slurred speech, decreased saliva, difficult swallowing and general weakness.

33
Q

What occurs in about 20% of those who have botulism food poisoning?

A

Flaccid paralysis with accompanying respiratory failure which can be fatal

34
Q

When can botulism food poisoning be observed following ingestion of heat liable preformed toxin?

A

18-36 hours

35
Q

How is botulinum toxin expressed?

A

Expressed as an inactive 150 kDa polypeptide comprising a 100kDa heavy chain (HC) and a 50 kDa light chain (LC) linked through a disulphide bridge

36
Q

What does the heavy chain of botulinum toxin bind to?

A

Binds to the presynaptic receptor; toxin enters the cell & the disulphide bond is cleaved

37
Q

What happens when the disulphide bond in the botulinum toxin is cleaved?

A

Cleavage liberates the light chin into the cytoplasm and endosomal compartment

38
Q

What does the light chain of botulinum toxin act as?

A

Acts as a zinc endopeptidase, cleaving the synaptosomal-associated protein (SNAP), vesicle-associated membrane protein (VAMP) and syntaxin. This prevents fusion of acetylcholine vesicles at the cell membrane

39
Q

What is Clostridium tetani the causative agent of?

A

Tetanus (lockjaw)

40
Q

What does tetanus result from?

A

Trauma or a puncture wound leading to tissue contamination

41
Q

What is tetanus caused by?

A

By the release of a single antigenic type of exotoxin by C. tetani

42
Q

The tetanus exotoxin circulates in the blood, but what does it adhere to?

A

Neuronal receptors
Specifically, the toxin fixes to gangliosides thereby blocking the release of the neurotransmitters glycine and γ-amino butyric acid (GABA). Glycine normally prevents contraction of antagonistic muscles; therefore, muscle spasms and convulsions (lockjaw) may occur

43
Q

What is tetanus toxin synthesised as?

A

As a 150kDa polypeptide chain (100kDa heavy chain (HC) required for cell entry and a 50kDa light chain (LC) which causes disease

44
Q

What do the C-terminal domain and N-terminal domains of the heavy chain of the tetanus toxin to?

A

C-terminal domain binds to gangliosides
N-terminal domain allows the LC to cross into the cell cytoplasm

45
Q

Once in the cytoplasm, what does the light chain of the tetanus toxin interrupt?

A

The release of neurotransmitters LC is a zinc metalloprotease which cleaves synaptobrevin 2, a SNARE protein involved in the fusion of neurotransmitter vesicles with the neuronal membrane

46
Q

What does the cleaving of synaptobrevin 2 caused by the light chain of the tetanus toxin cause?

A

Vesicles containing GABA and glycine are not allowed to dock and hence no neurotransmitter can be released

47
Q

How does a host die from a C. tetani infection?

A

Host dies from respiratory paralysis though the underlying infection with C. tetani is located elsewhere & plays only an indirect role in the outcome of disease.

48
Q

What does the killing of a human host allow C. tetani to do?

A

To multiply as these anaerobic microbes can only grow in tissues that are not circulated with oxygenated blood

49
Q

Which group of people are particularly susceptible to tetanus?

A

Heroin addicts

50
Q

What are the properties of Clostridium difficile?

A

Gram positive, obligately anaerobic, spore former

51
Q

What can cause C. difficile infections?

A

Antibiotic use (e.g. β-lactam antibiotics) reduces concentration of normal microbiota. C. difficile overgrows

52
Q

What toxins are produced by C. difficile and what do these toxins cause?

A

Produces toxins A and B
Toxins cause diarrhoea & lesions on colon surface which coalesce forming extensive tissue damage – known as pseudomembranous colitis
Toxins A and B – large exotoxins that modify host cell membrane G proteins

53
Q

What is the mode of action of C. difficile?

A

Alter actin cytoskeleton of mammalian cells

54
Q

What mediates the mode of action of C. difficile?

A

Mediated by toxins glucosylating G-proteins
Glucosyl group (from UDP-glucose) added to specific threonine residue on the G protein

LF223: Microbial Pathogens (15 decks)

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