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medical microbiology > Bacterial Oxygen Requirements > Flashcards

Bacterial Oxygen Requirements Flashcards

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USMLE® Step 1 flashcards Biology 101 flashcards
1
Q

Oxygen is essential for

A - obligate aerobes

B - obligate anaerobes

A

A

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2
Q

Why is oxygen is essential for obligate aerobes?

A

Serves as the final electron acceptor in electron transport chains which produce most of the ATP in these organisms.

By contrast oxygen is a deadly poison for obligate anaerobes

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3
Q

How can oxygen be essential for one group of organisms and yet be a fatal toxin for others?

A

Neither gaseous atmospheric oxygen (O2) nor the covalently bound oxygen in compounds such as carbohydrates or water is poisonous
The forms of oxygen that are toxic are those that are highly reactive.

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4
Q

Why are highly reactive forms of oxygen toxic?

A

because in the same way that oxygen is the final oxygen acceptor for aerobes, they are excellent oxidizing agents,
i.e. they steal electrons from other compounds

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5
Q

How toxic forms of oxygen cause a chain reaction that damages cells?

A

The electron depleted compounds then steal electrons from other compounds
Resulting in a chain of vigorous oxidation
Causing irreparable damage to cells by oxidising important compounds including proteins and lipids

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6
Q

Singlet oxygen (1O2)

A

toxic form of oxygen - A very reactive oxidizing agent

Molecular oxygen – electrons boosted to a higher energy state – during aerobic metabolism

Phagocytic cells – certain human white blood cells use it to oxidize pathogens

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7
Q

Superoxide radical (O2-)

A

Superoxide radicals form

  • during incomplete reduction of O2 during electron transport in aerobes
  • during metabolism by anaerobes in the presence of oxygen
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8
Q

How aerobic organisms detoxify superoxide radicals

A

Produce superoxide dismutase – lacking in anaerobes

Have active sites that contain metal ions e.g. Zn2+ and Fe2+

Combine 2 superoxide radicals and 2 protons to form hydrogen peroxide (H2O2) and oxygen

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9
Q

Peroxide anion O22-

A

Hydrogen peroxide produced during reactions catalysed by superoxide dismutase

Peroxide anion makes hydrogen peroxide an antimicrobial agent

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10
Q

What do aerobes have to detoxify the peroxidase anion?

A

either catalase or peroxidase

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11
Q

Hydroxyl radical (OH)

A

Hydroxyl radicals result from ionising radiation and from incomplete reduction of hydrogen peroxide

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12
Q

What is the most reactive of the 4?

A

Hydroxyl radicals

Due to catalase and peroxidase effect eliminated in aerobes

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13
Q

other antioxidants aerobes can use

A

Vitamin C & E

Again, provide electrons that reduce toxic forms of oxygen

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14
Q

Facultative anaerobes

A

Can live in various oxygen concentrations

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15
Q

How do facultative anaerobes maintain life

A

Can maintain life via fermentation or anaerobic respiration

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16
Q

Metabolic efficiency in absence of oxygen for facultative anaerobes

A

reduced

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17
Q

Example of facultative anaerobe

A

E.coli

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18
Q

Aerotolerant anaerobes

A

Do not use aerobic metabolism

Have some detoxifying enzymes

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19
Q

example of a Aerotolerant anaerobe

A

lactobacilli

20
Q

Microaerophiles

A

Microaerophiles are damaged by the 21% concentration of atmospheric oxygen

Some organisms require oxygen levels of 2% to 10%

21
Q

Example of microaerophile

A

Helicobacter pylori

ulcer causing pathogen

concentration of oxygen in stomach 2-10%

22
Q

Injecting Drug User (IDU) Infections

A

40% of IDU hospital admissions due to infections, 20% result in death

Intravenous, intra-muscular or subcutaneous injection

Minor bacterial infections usually result in local abscess formation

23
Q

Severe IDU infections

A

Severe illness if the injected material or paraphernalia, are contaminated with certain clostridial spores

24
Q

Clostridia

A

Gram positive anaerobic spore-forming rods

25
Q

Where is Clostridia found?

A

Widely distributed in soil and gut

26
Q

How does Clostridia exist?

A

exo-spores

Resistant to environmental conditions

Spores germinate when introduced into an oxygen-reduced environment

27
Q

Pathogenic Clostridia species

A
  • C. perfringens*
  • C. septicum*
  • C. sordellii*
  • C. novyi*
  • C. histolyticum*
  • C. tetani*
  • C. botulinum*
  • C. difficile*
28
Q

IDU Outbreak, 2000

A

Cases of serious illness and deaths amongst IDUs recorded in parts of UK

60 IDUs in Scotland acquired a severe infection at or near an injection site

29
Q

clinical manifestations of IDU outbreak

A

Spread rapidly

Extensive skin and muscle damage

Hypotension

Multi-organ failure

23 deaths

30
Q

origin of IDU outbreak

A

Association with a batch of heroin in circulation at the time and the practice of skin or muscle “popping”

31
Q

Clinical Presentation of Clostridia infection; Soft tissue inflammation at injection site

A

Abscess, Cellulitis, Fasciitis, Myositis

32
Q

Clinical Presentation of Clostridia infection; Local inflammatory reaction has varied

A

Minimal pain and swelling at injection site

33
Q

Clinical Presentation of Clostridia infection; Severe local symptoms

A

Extensive swelling, Pain, Oedema, Erythema with blackening/blistering at centre, Extensive necrosis, Necrotising fasciitis

34
Q

C.novyi Type A

A

Widely distributed in soil

Gram-variable rods, some with sub-terminal spores

35
Q

Identifying C.novyi Type A

A

Examine anaerobic cultures after 24h incubation for small, flat, rough or rhizoidal, translucent, haemolytic colonies with a spreading edge

Exposure to air toxic to micro-colonies that haven’t begun sporulation

After 48-72h, colonies often coalesce to give a fine spreading growth

Unreactive in commercial anaerobe identification kits (API Anaerobe)

36
Q

C.perfringens

A

Post-mortem contaminant

Straight-sided, gram variable rods, no spores

37
Q

Identifying C.perfringens morphological appearance

A

Large discrete colonies after 24h incubation

Flat and rough-edged, or smooth and domed

Non-haemolytic or with a narrow zone of complete haemolysis inside a larger zone of partial haemolysis

38
Q

C.septicum

A

Gram variable rods, numerous sub-terminal spores

Most common source of isolates from blood cultures of patients with malignancies of the colon

39
Q

Culture growth of C.septicum

A

Grows rapidly

Thick, swarming growth, haemolytic

40
Q

C.botulinum

A

Profuse sub-terminal and free spores, gram variable bacilli

Implicated in food-borne illnesses and cases of wound botulism

41
Q

Culture growth of C.botulinum

A

Proteolytic types A,B and F initially produce discrete rhizoidal colonies that spread and coalesce

Haemolysis is variable

42
Q

C.tetani history in human disease

A

Uncommon in recent decades

Outbreak between July ‘03 and March ‘04, 22 cases in IDUs

43
Q

Culture growth and identification of C.tetani

A

Colonies may produce a fine swarming growth

Gram stain of overnight cultures can give readily over-decolorised long bacilli without spores

Classical ‘drumstick’ appearance of cells with terminal, round spores after further incubation

44
Q

Outbreak of C.histolyticum

A

December ’03 to March ’04

First case, 35 year old female IDU from Glasgow, presented at hospital with a necrotic lesion at injection site

Outbreak on-going

45
Q

Identifying the organism responsible for the outbreak

A

Molecular typing and PCR ribotyping revealed them all to be indistinguishable

Isolated organism referred to Anaerobe Reference Lab(ARL), identified as C.histolyticum

46
Q

C.histolyticum

A

Member of the gas gangrene group, may be isolated from soil, bone-meal and gelatin

Produces potent exotoxins causing severe localized necrosis

47
Q

Treatment of C.histolyticum infection

A

Early surgical intervention

  • Exploration, Drainage, Extensive debridement

Microbiological sampling

Patients presenting with compartment syndrome

  • Urgent decompression, Excision of surrounding oedematous tissues

Antimicrobial therapy

  • Penicillin, Metronidazole, Clindamycin

medical microbiology (29 decks)

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