Bacterial Oxygen Requirements Flashcards
Oxygen is essential for
A - obligate aerobes
B - obligate anaerobes
A
Why is oxygen is essential for obligate aerobes?
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
How can oxygen be essential for one group of organisms and yet be a fatal toxin for others?
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.
Why are highly reactive forms of oxygen toxic?
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
How toxic forms of oxygen cause a chain reaction that damages cells?
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
Singlet oxygen (1O2)
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
Superoxide radical (O2-)
Superoxide radicals form
- during incomplete reduction of O2 during electron transport in aerobes
- during metabolism by anaerobes in the presence of oxygen
How aerobic organisms detoxify superoxide radicals
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
Peroxide anion O22-
Hydrogen peroxide produced during reactions catalysed by superoxide dismutase
Peroxide anion makes hydrogen peroxide an antimicrobial agent
What do aerobes have to detoxify the peroxidase anion?
either catalase or peroxidase
Hydroxyl radical (OH)
Hydroxyl radicals result from ionising radiation and from incomplete reduction of hydrogen peroxide
What is the most reactive of the 4?
Hydroxyl radicals
Due to catalase and peroxidase effect eliminated in aerobes
other antioxidants aerobes can use
Vitamin C & E
Again, provide electrons that reduce toxic forms of oxygen
Facultative anaerobes
Can live in various oxygen concentrations
How do facultative anaerobes maintain life
Can maintain life via fermentation or anaerobic respiration
Metabolic efficiency in absence of oxygen for facultative anaerobes
reduced
Example of facultative anaerobe
E.coli
Aerotolerant anaerobes
Do not use aerobic metabolism
Have some detoxifying enzymes
example of a Aerotolerant anaerobe
lactobacilli
Microaerophiles
Microaerophiles are damaged by the 21% concentration of atmospheric oxygen
Some organisms require oxygen levels of 2% to 10%
Example of microaerophile
Helicobacter pylori
ulcer causing pathogen
concentration of oxygen in stomach 2-10%
Injecting Drug User (IDU) Infections
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
Severe IDU infections
Severe illness if the injected material or paraphernalia, are contaminated with certain clostridial spores
Clostridia
Gram positive anaerobic spore-forming rods
Where is Clostridia found?
Widely distributed in soil and gut
How does Clostridia exist?
exo-spores
Resistant to environmental conditions
Spores germinate when introduced into an oxygen-reduced environment
Pathogenic Clostridia species
- C. perfringens*
- C. septicum*
- C. sordellii*
- C. novyi*
- C. histolyticum*
- C. tetani*
- C. botulinum*
- C. difficile*
IDU Outbreak, 2000
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
clinical manifestations of IDU outbreak
Spread rapidly
Extensive skin and muscle damage
Hypotension
Multi-organ failure
23 deaths
origin of IDU outbreak
Association with a batch of heroin in circulation at the time and the practice of skin or muscle “popping”
Clinical Presentation of Clostridia infection; Soft tissue inflammation at injection site
Abscess, Cellulitis, Fasciitis, Myositis
Clinical Presentation of Clostridia infection; Local inflammatory reaction has varied
Minimal pain and swelling at injection site
Clinical Presentation of Clostridia infection; Severe local symptoms
Extensive swelling, Pain, Oedema, Erythema with blackening/blistering at centre, Extensive necrosis, Necrotising fasciitis
C.novyi Type A
Widely distributed in soil
Gram-variable rods, some with sub-terminal spores
Identifying C.novyi Type 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)
C.perfringens
Post-mortem contaminant
Straight-sided, gram variable rods, no spores
Identifying C.perfringens morphological appearance
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
C.septicum
Gram variable rods, numerous sub-terminal spores
Most common source of isolates from blood cultures of patients with malignancies of the colon
Culture growth of C.septicum
Grows rapidly
Thick, swarming growth, haemolytic
C.botulinum
Profuse sub-terminal and free spores, gram variable bacilli
Implicated in food-borne illnesses and cases of wound botulism
Culture growth of C.botulinum
Proteolytic types A,B and F initially produce discrete rhizoidal colonies that spread and coalesce
Haemolysis is variable
C.tetani history in human disease
Uncommon in recent decades
Outbreak between July ‘03 and March ‘04, 22 cases in IDUs
Culture growth and identification of C.tetani
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
Outbreak of C.histolyticum
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
Identifying the organism responsible for the outbreak
Molecular typing and PCR ribotyping revealed them all to be indistinguishable
Isolated organism referred to Anaerobe Reference Lab(ARL), identified as C.histolyticum
C.histolyticum
Member of the gas gangrene group, may be isolated from soil, bone-meal and gelatin
Produces potent exotoxins causing severe localized necrosis
Treatment of C.histolyticum infection
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
