mechanisms of bacterial resistance against mercury toxicity
TRANSCRIPT
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Mechanisms of Bacterial
Resistance Against Mercury
Toxicity
Supervisor: Prof Erick StraussBy Monique Smit
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Introduction
• Why is Mercury so TOXIC?– Mercury– Mercury Poisoning
• Different Enzymatic Mechanisms of Mercury Detoxification– General Mechanism–Adaptations of Different Bacteria
• Conclusions
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Mercury ‘Hydrargyrum’
‘hydr’: watery‘argyros’: silver
http://www.ecorazzi.com/wp-content/uploads/2009/06/depp_madhatter.jpg
‘Mad as a Hatter !’ Roman god, Mercury
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Why is Mercury so TOXIC?
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Mercury: PropertiesProperty Description
PHYSICALType Heavy MetalColour Silver
Form Liquid at Room Temperature
Density 13.534 g.cm3
CHEMICAL
Melting Temperature - 38.83 °C
Boiling Point 356.73 °C
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Mercury: Occurrence
• Earth’s crust:– Lower crust: 21 ppb– Upper crust : 56 ppb
• Forms: – Elemental– Deposits; Cinnabar (HgS, Binary Minerals)
• Transformation available (three redox states)
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Mercury: General Useshtt
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ThermometersBarometersManometers
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Mercury: Applications to Different Sectors
Sector ExampleMEDICAL Dental Amalgams
ThiomersalMercurochrome
INDUSTRIAL Gold extraction
AGRICULTURAL InsecticidesHerbicidesFungicidesBactericides
Google Im
ages
Growing CONCERN?
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Mercury PoisoningExposure : High toxicity: mercurialism
Acrodynia (‘Pink Disease’)-Pain-Pink discolouration
Minamata Disease-Ataxia-Impairment of speech & hearing
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Different Methods to Exposure
Ingestion Inhalation Skin Contact & Absorption
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Google Images
Effects on Health
• Skin & mucous membrane corrosion
• Nervous system deterioration
• Hearing, speech, vision impairment
• Involuntary muscle movements
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Mercury Poisoning• Exposures to different forms• Inorganic & Organic Mercury
Form Symbol FormINORGANICMetallic HgMercurous HgMercuric Hg
Hg(0)Hg(I)Hg(II)
Vapour or Liquid metalInorganic saltInorganic salt
ORGANICMethyl, Dimethyl, & Ethyl Hg
-CH3, -CH2CH3 &-C2H5
Organic salt
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Mechanism of ActionHg binds to Sulfhydryl
groups
INCAPACITATION of key enzymes
Severely Damaged Nervous System
NO:- Involvement in Cellular Stress Response- Protein Repair- Oxidative Damage Prevention
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Different EnzymaticMethods Of Mercury
Detoxification
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Bacteria to the rescue?
DEFENCE Mechanisms Reduction of Hg
Natural & Engineered role in bioremediation???
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General Mechanism
• Common Mechanism• ‘mer’ Operon, respective Mer proteins• Mercury Resistance Genes:
*Gram NEG. bacteria
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Mercury Resistance Genes
Mercury Resistance Genes
Function
MerR Hg responsive regulator; controls MerA & MerT
MerP Scavenger; passes Hg(II) to MerT
MerT Transport of Hg(II)
MerA Reduction of Hg(II) to Hg(0)
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General Mechanism
Detoxification Process:
Transport of Hg(II) into the cell
Delivery of Hg(II) to Reductase
Reduction
Export of reduced Hg(II)43
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N.L Brown et al. (2002)
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2
3
4
CELL MEMBRANE
CYTOPLASM
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Adaptations of Different Bacteria
Diversity of Hg Resistance Loci
Variation in mer operon structure
Mer as part of mobile genetic elements
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Diversity of Hg Resistance Loci
Variation in operon structure
Difference between Gram+ and Gram- Bacteria:
An additional enzyme, MerB seldom found in Gram- bacteria
but dominantly found in Gram+ bacteria
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Diversity of Hg Resistance Loci
Mer operon as part of mobile genetic elements
Mer resistance operon transposons
Transposons carry integrons with 1/more antibiotic resistance genes
HOW? Horizontal Gene Transfer & RecombinationEvents
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Bacteria used in bioremediation?