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Mechanisms of Bacterial

Resistance Against Mercury

Toxicity

Supervisor: Prof Erick StraussBy Monique Smit

Introduction

• Why is Mercury so TOXIC?– Mercury– Mercury Poisoning

• Different Enzymatic Mechanisms of Mercury Detoxification– General Mechanism–Adaptations of Different Bacteria

• Conclusions

Mercury ‘Hydrargyrum’

‘hydr’: watery‘argyros’: silver

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‘Mad as a Hatter !’ Roman god, Mercury

Why is Mercury so TOXIC?

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

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

Mercury: Applications to Different Sectors

Sector ExampleMEDICAL Dental Amalgams

ThiomersalMercurochrome

INDUSTRIAL Gold extraction

AGRICULTURAL InsecticidesHerbicidesFungicidesBactericides

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Growing CONCERN?

Mercury PoisoningExposure : High toxicity: mercurialism

Acrodynia (‘Pink Disease’)-Pain-Pink discolouration

Minamata Disease-Ataxia-Impairment of speech & hearing

Different Methods to Exposure

Ingestion Inhalation Skin Contact & Absorption

Google Images

Effects on Health

• Skin & mucous membrane corrosion

• Nervous system deterioration

• Hearing, speech, vision impairment

• Involuntary muscle movements

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

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

Different EnzymaticMethods Of Mercury

Detoxification

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

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)

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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CELL MEMBRANE

CYTOPLASM

Adaptations of Different Bacteria

Diversity of Hg Resistance Loci

Variation in mer operon structure

Mer as part of mobile genetic elements

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

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

Bacteria used in bioremediation?