Metal Resistance of Microorganisms

Amy Dahl

Advanced Environmental Chemistry II

Instructed by Jean-Francois Gaillard

Winter Quarter 2000

IntroductionMicrobes have had to interact with metals since life began in a metal-abundant world.

In more recent years, humans have greatly altered metal availability in the environment.

I will present an overview of how microbes interact and deal with metal stress, focusing on metal resistance.

There are many ways microbes use or interact with metals. For example:

• Metalloenzymes or enzyme activation

• Energy metabolism• Biocorrosion• Passive binding to cell

• Metal leaching• Extracellular

precipitation• Metal resistance

– Redox

– Complexation

– Efflux

Focus: Metal Resistance

• Some metals are required nutrients– e.g. Fe, Zn, Ni, etc.

• Others are toxic– e.g. Pb, As, Hg, Cd, etc.

• Even required metals can be toxic at high levels!!

Uptake Transport Mechanisms:

• Fast and unspecific– MIT

– Pit

– Sulfate Uptake

• Slow and specific– P-type ATPase

– ABC

– HoxN

Examples of Uptake and Efflux Mechanisms

How metals can be toxic to cells:

• “Open Gate”

• Metals can bind to SH groups and inhibit enzyme activity.

• Toxic metal may interfere with function of physiological cation.

• Successive binding with glutathione causing oxidative stress.

• Metal oxyanions may interfere with non-metal oxyanions and subsequent reduction of metal oxyanion leads to radicals.

Toxic metal resistance systems arose early on in evolution.

Strong selective pressures to transport and accumulate needed inorganic nutrients and removal or detox of toxic cations and oxyanions.

Polluted environments have also exerted selection pressure for metal resistance.

3 Mechanisms for Metal Resistance

• Reduction of metal cation to a less toxic or easier to expell substance– The redox potential of the metal must be

between H2/H+ and O2/H2O.– Most reduced metals need an efflux system.

Only mercury doesn’t because it can diffuse out.

– Reduction requires some energy in the form of electrons.

3 Mechanisms for Metal Resistance

• Intracellular complexation or precipitation– Well known complexes are formed with

sulfide, glutathione, and metallothionein.– This method of resistance costs the organism a

lot of energy!– The complexes or precipitates can either be

stored intracellularly (most common with eukaryotes) or expelled with an efflux system (generally done by prokaryotes).

3 Mechanisms for Metal Resistance

• Efflux Systems– Most common efflux systems are:

• ATP-binding cassettes (ABC)

• P-type ATP

• A-type ATP

• Resistance-, nodulation-, cell division (RND)-driven transenvelope transporter

• Cation-diffusion facilitator (CDF) transporters

Efflux Systems

• Relatively inexpensive, only 1 ATP

• Can be used in conjunction with reduction or complexation.

• A futile cycle of uptake and efflux may form, so complexation may be cheaper in the long run in low concentration environments.

Let’s look at lead resistance:• Lead exclusion with exopolymer• Extracellular lead methylation• Intracellular precipitation of lead phosphate• Efflux - CadA p-type ATPase

Conclusion

• Metal resistance is extremely complex!

• But, this is a very hot topic!

• Lot’s of opportunities for more research!