Hello Jaqueline,

You mention that E. Coli bacteria do not conduct electricity as well as other electroactive bacteria. I am not sure how you are measuring the electrical conductivity by using a multimeter to make voltage and current measurements. In other words, usually one measures electrical conductivity of a solution by imposing an external voltage difference from a power supply or battery across the solution using inert electrodes connected to the voltage source and measures the resulting current that flows through the solution. The quotient of the measured current divided by the applied voltage difference would be the electrical conductivity.

Can you reference one of the papers you are reading and provide a link to it so that the readers of your question can understand the source of your experimental protocol?

Regards,

Tom Cuff

Hi Jaqueline,

even without knowledge of your setup but provided that your equipment itself is faultless, I guess it's safe to say that the fluctuations of the current are not caused by the multimeter.

My first suspicion is that your setup for voltage measurement causes a very low current through the solution (or, in an erraneous setup, the amount of current might have no impact at all, sorry for the imputation ;-) while during current measurements a considerable current is flowing. If so, this current could cause a change of the density of bacteria (and/or of other ingredients) in the neighborhood of the electrodes, resulting in a fluctuation of the mean conductivity.

In this case, gentle stirring of the solution (not manually but by mechanical means) might slightly affect the results but largely eliminate the fluctuations.

Hello Jaqueline,

I agree with Joerg in general, but I have some reservations. With respect to measuring the voltage difference across your electrochemical cell, assuming your multimeter is in good repair, I agree that the current drawn by the multimeter will be negligible since the internal resistance of the multimeter in the DC voltage measuring mode is minimum of 10 mega-ohms and maximum for the small voltage ranges it can approach 10 giga-ohms, see,

Clyde F. Coombs, Jr.; Electronic Instrument Handbook, Third Edition; McGraw-Hill Companies, Inc.; 2000; p. 13.4 - 13.5.

Therefore, if you were measuring 1 volt DC across the cell, then the multimeter would only be drawing a maximum of 0.1 microamps. Thus, you are measuring the open circuit voltage difference of the electrochemical sell.

However, connecting a multimeter in its current measuring mode across a battery (electrochemical cell) or a power supply is something multimeter manufacturers expressly warn against. That is because in the current measuring mode, the multimeter presents essentially zero resistance, which means it can draw an unlimited amount of current. In fact, all multimeters are protected against such a mistake by one or more internal fuses, which will blow to protect the meter, see pp. 13-11 - 13.12 of Coombs. The only thing maybe protecting your instrument is a very high internal resistance inside the electrochemical cell. I agree that normally a properly functioning multimeter would not be the source of the current fluctuations you are seeing, but what if the meter is being overloaded by the current it is drawing?

It is, of course, possible that when the multmeter in the current measuring mode is connected across your cell that it is drawing so much current that it is causing polarization at the cell's electrodes, which then limits the current. In any event, connecting multimeters in their current measuring mode across voltage sources is not a good idea.

Again, I am not familiar with your experimental setup or the source of your experimental protocol. It would help me, if you would provide this information.

Regards,

Tom Cuff