Zeta Potential as a measurement system
Zeta potential measurement is a comparatively new and underused method in the analysis of cell cultures. Zeta potential measurement employs the use of optoelectric methods to measure the zeta potential of the (colloidal) components in the solution/colloid/suspension to be analysed.
Principle
Zeta potential is the specific term for electrokinetic potential in colloidal dispersions[1]. Measured in volts(V), it is a measure of the potential gap in the Helmholtz double layer found in colloids.
Fig.1 - Potential Field around a colloidal particle
Zeta Potential of Cell Cultures
Cell cultures, being colloidal solutions, also have a measurable Zeta potential. Also, being stable colloids, the zeta potential's absolute value is usually greater than about 30mV. The excellent sensitivity of zeta potential towards the changes in membrane proteins and cell surface charges allows us to distinguish bacteria on the basis of their potentials. The intensity obtained for the measurement is also positively correlated with the concentration of the specific bacteria in the culture. This makes measurement of zeta potential an excellent method for cell counting. Hence, we decided to try to incorporate this measurement method into our system.
Zeta Potential in Co-cultures
Now, convinced that we will use this measurement method, we went forward with looking at the presently available research.
Available Research and Methods
Until now, not a lot of research has been done on the measurement methods of bacteria in co-cultures (mostly because of the lack of availability of dynamic control methods). Due to this, the only papers we found were on the characterization of mixed cultures. The data and plots available in those papers show promise towards the implementation of the method for measurement of ratios in co-cultures.
Some of the data from a larger dataset is presented here.
Fig.2 - Zeta Potentials of mixed cultures of S. aureus and E. coli[2]
The excellent stability of the peak even in different ratios of mixing is observed here. This is one of the datasets which shows great promise.
Here, although some research is done, zeta potential measurements are greatly dependent on precision machinery which is not very stable to perturbations. The machinery used is also moderately bulky and not conducive towards the creation of portable systems.
Expected Changes in Co-cultures
In c-ocultures, we can expect the zeta potential to change because bacteria show slightly different zeta potentials as it is known that bacteria express proteins distinct from the ones on their cell wall when grown in a co-culture. This, coupled with the sensitivity of the zeta potential method can provide insight into the changes in the bacteria post transformation/ in co-cultures.
Our Conclusions
We from our research on the methods of measurement and previous work came to a conclusion that making a portable system that can be put inside a shaker is not feasible if we use zeta potential measurements. Still, if microfluidic systems are considered, the method has potential and will be explored in the future.
References
[1]ISO International Standard 13099, Parts 1,2 and 3, “Colloidal systems – Methods for Zeta potential determination", (2012)
[2]Kłodzińska, E. , Szumski, M. , Dziubakiewicz, E. , Hrynkiewicz, K. , Skwarek, E. , Janusz, W. and Buszewski, B. (2010), Effect of zeta potential value on bacterial behavior during electrophoretic separation. ELECTROPHORESIS, 31: 1590-1596. doi:10.1002/elps.200900559
