1) A portable biosensor system has been developed to measure bacterial concentration in liquid and semi-liquid media through electrical impedance measurements. The system uses disposable electrodes in contact with the sample to measure the electrical parameters Rs and Cs over time. 2) As the bacterial concentration increases over time, the electrical parameters deviate from their baseline values, allowing the detection time (DT) to be calculated. Shorter DT indicates higher initial bacterial concentration. 3) The biosensor system has been used to measure bacterial concentration in samples like ice cream, raw milk, and water. It provides results faster than traditional plate counting methods while maintaining accuracy.

1. A Portable Biosensor System for Bacterial Concentration
Measurement in Liquid and Semi-Liquid Media
Marco Grossi
Department of Electrical Energy and Information Engineering
University of Bologna, Italy
http://www.researchgate.net/profile/Marco_Grossi

2. Bacterial reproduction happens by means of binary scission.
• duplication of the bacterial chromosome
• elongation of the cell
• formation of a transverse septum centrally located
• subdivision of a copy of the chromosome and cytoplasm in
the two daughter cells
(A) Lag Phase
(B) Exponential Phase
(C) Stationary Phase
(D) Death Phase

3. The standard reference technique for bacterial concentration
measurement is the Plate Count method.
• It is the reference method
• Accurate and reliable results
• Long response times
• It is a laboratory method
Advantages Drawbacks

4. A system composed of a liquid medium in direct contact with a
couple of electrodes can be modeled with the following circuit.
Rm : medium resistance
Cm : medium capacitance
Ri : interface resistance
Ci : interface capacitance
When the frequency of the applied signal is lower than 1MHz
Rs = Rm + Ri Cs = Ci

5. • a sine wave voltage Vin(t) is applied to the electrodes
• the current through the electrodes Iin(t) is measured
• the electrical parameters Rs and Cs are calculated

6. Detect Time (DT): time needed for the bacterial population to
reach the critical concentration CTH = 107
cfu/ml
The liquid sample is placed in a sensor in direct contact with a
couple of electrodes. The electrical parameters Rs and Cs are
measured at regular time intervals of 5 minutes.
Conc. < CTH: the
electrical parameters
Rs and Cs are almost
constant.
Conc. > CTH: the
electrical parameters
Rs and Cs deviate
from baseline value.

7. Low contaminated samples feature high values of DT.
Higher contaminated samples feature lower values of DT.

8. • Shorter response time
(2−12 hours vs 24−72 hours)
• The technique can be easily
automated as part of an
embedded system
• Need of calibration for the
different sample types and
different bacterial strains
• The electrical parameters
are very sensitive to
temperature variations
Advantages Drawbacks
The impedance technique has advantages and drawbacks if
compared with the reference Plate Count method.

9. Different commercial systems have been produced that measure
the bacterial concentration with the impedance technique.
Bactometer
(Biomerieux, France)
• 2ml sensor cell
• up to 128 samples
• weight 130.2 kg
Bac Trac
(Sy-Lab, Austria)
• 20ml sensor cell
• up to 64 samples
• weight 20 kg

10. A portable biosensor system that measures bacterial
concentration using the impedance technique has been built.
• Electronics boards: impedance measures and thermoregulation
• Incubation chamber to store the sample
• Both serial and wireless communication

12. • The sample is loaded in the sterile incubation chamber
• Thermoregulation is enabled
• Electrical parameters are measured every 5 minutes
• At the end of the assay, DT is calculated and the bacterial
concentration is estimated

13. Bacterial concentration measure in ice-cream samples
Incubation temperature 35 °C

14. Bacterial concentration measure in raw milk samples
Two incubation temperatures (a) 18 °C and (b) 30 °C
Shorter response time when incubated at 30°C
Better correlation when incubated at 18°C
A trade off between accuracy and measure time is needed

15. Bacterial concentration measure in water samples
Incubation temperature 37 °C

16. Bibliography
[1]
Grossi M., Lanzoni M., Pompei A., Lazzarini R., Matteuzzi D., Riccò B. (200
Detection of microbial concentration in ice-cream using the impedance tech
[2]
Grossi M., Pompei A., Lanzoni M., Lazzarini R., Matteuzzi D., Riccò B. (200
Total bacterial count in soft-frozen dairy products by impedance biosensor s
[3]
Grossi M., Lanzoni M., Pompei A., Lazzarini R., Matteuzzi D., Riccò B. (201
sor system for bacterial concentration detection. Biosensors & Bioelectronic
[4] Grossi M., Lanzoni M., Pompei A., Lazzarini R., Matteuzzi D., Riccò
B. (2011). A portable biosensor system for bacterial concentration
measurements in cow’s raw milk. 4th
IEEE International Workshop on
Advances in Sensors and Interfaces, 132-136.
[5] Grossi M., Lazzarini R., Lanzoni M., Pompei A., Matteuzzi D., Riccò
B. (2013). A portable sensor with disposable electrodes for water
bacterial quality assessment. IEEE Sensors Journal, 13 (5), 1775-1781.