A Portable Biosensor System for Bacterial Concentration Measurement in Liquid and Semi-Liquid Media

Marco GrossiDepartment of Electrical Energy and Information Engineering

University of Bologna, Italyhttp://www.researchgate.net/profile/Marco_Grossi

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

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

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

• 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

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.

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

• 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.

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

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

• 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

Bacterial concentration measure in ice-cream samplesIncubation temperature 35 °C

Bacterial concentration measure in raw milk samplesTwo 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

Bacterial concentration measure in water samplesIncubation temperature 37 °C

Bibliography

[1] Grossi M., Lanzoni M., Pompei A., Lazzarini R., Matteuzzi D., Riccò B. (2008). Detection of microbial concentration in ice-cream using the impedance technique. Biosensors & Bioelectronics, 23, 1616-1623.[2] Grossi M., Pompei A., Lanzoni M., Lazzarini R., Matteuzzi D., Riccò B. (2009). Total bacterial count in soft-frozen dairy products by impedance biosensor system. IEEE Sensors Journal, 9 (10), 1270-1276.[3] Grossi M., Lanzoni M., Pompei A., Lazzarini R., Matteuzzi D., Riccò B. (2010). An embedded portable biosensor system for bacterial concentration detection. Biosensors & Bioelectronics, 26, 983-990.[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.