A portable biosensor system for bacterial concentration measurement in liquid and semi liquid media

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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_Gr
ossi
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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
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The standard reference technique for bacterial
concentration measurement is the Plate Count
method.
Advantages
Drawbacks

• It is the reference method

• Long response times

• Accurate and reliable results

• It is a laboratory method

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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
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• 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

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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. lt CTH the electrical parameters Rs and Cs
are almost constant.
Conc. gt CTH the electrical parameters Rs and Cs
deviate from baseline value.
Detect Time (DT) time needed for the bacterial
population to reach the critical concentration
CTH 107 cfu/ml
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Low contaminated samples feature high values of
DT. Higher contaminated samples feature lower
values of DT.
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The impedance technique has advantages and
drawbacks if compared with the reference Plate
Count method.
Advantages
Drawbacks

• Shorter response time (2-12 hours vs 24-72 hours)

• Need of calibration for the different sample
  types and different bacterial strains

• The technique can be easily automated as part of
  an embedded system

• The electrical parameters are very sensitive to
  temperature variations

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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

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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

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• 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

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Bacterial concentration measure in ice-cream
samples Incubation temperature 35 C
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Bacterial concentration measure in raw milk
samples Two incubation temperatures (a) 18 C and
(b) 30 C
Shorter response time when incubated at
30C Better correlation when incubated at
18C A trade off between accuracy and measure
time is needed
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Bacterial concentration measure in water
samples Incubation temperature 37 C
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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 cows
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.