WO2011086216A1

WO2011086216A1 - Bacteria-detecting inductive device

Info

Publication number: WO2011086216A1
Application number: PCT/ES2011/070012
Authority: WO
Inventors: Eva Baldrich Rubio; Francisco Javier MUÑOZ PASCUAL; Susanna MARTÍNEZ MENDIZABAL
Original assignee: Consejo Superior De Investigaciones Científicas (Csic)
Priority date: 2010-01-14
Filing date: 2011-01-11
Publication date: 2011-07-21
Also published as: ES2363289A1; ES2363289B1

Abstract

The invention relates to a novel device (1) for the detection of bacteria, comprising two similar inductive transducers configured such that one can remain idle while the other is exposed to a solution and designed to be connected to an impedance analyser, allowing the presence or absence of bacteria in a solution to be deduced on the basis of the variation in the impedance of the inductive transducer upon exposure to said solution.

Description

BACTERIAL PRESENCE DETECTOR INDUCTIVE DEVICE

OBJECT OF THE INVENTION A first object of the present invention is an inductive device for detecting bacterial presence in a sample quickly and easily.

A second object of the present invention is a method for detecting bacterial presence in a sample using the above detection device.

STATE OF THE TECHNIQUE Rapid and sensitive detection of bacterial presence is a key requirement for the effective prevention of a significant number of health and environmental problems. However, although simple in concept, this objective still faces great challenges. Most of the currently available detection methods are based on classical techniques such as culture, biochemical tests and enumeration by microscopy or flow cytometry, which although extremely sensitive, require long test times, adequate laboratories, expensive instrumentation and highly specialized staff.

Recent advances in alternative methods, including immunological techniques (immunoassays, immunomagnetic separations), polymerase chain reaction (PCR), mass spectrometry, as well as the burgeoning field of DNA microarrays, are laying the groundwork for development of commercial kits and portable detection systems from easier handling. However, many of these complex methods and require excessively long analysis times.

On the other hand, the determination of the conductivity of bacterial particles is an important source of information about the physiological state of the cell. The conductivity of the bacterial particles is mainly determined by the number of charged groups present in the cell wall (gel-porous structure) and by the mobility of the counterions from / to it. Their experimental values have been obtained by different techniques, among which the article by A. Vander Wal, M. Minor, W. Norde, A.J.B. Zehnder and J. Lyklema, J. Colloid Interface Sci., 1997, 186, 71-79. VanderWal and colleagues observed that the surface conductivity of bacteria in suspension tends to increase slightly with the concentration of electrolyte in solution. However, the conductivity of the cells was insignificant compared to the electrolyte solution at both the highest and the lowest electrolyte concentrations tested. While the first is the expected behavior for dilute suspensions of non-conductive and non-charged spherical particles, the second observation was attributed to ion leakage through the cytoplasmic membrane. Consequently, the estimates of these authors for the conductivity of the cells, which are equivalent to those of an electrolyte solution of about 1-5 mM, were more reliable measured in the presence of electrolytes at a concentration of 0.01-0.04 M.

Subsequently, it was shown that the conductivity of the bacterial periplasmic space increases with the square root of the ionic strength of the medium (Hotzel, Biochim. Biophys. Acta-Mol. Cell Res., 1999, 1450, 53-60).

More recently, Suehiro detected an increase in conductance after the concentration of E. coli by dielectrophoresis on the surface of interdigitated microelectrode arrays (J Suehiro, R. Hamada, D. Noutomi, M. Shutou and M. Hara, J. Electrost., 2003, 57, 157-168). In addition, the increase in conductance was directly proportional to the amount of bacteria captured. DESCRIPTION OF THE INVENTION

The present invention describes a new strategy for the detection of bacterial presence in a solution using inductive transducers. In fact, the conductivity of the bacterial wall helps to increase the overall conductivity of a solution, which causes a change of the magnetic field around the inductive transducer and, therefore, also of its inductance. A simple and rapid measurement of the inductance of an inductive transducer exposed to a solution thus provides information about the conductivity of said solution. The advantages of this measurement strategy are an extremely fast response time, a test format without reagent or marker, and the exploitation of highly compatible devices with mass production and low cost. In particular, the inductive transducers employed in the present invention are inductive coils. The inductance L of a coil depends on its composition and design, although it also changes in the presence of metals in its vicinity or when changes occur in external factors such as light or temperature. These changes in the inductance make it difficult to acquire coherent or reproducible signals over time (for example, obtained on different days or in disparate locations), thus hindering its use as a sensor in certain applications. In addition, microfabricated coils using CMOS technology and located in remote positions within the same silicon wafer tend to have slight differences in composition and therefore different behavior. However, it has been found that the relationship between the inductances of adjacent coils in the wafer remains practically constant between certain temperature limits. This is due in part to the fact that adjacent coils tend to show similar characteristics as they have been manufactured at the same time and in similar conditions. In addition, the effect of external factors is similar in two nearby coils and conditions the response of both in the same way, something that translates into close values of inductance and resistance. The device of the present application takes advantage of these characteristics to perform differential inductance measurements: a first coil or active coil is contacted with a solution and its inductance is measured, this measurement is corrected with respect to that of a second coil or reference coil to compensate for changes in environmental conditions, and finally the inductance measurement of the active coil already corrected correlates with the concentration of bacteria in a solution.

According to a first aspect of the invention, a bacterial presence detection device is described which comprises two inductive transducers similar to each other configured so that one of them can be free while the other is exposed to a solution. In this document, we will refer to the inductive transducer as a reference to the transducer that remains free during the measurement procedure, while the inductive transducer exposed to the solution will be called an active transducer. In addition, the device of the invention is adapted for connection to an impedance analyzer to carry out the inductance measurements of the inductive transducers from which the bacterial presence or not in the problem solution will be deduced and, where appropriate , the concentration of bacteria according to the procedure that will be described later.

Although the device of the invention can be made using any type of inductive transducer whose inductance varies when modifies the magnetic field around it, according to a preferred embodiment of the invention inductive transducers are inductive coils covered by an insulating layer. The inductive coils can be individualized, that is, each formed on a different substrate to facilitate the deposition of a solution only on the active coil or to allow differential immersion measurements of the two coils in a solution, or be formed adjacent and coplanar on the same substrate. It is understood that, although the basic configuration of this device is carried out with a minimum of two coils, one reference and one active, it is possible to manufacture a device that has a variable number of reference coils and a variable number of active coils to facilitate the simultaneous taking of several measures. On the other hand, the coils of the invention can be of any size as long as they allow a volume of solution to completely cover the active coil, thus modifying its inductance to perform the measurement. Similarly, the distance between the two coils should be sufficient to expose the active coil to a solution without the solution affecting the reference coil. According to a preferred embodiment of the invention, however, the coils have a radius between 100 μηι and 1000 μηπ, being in this case normally manufactured using microfabrication techniques or by usual techniques in microelectronics, such as chemical vapor deposition, sputtering or the like.

Finally, in yet another preferred embodiment, the device of the invention comprises a guard ring connected to ground around each coil to reduce possible current leakage.

According to a second aspect of the invention, a method of detecting bacterial presence in a solution is described problem using a device comprising an active coil (A) and a reference coil (B) covered by an insulating layer and connected to an impedance analyzer. In this document, we will call the solution whose bacterial concentration one wishes to measure a "problem solution", and "reference solution" the solution without bacteria used as a reference. The procedure, therefore, comprises the following steps:

1) Perform at least one inductance measurement with both coils in contact with a reference solution without bacteria, obtaining the reference inductances LAref, Leref-

2) Measure the LAmed, Lemed inductances of the coils (2 _A , 2 _B ) with the active coil (A) in contact with the test solution and the reference coil (B) in contact with the reference solution without bacteria.

3) Deduce the presence and / or concentration of bacteria in the test solution based on the change of inductance of the active coil (A).

Preferably, the change of inductance of the active coil (A) is obtained by subtracting its reference inductance LAref from its corrected measurement inductance LAmed to compensate for environmental changes. Indeed, the LAmed measurement inductance is affected not only by the presence of the problem solution, but also by the change in environmental conditions. To compensate for this change, according to a preferred embodiment, the measurement inductance LAmed is multiplied by a factor obtained from the relationship between the values of the inductance of the reference coil (B) under the conditions of the first measurement and those of the second measure. Mathematically, it can be expressed as: ΔΙ_Α - L.Aref "l-Amed ^" l-Bref / l-Bmed

Thus, it is verified that changes in the corrected inductance of the active LA coil indicate the presence of bacteria in the test solution. Moreover, it is possible to perform a previous calibration of the LA values in the presence of solutions with known bacterial concentrations, thus being able to obtain an estimate of the concentration of bacteria in the test solution. Furthermore, it has been found that the inductance measurements performed by the impedance analyzer are preferably performed at frequencies between 4 MHz and 20 MHz, and more preferably even at frequencies between 4 MHz and 10 MHz. Preferably, in order to improve The accuracy of the measurements is made a plurality of inductance measurements with the reference solution and the reference inductances l-Aref, l-Bref are taken as the average of all of them. Preferably the reference solution without bacteria is a saline solution, more preferably a PBS saline solution, or a bacterial culture medium. In turn, the saline solution can be any solution whose conductivity has been adjusted by the addition of salts. Similarly, the test solution may be a dilution of the bacterial sample in a saline solution more preferably a PBS saline solution, or in a bacterial culture medium. In turn, the saline solution can be any solution whose conductivity has been adjusted by the addition of salts. Finally, according to another preferred embodiment, the described bacterial presence detection method uses as a solution for reference the matrix of the problem solution, which has previously been subjected to bacterial presence depletion.

BRIEF DESCRIPTION OF THE FIGURES

Figures 1 a and 1 b show respectively a perspective view and a cross section of the device of the invention.

Figures 2a and 2b show the average of the data obtained for each concentration of bacteria diluted in PBS and water respectively.

Figures 3a and 3b show the changes in inductance as a function of the salt concentration tested.

Figure 4 shows the changes in inductance as a function of the concentration of bacteria in mineral water samples, supplemented or not with salts to adjust the conductivity. DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred embodiment of the invention is described with reference to the attached figures. Fig. 1 a shows a perspective view of the device (1) of the invention where two microbobbins (2 _A , 2 _B ) formed on a substrate (3) can be seen. The detail view of Fig. 1 b shows a sectional view of one of the microbobbins (2 _A , 2 _B ) where the layers that form it are observed: a silicon oxide coated silicon layer as an insulator forms the base for the microbobbins themselves (2 _A , 2 _B ), which are protected by an insulating coating made in this example of Si0 ₂ and Si ₃ N ₄ .

EXAMPLE 1: Detection of bacteria in PBS saline medium The validity of this invention has been demonstrated by determining the presence of Escherichia coli (E. coli) bacteria, a recognized indicator of faecal contamination in water sources, in a PBS saline solution. For this, serial dilutions were made in PBS or mineral water, previously sterilized, from a stock solution of known concentration. This stock solution consisted of a night culture in LB culture medium, subjected to centrifugation for 10 minutes at 12,000 rpm, removal of the supernatant medium and resuspension of the precipitate in PBS or in water.

Pairs of microbobins (2 _A , 2 _B ) of 500 μηι radius manufactured by CMOS technology were used using 2 aluminum levels with a section of 5 μηι (thickness) x 1 .5 μΐη (height) and 49 turns each. Within each device, the two microbobbins (2 _A , 2 _B ) were located coplanarly and encapsulated in a T08 configuration. The microbobine contacts (2 _A , 2 _B ) moved as far as possible (avoiding excess resistance) from their active area to facilitate the deposition of the sample to be studied. Each micro coil (2 _A , 2 _B ) was surrounded by a protective ring (guard ring) to reduce possible current leakage.

To obtain measurements, a 0.5 μ 0.5 volume drop of sterile PBS or water as a reference solution, or a 0.5 μ gota volume drop of each of the bacterial concentrations was deposited on the active microcoil (2 _A ) prepared in PBS or sterile water. Next, the inductance in a frequency range between 4 and 20 MHz was measured and the change in the generated inductance L _A / L _B was calculated. For each concentration, four independent measurements were made (corresponding to independent sample depositions on the sensor coil), and for the reference solution a minimum of 10 independent measurements were made (corresponding to a minimum of 10 independent depositions of reference solution on the coil sensor). Each measurement was followed by washing with saline and water, as well as drying under a flow of nitrogen. The average of the data obtained for each concentration of bacteria was then compared with those obtained in reference solution (PBS or sterile water in the absence of bacteria) (Figures 2a and 2b).

In all experiments carried out in PBS, the presence of bacteria in the medium generated a detectable increase in inductance for E. coli concentrations of 500 to 1 x 10 ⁵ cells / μΙ (Figure 2a). Taking into account that the volume of drop deposited is 50 microliters, we are able to detect around 200 bacteria with this novel method. The change in inductance was most evident at the lowest frequencies studied (4-10 MHz). However, the presence of bacteria in mineral water does not generate significant inductance changes.

The dielectric constant of some particle suspensions increases significantly as the measurement frequency decreases, probably due to the tangential displacement of the contractions with respect to the surface of the charged particles. The "relaxation" of the counterions is one of the mechanisms responsible for the apparent capacitive behavior of the membrane of living cells. In this sense, living cells exposed to an electric field behave like small capacitors and accumulate electrical charges on the surface. The resulting permittivity would result from the accumulation of charges around cell membranes. Therefore, cell surfaces are characterized in solution by an increase in ion density in their immediate environment. This is mainly due to the characteristics of the wall surrounding the cell, which is basically composed of peptidoglycan, a component that is highly charged to physiological conditions and contributes significantly to the ion attraction of the solution to its proximity. EXAMPLE 2: Detection of salinity / conductance of a solution

As described in the previous sections, the impedance of an inductive coil (2 _A , 2 _B ) is sensitive to a number of external factors. As described in this invention, one of these factors is the conductance / salinity of the solution that is deposited on its surface. For this reason, we decided to investigate the minimum conductivity values necessary to activate the response of the microbobins (2 _A , 2 _B ) described above. The inductance changes recorded for serial dilution of a PBS saline solution are summarized in Figures 3a and 3b (the PBS solution contains 10 mM of PO ₄ buffer, 140 mM NaCI and 3 mM KCI, at pH 7.4). As seen in Figures 3a and 3b, the changes in inductance are correlated with the salt concentration tested. The linear detection range covers concentrations of PBS between 5 and 75 mM and the undiluted PBS (approximately 150 mm of salt) generates values close to the saturation of the device. PBS dilutions above 1: 64 (equivalent to a salt concentration of 4.87 mM) do not generate any detectable change in inductance. The lower limit of detection (LOD) for this salt mixture is 4.13 mM, calculated as the average of 10 independent targets (obtained in water without salt), plus 3 times its standard deviation.

EXAMPLE 3: Detection of bacteria in water samples The applicability of the invention to the study of real samples has been carried out by determining E. coli, inoculated at different concentrations (0-10 ⁶ cells / μΙ), in mineral water . The samples were then supplemented with salts (concentrated PBS) to a final concentration of 0-150 mM. As illustrated in Figure 4, the bacterial presence is not directly detectable in water, nor in the most dilute saline buffers studied (1: 8 dilution of PBS, equivalent to 19 mM salt concentration). When water is supplemented with salts up to a concentration equivalent to 1: 4 PBS (38 mM), the presence of bacteria can be detected above 10 ³ cells / μΙ but the limit of detection of the assay is above 10 ⁶ cells / μΙ. Only at equivalent conductivity or above that of the PBS can we detect E. coli in a range between 10 ² and 10 ⁶ cells / μΙ. Therefore, the present invention allows the study of water samples, once their conductivity has been properly adjusted.

Claims

one . Device (1) bacterial presence detector, characterized in that it comprises two similar inductive transducers configured so that one of them can remain free while the other is exposed to a solution, and which are adapted for connection to an impedance analyzer .

2. Device (1) bacterial presence detector according to claim 1, characterized in that the inductive transducers are coils (2 _A , 2 _B ) covered by an insulating layer.

3. Device (1) bacterial presence detector according to claim 2, wherein the coils (2 _A , 2 _B ) are arranged adjacently and coplanarly on the same substrate (3).

4. Device (1) according to any of claims 2-3, further comprising a guard ring connected to ground around each coil (2 _A , 2 _B ) to reduce possible current leakage.

5. Device (1) according to any of claims 2-4, wherein the radius of the coils (2 _A , 2 _B ) is between 100 μηι and 1000 μιτι.

6. Method for detecting bacterial presence in a problem solution using a device (1) comprising a pair of coils (2 _A , 2 _B ), respectively active coil and reference coil, covered by an insulating layer and connected to an analyzer of impedances, characterized in that it comprises the following steps:

- perform at least one inductance measurement with both coils

(2 _A , 2 _B ) in contact with a reference solution without bacteria, obtaining the reference inductances l-Aref, l-Bref of the coils (2 _A , 2 _B );

- measure the inductances l-Amed, l-Bmed of the coils (2 _A , 2 _B ) with the active coil (2 _A ) in contact with the problem solution and the reference coil (2 _B ) in contact with the reference solution without bacteria; Y

- deduce the presence and / or concentration of bacteria in the test solution based on the change in inductance of the active coil (2 _A ).

7. Method according to claim 6, wherein the change of inductance of the active coil (2 _A ) is obtained by subtracting its reference inductance l-Aref from its corrected l-Amed measurement inductance to compensate for environmental changes.

8. Method according to claim 7, wherein the change of inductance of the active coil (2 _A ) is obtained according to the following formula:

ΔΙ_Α = l-Aref "l-Amed ^" l-Bref / l-Bmed

9. Method according to any of claims 6-8, wherein the inductance measurements are performed at frequencies between 4 MHz and 20 MHz.

10. Method according to claim 9, wherein the inductance measurements are performed at frequencies between 4 MHz and 10 MHz.

1. Method according to any of claims 6-10, wherein the reference solution without bacteria is a saline solution or a bacterial culture medium.

12. Method according to claim 1, wherein the saline solution is a PBS saline solution.

13. Method according to any of claims 6-12, wherein the test solution is a saline solution or a bacterial culture medium.

14. Method according to claim 13, wherein the saline solution is a PBS saline solution.

15. Method according to any of claims 6-14, wherein the reference solution is obtained from the matrix of the test solution previously subjected to bacterial presence depletion.