Patent Publication Number: US-2021172002-A1

Title: Detecting cell vitality

Description:
FIELD OF THE INVENTION 
     The invention relates to detecting cell vitality, i.e. determining whether cells are alive (vital) or dead (non-vital). 
     BACKGROUND 
     Conventional methods of detecting the presence of vital, or viable, cells in a sample generally involves culturing the sample in a medium to determine whether, and to what extent, the cells reproduce. This type of test is typical for detecting the presence and quantity of bacteria in a sample. The requirement to culture a sample involves significant delays to allow a sample to be cultured, which may be of the order of days before a result is available. 
     Some fluorescent dyes, such as the Sytox® family of nucleic acid stains, can be used to detect cells with compromised membranes. This is because the dye is hydrophilic or charged and has no route by which it would ordinarily cross the lipid membrane. Cells with non-specific pores, such as those created by electroporation, will provide a path by which the dye can cross the membrane, bind to DNA and consequently produce a fluorescent signal. Cells with intact membranes, however, produce no signal, and these dyes cannot be used to detect vital cells. An indication of cell vitality can therefore only be made by discounting disintegrated cells. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect, there is provided a method of detecting vitality of cells in a sample, comprising:
         adding a fluorescent dye to the sample;   disposing a portion of the sample in a test volume between a pair of electrodes;   applying a voltage across the pair of electrodes to generate an electric field across the portion of the sample;   illuminating the test volume;   measuring a fluorescence response from the test volume over a period of time after applying the voltage; and   detecting cells in the sample to be vital dependent on a change in their fluorescence response over the period of time.       

     The change in fluorescence response may for example be an increase or a decrease in response over the period of time, depending on the type of fluorescent dye used to detect the cellular membrane potentials. 
     The method addresses the problem of how to rapidly and reliably detect vital cells in a sample. The method is capable of detecting such cells in a much shorter time, and directly on the sample in question, compared with conventional techniques involving culturing. 
     The electric field in typical applications may be less than 1 kV/cm. The aim in this case is not to open pores in the cell membrane, as with electroporation, but instead to preferentially accelerate introduction of the fluorescent dye for vital cells but not for non-vital cells. The electric field may in typical applications be greater than 100 V/cm, with an example range between around 100 V/cm and 800 V/cm or between around 400 and 800 V/cm. The method preferably does not involve electroporation of the cells. In other words, the electric field is preferably selected such that electroporation of the cells does not occur while the voltage is applied. 
     The electric field may be alternating, for example applied in the form of one or more pulses, so that electrolysis of the sample is reduced or avoided. The voltage may be applied for only a short period such as less than 10 s or less than 5 s. A minimum time period over which the voltage is applied may be as short as 0.1 s. 
     The method may further comprise the step of detecting cells in the sample to be non-vital cells if their fluorescence response decreases or does not rise over the period of time. 
     The period of time over which detection is carried out may for example be up to 30 minutes, or in some cases may be less than 60 s or 30 s, and may be less than 10 s, and optionally greater than 0.1 s. 
     The fluorescent dye may be a substance which produces a fluorescent response that indicates a changing electrical field or ion gradient across a cellular membrane. This may be a Nernstian dye, for example a flavin, such as thioflavin T. The dye does not, however, need to be Nernstian, and may instead be a substance that can travel or change its conformation as a result of membrane potential changes in response to voltage stimulation. 
     In some implementations, the sample may flow through the test volume from a first location to a second location of the test volume over the period of time, the steps of illuminating the test volume with light and measuring a fluorescent response being repeated at the first and second locations. 
     The step of detecting may comprise detecting vital cells in the sample if their fluorescence response increases, for example by more than a factor of two, over the period of time. 
     The method preferably relates to detection of vitality or viability of bacterial cells, i.e. the ability of such cells to proliferate. The method may therefore be used to detect a strain of bacteria in a sample, by first treating the sample with an antibiotic, and then performing the method to detect bacteria resistant to the antibiotic. The bacteria may for example be a coliform such as  Escherichia coli , which is resistant to certain antibiotics including penicillin and vancomycin. 
     In accordance with a second aspect, there is provided an apparatus for detecting vitality of cells in a sample, the apparatus comprising:
         a sample holder for containing the sample, the sample holder comprising a pair of electrodes on opposing sides of a test volume;   a voltage generator connected to the pair of electrodes;   a light source arranged to illuminate the test volume; and   a light detector arranged to detect a fluorescence response from the test volume upon illumination by the light source.       

     The sample holder may comprise a transparent substrate having a surface on which the pair of electrodes is provided. 
     The light source may be arranged to illuminate the sample from an opposing side of the test volume to the light detector. 
     The apparatus may comprise an optical fibre plate between the sample holder and the light detector, wherein the light detector is an image sensor. A numerical aperture of the optical fibre plate is preferably less than 1, such that incident illumination (excitation) light from a shallow angle relative to the substrate is not transmitted through the optical fibre plate to the detector for fluorescently emitted light. 
     If the light source is a first light source arranged to illuminate a first portion of the test volume and the light detector is a first light detector arranged to detect fluorescence light emitted from the first portion of the test volume, the apparatus may comprise a second light source arranged to illuminate a second portion of the test volume and a second light detector arranged to detect fluorescence emitted from the second portion of the test volume, the sample holder configured to allow the sample to flow through the test volume from the first portion to the second portion. 
     The method according to the first aspect is preferably at least partially automated by being carried out under the control of a suitably programmed computer controller. The method may therefore be alternatively defined as a method of detecting vitality of cells in a sample, the method comprising:
         applying a voltage across a pair of electrodes in a test volume containing the sample and a fluorescent dye to generate an electric field across the portion of the sample;   illuminating the test volume;   measuring a fluorescence response from the test volume over a period of time after applying the voltage; and   detecting cells in the sample to be vital dependent on a change in their fluorescence response over the period of time.       

     Other optional and advantageous features of relating to the method of the first aspect may also apply to the above alternatively defined method. 
     The apparatus according to the second aspect may comprise a controller that is configured to perform the above alternatively defined method. 
     In accordance with a third aspect therefore, there is provided a computer program for instructing a computer to perform the method according to the above alternative aspect. The computer program may be embodied on a non-transitory storage medium such as a read-only memory. 
    
    
     
       DETAILED DESCRIPTION 
       The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of an example apparatus for measuring the vitality of cells in a sample; 
         FIG. 2  is a schematic plan view of an array of interdigitated electrodes for use in the apparatus of  FIG. 1 ; 
         FIG. 3  is a micrograph of a sample of cells after application of an electric field in the presence of a fluorescent dye; 
         FIGS. 4 a , 4 b  and 4 c    are images of fluorescence from cells at different times during the application of an electric field; 
         FIG. 5  is a plot of relative fluorescence response over time during the application of an electric field for two selections of cells in a sample; 
         FIG. 6  is a further plot of relative fluorescence response over time during the application of an electric field for two selections of cells in a sample; 
         FIG. 7  is a schematic diagram of an alternative example apparatus for measuring the vitality of cells in a sample; 
         FIG. 8  is a schematic diagram of a further alternative example apparatus for measuring the vitality of cells in a sample; 
         FIG. 9  is a schematic flow diagram illustrating an example method for detecting vitality of cells in a sample; 
         FIG. 10  is a schematic diagram of an example apparatus for measuring the vitality of cells in a sample; 
         FIG. 11  is a schematic diagram of a circuit arrangement for the apparatus of  FIG. 10 ; 
         FIG. 12  is a schematic sectional view of an example optical assembly for the apparatus of  FIG. 10 ; 
         FIG. 13  is a plan view of an example electrode layout for a sample holder; 
         FIG. 14  is a plan view of a portion of the example electrode layout of  FIG. 13 ; and 
         FIG. 15  is a schematic diagram of an arrangement of a sample holder with the electrode layout of  FIG. 13 . 
     
    
    
     The change in membrane potential by an external electrical field (ΔΨ) is quantitatively described by the Schwan equation: ΔΨ=1.5 r E cos θ. Here, r is the radius of an idealized spherical cell, E is the field strength of external electrical field, and θ is the angle to the electrical field. 
     According to this equation, an external electrical field can open voltage-gated potassium channels by altering the transmembrane potential of a cell. The Schwan equation enables a coarse estimation that the field strength required to cause the gating of a voltage-sensitive channel on a bacterial membrane is approximately in the range of 400˜800 V/cm. 
     Opening of potassium channel lead to hyperpolarization of the cellular membrane when cells are vital because vital cells store a large amount of potassium in the cytoplasm. In non-vital cells, metabolism is limited or non-existent so there is no metabolic energy available to restore the potassium ion gradient. This means the membrane potential collapses, i.e. depolarises, when potassium channels open upon stimulation by an electrical pulse. The addition of a fluorescent dye that responds to a change in membrane polarization may therefore be used to determine whether cells are vital depending on how the dye responds to the cell membrane being subject to an external electric field. Hyperpolarization may therefore be shown as a fluorescence increase when a Nernstian dye is used, or may be shown as a decrease in fluorescence signal for other types of dyes. A fluorescent dye such as FluoVolt®, for example, decreases its fluorescence signal when hyperpolarized. Other fluorescence indicators for membrane potential may change their spectra according to membrane polarity, such as di-4-ANEPPS dyes. 
     Although the primary mechanism involved in introducing molecules is considered to be the opening of selective ion channels due to an applied electric field, other mechanisms may be operating. For example, an electrical pulse may temporarily change the ion concentration in the extracellular medium, changing the ion gradient across the cell membrane and causing efflux, such that there is no change in transporter state but a change in gradient that drives transport. The response could also be in part be due to the induction of stress in the cells, which can also trigger ion channel opening, especially potassium channels. This stress could be due to the electrical signal or chemical substances generated electrochemically during application of an electrical pulse. The response of the cells could also be in part due to the production of reactive species by an electrochemical reaction caused by the flow of electrical current between the electrodes. These species may modify the channels or trigger a stress response. Such species may be reactive oxygen species or oxidising chlorine compounds. All of the above described mechanisms have the common property that they involve modifying the conductivity of ion channels in the cell membrane or the rate of transport of ions, and consequently the movement of a molecule into the cell. 
       FIG. 1  illustrates an example apparatus  100  for measuring the vitality of cells in a sample. The sample  101  is placed within a sample chamber  102  on the surface of a substrate  103 , on to which electrodes (not shown in  FIG. 1 ) have been applied. The sample  101  contains a medium within which is provided cells and a fluorescent dye that may be absorbed through the cell membranes. The medium may for example be in the form of a liquid or gel medium. 
     Optics  104  are provided adjacent the substrate  102 , the optics  104  comprising a light source  107  arranged to direct light on to the sample  101  and a light detector arranged to detect light fluorescing from the sample  101  in response. In the illustrated example the light source  107  is positioned on the same side as the optics for viewing the sample  101 . In alternative embodiments, the light source may be positioned on the opposite side to the optics. The light source may comprise a ring of sources, such as LEDs or lamps, arranged in a ring or above the sample. The angle of the incident light in each case is preferably such that an angle of incidence of light on to the sample  101  is sufficiently low to avoid direct entry of light into the optics for detecting fluorescence. This minimises direct illumination of the detector by the fluorescent excitation light source. 
     A voltage generator  105  is connected to the electrodes on the substrate  103 , for providing a controllable electrical field across portions of the sample  101 . An example of the type of electrodes that may be provided on the surface of the substrate  103  is illustrated in  FIG. 2 . The electrodes  201 ,  202  are interdigitated, providing regions between adjacent opposing fingers  203 ,  204  of the electrodes within which a controllable electric field may be applied to a portion of the sample  101  placed on the surface of the substrate  103  over the electrodes  201 ,  202 . 
     A controller  108  is connected to the voltage generator  105 , light source  107  and optics  104  and configured to control the voltage generator  105  and acquire images from the optics  104  over a defined period of time a voltage signal is applied to the sample  101 . The controller  108  may for example acquire a first image at the onset of the period of time before the voltage signal is applied, then a second image at the end of the period of time the signal is applied, and calculate a third image based on a difference between the first and second images, such as differences in luminance levels, outputting the third image to determine cells that show a change in fluorescence over the period of time. 
     To make the sample chamber, an array of interdigitated or evenly spaced signal and ground electrodes, which may for example be made of Ti—Au, indium tin oxide (ITO) or another conductive material, may be deposited onto a glass or other transparent hard substrate by a deposition technique such as physical or chemical vapour deposition or by screen printing. 
     The substrate  103  may be of a type allowing clear optical imaging at high resolution using common microscope objectives of a high numerical aperture from the reverse side of the transparent substrate, as shown in the arrangement in  FIG. 1 . This means the substrate will typically be optically clear and thinner than 0.2 mm. Multiple sets of electrodes may be deposited on the substrate  103  to allow multiple samples to be analysed either consecutively or simultaneously. 
     External circuitry such as the voltage supply  105  may be connected to the substrate  103  using contact pads and standard electrical connectors. 
     Spacing between the electrodes will typically be less than 100 microns, with a preferred spacing of around 50 microns. A common spacing may be used across multiple electrode pairs so that a constant electric field can be applied with the same voltage. Using this range of electrode spacing, an electrical field of around 60 V/mm can be applied without the total voltage applied between the electrodes causing electrolysis of the sample. 
     The substrate  103  may be encased in a gas and/or moisture tight housing  106 . The housing  106  may be transparent to allow light to pass through the sample from above and below. The light source may be provided on either side of the substrate in the embodiment in  FIG. 1 . 
     This voltage source  105  may be configured to generate electrical signals as well as recording these signals, and may be arranged to selectively apply and monitor voltage signals on multiple sets of electrodes on the substrate, allowing voltages supplied to different samples to be controlled. 
     In an experimental setup, bacterial cells were inoculated on agarose pads and placed on the electrode surface, enabling monitoring of cells at single-cell resolution by microscopy. The membrane potential dynamics of cells was monitored using the fluorescent Nernstian indicator dye, Thioflavin T (ThT), which has been used extensively with bacteria. 
     When a series of electrical pulses (±1.5 V AC 0.1 kHz for 2.5 seconds) were applied to  E. coli  (K12 strain) cells placed in the electrode gaps, the pulses caused hyperpolarization (seen by an increase of ThT fluorescence signal) in vital cells.  FIG. 3  illustrates the effect of applying an electric field across a sample  304  as viewed between a pair of opposing electrodes  301 ,  302  in an apparatus of the type illustrated in  FIG. 1 . Before the application of an electric field, a portion of the sample was subjected to ultraviolet radiation sufficient to kill cells within the defined region  303  of the sample holder. An electric field was then applied, and the image shown represents the difference in fluorescence over a period of time after the application of the electric field. As can be seen, the change in fluorescence in the region  303  that was subjected to UV radiation can be distinguished from the change in fluorescence for the rest of the sample  304 . Whereas the region of the sample unaffected by UV radiation showed an increase in fluorescence, the region  303  affected by UV did not. This provides a clear indication that the change in fluorescence is an indication of the vitality of cells within the sample. 
       FIGS. 4 a  to 4 c    shows microscope images at different times during the application of an electric field to a sample of  Bacillus subtilis . A signal of 3V in amplitude was applied to the sample held between electrodes with a spacing of 50 μm, giving a peak electrical field strength of between 400 and 800 V/cm. This compares with electroporation of bacteria, which may use a minimum of 3,000-24,000 V/cm.  FIG. 4 a    shows the sample within 1 second of the electric field being applied, showing a single bacterium  401  fluorescing.  FIG. 4 b    shows the sample after a further second, and two further bacteria  402  can now be seen to be fluorescing, while the strength of fluorescence of the first bacterium  401  decreases.  FIG. 4 c    shows the sample after a further two seconds, and the two further bacteria are now strongly fluorescing while fluorescence from the first bacterium has remained low. This example shows that a measure of vitality of cells can be obtained in a matter of seconds, demonstrating that the period of time the electric field needs to be applied before a measure of vitality is possible may be less than 10 seconds, and could be less than 5 seconds or even less than 3 seconds. 
     Living cells accumulate a Nernstian dye to much higher levels than the surrounding medium. This process is selective for Nernstian dyes which can already cross the cell membrane and excludes dyes such as Sytox®, which do not pass through living cell membrane. The process does not cause transport of the dye by electroporation of the cells, which is only achieved at considerably higher electric field levels, but instead through enhancement of transport across a living cell membrane with an existing cell potential. 
       FIG. 5  shows the results of subjecting a sample of  Bacillus subtilis  to selective UV radiation treatment and subsequent treatment by the application of an electric field. The sample (shown in  FIG. 3 ) was subjected to alternating 3V pulses at a frequency of 100 Hz for 2.5 seconds. The mean response  501  of ten UV-treated cells is shown in  FIG. 5  alongside the mean response  502  of ten non-UV-treated cells. A cell validity index, being a logarithmic scale of the ratio between current and initial fluorescence shows a relative increase or decrease in fluorescence, indicating clearly the difference in response between live and dead cells. Live cells in this example showed a substantial increase, whereas dead cells showed a substantial decrease. 
       FIG. 6  shows the log of a fluorescent response of the two cells types,  B. subtilis  and  E. coli  in the presence of vancomycin. Vancomycin is inactive against coliforms such as  E. coli , but targets almost all other types of bacteria. In the mixed culture. While vancomycin treated  B. subtilis  cells showed depolarization  601 ,  E. coli  cells were hyperpolarized  602 . This demonstrates that the process can selectively show the vitality of coliforms in a mixed culture. The presence of  E. coli  or other coliforms could therefore be confirmed through fluorescence analysis of a sample treated with a selective antibiotic such as vancomycin. 
       FIG. 7  shows a further example of an apparatus  700  for detecting vitality of cells in a sample. In this case the apparatus comprises a test volume  701  for containing a sample between a transparent lid  702  and an optical fibre plate  702 . Electrodes are provided on one or both of the surfaces  703 ,  704  of the lid  702  and optical fibre plate  702  defining opposing faces of the test volume  701 . The electrodes may for example be in the form of pairs of electrodes on one of the plates, an interdigitated electrode pattern, or may be provided on both plates in the form of transparent electrodes, where the gap between the plates defines the electric field across the sample. 
     A voltage generator  705  is connected to the electrodes, and a light source  706  arranged to illuminate the test volume  701  through the lid  702 . The light source  706 , two of which are shown in  FIG. 7 , may be arranged to illuminate the test volume  701  at a shallow angle, for example at less than 45° to the surfaces  703 ,  704 . The effect of this is to avoid light passing directly into the optic fibre plate  702  due to the numerical aperture of the plate  702  limiting the angle at which light can be passed into the fibres of the plate  702 . 
     A light detector  706  is provided on an opposing side of the optical fibre plate  702 . The light detector  706  may be in the form of an imaging sensor such as a CCD. Interposing the optical fibre plate  702  between the imaging sensor  706  and the test volume  701  allows the sensor  706  to directly image the cells within the test volume without the need for intervening lens arrangements. 
     A filter  707  may be provided between the imaging sensor  706  and the test volume  701 , in this case between the imaging sensor  706  and an adjacent face of the optical fibre plate  702 . The filter  707  may selectively filter out a range of wavelengths of light including that emitted by the light source  706 . The light source may for example emit blue light, and the filter  707  may allow transmission of green light to allow fluorescence from the test volume to be passed to the imaging sensor  706 . 
     A controller  708  is connected to the voltage generator  705 , light source  706  and imaging sensor  706  and configured to control the voltage generator  705  and acquire images from the imaging sensor  706  over a defined period of time a voltage signal is applied to the sample  701 . The controller  708  may for example acquire a first image at the onset of the period of time before the voltage signal is applied, then a second image at the end of the period of time the signal is applied, and calculate a third image based on a difference between the first and second images, such as differences in luminance levels, outputting the third image to determine cells that show a change in fluorescence over the period of time. 
     Further details of example apparatus and experimental results for  B. subtilis  and  E. coli  are provided in GB application 1817435.9, to which this application claims priority and which is incorporated herein by reference in its entirety. 
       FIG. 8  illustrates a further alternative apparatus  800  for detecting the vitality of cells in a sample. The apparatus comprises a sample holder  801  in the form of a flow channel in a microfluidic device  802 , arranged to flow a sample from an inlet  803  to an outlet  804 . The channel  801  may be sized to allow cells to flow through the device  802  one at a time so that fluorescence measurements can be taken on individual cells. A pair of electrodes may be provided on opposing walls of the flow channel, the electrodes connected to a voltage generator  805 . 
     The apparatus  800  comprises two sets of light sources and light detectors. A first light source  806   a  transmits (excitation) light to illuminate a first part of the flow channel  801 , the light being transmitted via a dichromic reflector  807   a  and into an optical fibre  808   a  via a collimating lens  811   a  to the flow channel  801  via a further lens  812   a . Fluorescence emission light from the flow channel  801  at a different wavelength is transmitted back along the optical fibre  808   a  and through the dichromic reflector to a light detector  809   a  via a filter  810   a.    
     A second light source  806   b  transmits light to illuminate a second part of the flow channel  801  downstream from the first part, the light again being transmitted via a dichromic reflector  807   b  and into an optical fibre  808   b  via a collimating lens  811   b  to the flow channel  801  via a further lens  812   b . Fluorescence light from the flow channel  801  at a different wavelength is transmitted back along the optical fibre  808   b  and through the dichromic reflector to a light detector  809   b  via a filter  810   b.    
     Excitation light may for example be provided by a 405 nm laser diode, and a beam focused onto a 10-20 um spot on the flow channel  801  in the microfluidic device  802 . If a cell is present, fluorescence light is emitted which retraces the path of the excitation light back through the fibre towards the source, but instead of being reflected into the source, passes the dichroic mirror to the photodetector  809   a . Two identical setups are used to quantify cell fluorescence by allowing a comparison between fluorescence as a cell passes the first part of the flow channel and then as the same cell passes the second part of the flow channel. Knowing the speed at which the cells pass through the flow channel allows a measure of relative change in fluorescence to be determined on a cell-by-cell basis. Each light detector  809   a ,  809   b  may for example be a photomultiplier tube or an avalanche photodiode. 
     A controller  818  is connected to the voltage generator  805 , light sources  806   a ,  806   b  and detectors  809   a ,  809   b  and configured to control the voltage generator  805  and acquire signals from the detectors  809   a ,  809   b  over a defined period of time a voltage signal is applied to a sample flowing through the device  802 . The controller  808  may for example store successive readings of fluorescence from the detectors  809   a ,  809   b  at time intervals corresponding to a time taken for a part of the sample to flow between the first and second parts of the flow channel  801 , and output a difference between these readings that corresponds to the change in fluorescence. 
       FIG. 9  illustrates a schematic flow diagram showing a method of detecting vitality of cells in a sample. The method starts by providing a sample comprising cells and a fluorescent dye (step  901 ). The sample, or a portion thereof, is disposed in a test volume between a pair of electrodes (step  902 ). A voltage is then applied (step  903 ) between the pair of electrodes to generate an electric field across the portion of the sample in the test volume. The test volume is illuminated (step  904 ) and a fluorescence response is measured (step  905 ) over a period of time after applying the voltage across the electrodes. Cells in the sample are then detected as being vital (step  906 ) if their fluorescence response increases over the period of time. At least steps  903  to  906  may be automated and performed by a suitably programmed controller. 
       FIG. 10  illustrates an example apparatus  1000  for measuring the vitality of cells in a sample. The sample is contained in a holder  1001  mounted on a translation stage  1002 , which allows the sample holder  1001  to be moved in two directions in an x-y plane as with a conventional microscope translation stage. The stage  1002  preferably allows for movement of a sample slide (not shown) attached to the holder  1001  to allow sequential readings to be taken in two or more individual positions on the slide. An example slide  1501  and holder  1001  are shown in further detail in  FIG. 15 , the slide  1501  in this example having three locations where a sample may be deposited. The slide  1501  inserts into the holder  1001  and comprises an electrode layout with edge contacts  1502  for attaching to a PCB edge connector  1503 . 
     Referring again to  FIG. 10 , the apparatus  1000  comprises an optical assembly  1010  having a lens assembly  1003  and image sensor  1004 , which are translatable relative to the sample holder  1001  in the z direction, i.e. orthogonal to the x-y plane across which the sample holder  1001  is translatable. Translation of the optical assembly  1010  allows for focusing of the image sensor  1004  on to the sample in the sample holder  1001 . The image sensor may comprise a CMOS, CCD or other type of sensor for providing an electrical signal from a spatially structure light input. A controller  1005 , which may be part of, or separate from, the optical assembly  1010 , comprises a microcontroller and a board level computer, which controls electrical stimulation to the sample through the edge connector  1503  and controls the image sensor  1004 , processing image data to provide a required output signal, as described above and in further detail in GB1817435.9, incorporated herein by reference. An interface with the controller  1005  may be provided via a screen  1006  on a casing  1007  of the apparatus  1000 , which contains all of the above mentioned components. The screen  1006  may for example be a touch screen to allow a user to control the apparatus and to display data, including bacterial cell counts. 
       FIG. 11  illustrates an example layout of the controller  1005  based around a Lattepanda minicomputer  1101 . The computer  1101  interfaces via an LCD screen output  1102  and touch screen input  1103 . An interface board  1104  connects the computer  1101  to a light source  1105 , which in this example is provided by multiple (e.g. six) LEDs, and to the sample slide via a connector  1106 . A signal generator circuit  1107  provides a driving signal to a DAC  1108 , which outputs signals to the light source  1105  and selectively to one or more electrodes of the sample slide. 
     A detailed view of an example optical assembly for illuminating a sample is shown in  FIG. 12 . A light source  1201 , for example an LED, provides incident light for the sample  1202 , the light passing through a lens  1203 , filter  1204  and light pipe  1205  before reaching the sample  1202 . Light from the sample is received via a lens assembly  1206 , which focuses incident light for the image sensor described above. In a general aspect, the test volume of the sample comprising cells and a fluorescent dye is illuminated by light from a light source  1201  that is incident upon the sample at an oblique angle to a plane of the sample holder. The incident light may be directed on to the sample via an optical guide  1205 . An advantage of this arrangement is that any stray light from the light source  1201  incident on the lens assembly is reduced or minimised. 
       FIG. 13  illustrates an example electrode layout for a sample slide. Three regions  1301   a - c  are provided on the layout for depositing samples. Each region comprises an interdigitated electrode array, a detailed view of which is shown in  FIG. 14 . The spacing in the electrode array may for example be around 0.05 mm, with each electrode track being around 0.07 mm wide. 
     Other embodiments are intentionally within the scope of the invention as defined by the appended claims.