GB2047401A

GB2047401A - Apparatus for performing an antibiotic assay

Info

Publication number: GB2047401A
Application number: GB8005895A
Authority: GB
Original assignee: Compagnie Generale dAutomatisme SA
Current assignee: Compagnie Generale dAutomatisme SA
Priority date: 1979-02-23
Filing date: 1980-02-21
Publication date: 1980-11-26
Also published as: GB2047401B; FR2449891A1; JPS55114284A; DE3006028C2; IT8067274A0; IT1128238B; FR2449891B1; DE3006028A1

Abstract

The apparatus is arranged to receive a transparent plastics tray (1) having rows of cells (4), each containing a mixture of a cultured micro-organism which has been incubated in the presence of an antibiotic under test. The mixture differs from one cell to the next in terms of antibiotic concentration, antibiotic used and/or micro-organism used. The tray is made to advance through the apparatus (e.g. on guides 22 and driven at 23). A parallel beam of light is directed down on each cell to give a spot size just smaller than the base of the cell. The cloudiness of the culture is measured as a function of the ratio between the directly transmitted light (photo-diode 10 Fig. 2) and light which is diffused obliquely to the direct light (photo-diodes 11 and 12). Electronic means are arranged to use the measurement as a pass/fail test for the antibiotic in each cell (clear = pass, cloudy = fail). <IMAGE>

Description

SPECIFICATION Apparatus for performing an antibiotic assay The invention relates to an apparatus for performing an antibiotic assay.

To study the efficiency of antibiotics on a given culture of micro-organisms, it is known to use a plastics tray which has a plurality of cells in each of which the effectiveness of a particular antibiotic with respect to said culture is tested, the antibiotic varying from one cell to another. After incubating the culture with the antibiotic, each cell is examined and its transparency or its cloudiness indicates the effectiveness or ineffectiveness of the corresponding antibiotic.

The above examination is made with the naked eye and the result which corresponds to each antibiotic is recorded in the form of a table. The table taken as a whole is called an "antibiogram".

Preferred embodiments of the present invention produce such an antibiogram, but automatically, i.e. examination of the cells after incubation and recording the results are no longer performed by eye and by hand, but by optical means associated with an electronic circuit and with read out means used for the purpose.

The invention provides apparatus for performing an antibiotic assay, said apparatus including means for receiving a transparent plastics tray which includes at least one row of identical cells each containing a culture of a micro-organism together with an antibiotic; said apparatus comprising: means for producing a parallel light beam which is directed perpendicularly to the surface of the contents of the cells, said light beam having a crosssection which is substantially equal to, but less than, the base area of each cell to avoid touching the side wall thereof; a photodiode support situated on the oposite side of the cell with respect to the means for producing the parallel light beam, said support supporting at least two photodiodes a first photodiode being disposed to measure intensity of the light after passing in a first direction directly through the cell, and a second photodiode being disposed to measure the intensity of the light beam propagating in a direction oblique to the first after diffusion by the culture, the centre of the light beam being aligned with the centre of the first photodiode; means for supporting said strip of cells and moving it in translation so that the cells cross the light beam; synchronizing means which start measurement by the photodiodes each time the centre of a cell coincides with the centre of the light beam; and electronic means for calculating the ratio between the two measured light intensities, said electronic means including a threshold circuit and being associated with read-out means which, for each cell, supply data which varies according to whether the threshold is reached or not, said data indicating the effectiveness or ineffectiveness of the correspond ing antibiotic.

Preferably, the photodiode support is directed such that said oblique measurement direction coincides with a direction in which the inherent diffusion of the bottom of the cell is at a minimum.

Advantageously, said oblique measurement is made by two symmetrically disposed diodes whose output signals are electrically combined, e.g. by parallel connection.

The synchronizing device may include a photo-electric arrangement which co-operates either with opaque marks on the strip which are in line with the centre of each cell or with orifices in the support of the strip, said orifices likewise being in line with the centres of the cells.

The invention will be better understood from the following description of an embodiment given hereinafter with reference to the accompanying drawings in which: Figures 1 and 2 are two mutually perpendicular schematic views of a device in accordance with the invention for producing an "antibiogram" automatically; Fig. 2 being a cross-section along ll-ll in Fig. 1; and Figure 3 is a circuit diagram of the device.

The device illustrated in Figs. 1 and 2 is designed to measure each antibiotic at two different concentrations C, and C2. It includes a tray 1 which has two rows 2 and 3 of cells 4. The cells 4 contain a gel containing an antibiotic. In the same row, e.g. 3, the cells each contain a different antibiotic at a concentration C, and in the next row 4, the cells contain respectively the same antibiotics as in the row 3, but at a higher concentration, namely, C2. When the measurement is made, a few drops of liquid containing bacteria or some other micro-organism are added to each cell and the strip is placed in an incubator.

When the strip 1 is removed from the incubator, it is placed on a support 5 which is shaped like a mould to receive the strip.

A photodiode support 6 is placed below the support 5.

The photodiode support 6 includes two series of three orifices 7,8,9 to accomodate the photodiodes for examining the contents of the cells of the row 2 and of the row 3.

In Fig. 2, which is a cross-section along Il-Il in Fig. 1, the central orifice 7 is equipped with a photodiode 10 and the oblique orifices 8 and 9 are respectively equipped with photodiodes 11 and 1 2. Of course, the orifices 7,8 and 9 in the photodiode support 6 which are situated below the row of cells 2 are also equipped with photodiodes 1 0,11 and 12.

A parallel light beam 1 3 is directed perpendicularly to the surface 14 of the contents of the cells. A diaphragm 1 5 divides said beam into two beams 1 6 and 1 7 so that the diame ters of these beams are substantially equal to the bases 1 8 of the cells 4 without the beams touching the side edges 1 9 of the cells when the cells are centred exactly below the beams 16 and 17.

The strip support 5 is fixed on a frame 20 which can be moved longitudinally in both directions shown by an arrow 21, by guide means 22 and by control means 23.

A mark 24 is cut on the tray 1 at the alignment point of the centres of each pair of cells (a pair of cells being constituted by a cell 4 of the row 2 and a cell 4 immediately next to it in the row 3). An orifice 25 is provided in the strip support 5 below each mark 24.

A photo-electric device formed by a light source 26 and a detector 27 is placed perma nently so that the light ray 28 is situated in the plane which passes through the axes of the light beams 16 and 17.

The measurement principle is as follows: If the antibiotic contained in a cell 4 is effective with respect to the bacteria considered, the contents of the cell remains clear after incubation and the light beam passes through the parallel-sided optical plate constituted by the contents of the cell without diffusing; the central photodiode 10 then receives a light intensity X, and the oblique photodiodes 11 and 1 2 receive a light intensity X2 which is almost zero.In contrast, if the bacteria proliferate due to the ineffectiveness of the antibiotic, the contents of the cell is cloudy and diffuses the light in all directions such that the light intensity received by the oblique photodiodes 11 and 12 is almost the same as that received by the central photodiode 1 0. Then the ratio between the light intensity received by the oblique photodiodes and that received by the central photodiodes is measured. The ration varies from one cell to another and a threshold is defined below which it is considered that the antibiotic is effective and above which it is ineffective. The measurement for each antibiotic is taken at two different concentration C, and C2 > C,; that is why there are two rows of cells. Therefore, the result for each pair of cells is recorded.

The result may be: -definitely positive if the antibiotic is effective both in the cell where the concentration is C, and in that where the concentration is C2; -definitely negative if the antibiotic is ineffective at both concentrations; and -intermediate between positive and negative if the antibiotic is effective at concentration C2 and ineffective at concentration C, < C2; and -erroneous if the antibiotic is found to be effective at concentration C, and ineffective at concentration C2.

Known methods based on electronic equipment are used to calculate the ratio between the light intensities and to compare the intensities with a threshold.

Likewise, logic equipment can used to generate a positive, negative, intermediate or erroneous signal according to the result which is recorded automatically by means of a small printer, or stored in a computer, or transmitted to any other user equipment.

The photo-electric arrangement 26, 27 acts as a synchronizer so as to take the measurement into consideration only when a ray 28 coincides with a mark 24 while the tray 1 is being driven continuously forward.

It is advantageous to place the two oblique photodiodes such as 11 and 1 2 in a direction perpendicular to the maximum diffusion direction of the plastics tray 1. Indeed, said strip is manufactured by punching out from a hot sheet which, in the majority of cases, is itself obtained by drawing or rolling. There is almost always a direction of the strip which provides optimum optical diffusion.

Further, the use of two oblique photodiodes 11 and 1 2 whose signals are combined re- duces the consequences of any possible slight defect of the synchronization or of the centring.

Now, with reference to Fig. 3 which is a block diagram of the circuit, the photo-electric cells 10 and 11 or 1 2 are connected respectively via connections abto adjustable gain amplifiers 30, 31 to compensate the individual differences in sensitivity of these cells. A circuit 32 of known type supplies an analog signal which is proportional to the ratio between the signals supplied by the amplifiers 30 and 31. A threshold circuit 33 which can be adjusted to the initial setting of the device supplies a logic signal whose value indicates whether the signal supplied by the circuit 32 is higher or lower than a given value.

The photo-electric cell 27 is connected via a connection cto a threshold circuit 35 which delivers a logic signal when the cells 4 are centred under the beams 16, 1 7. The output signal of said circuit 35 causes the output signal of the threshold circuit 33 to be stored in a memory 34, then starts the operation of read-out means 36 which, according to choice, may be a printer, a computer, telecommunication equipment or any other suitable means.

In the case where two rows 2,3 of cells are used as illustrated in Fig. 1, where said cells contain antibiotics at different concentrations, there is twice the number of measurement systems which are connected together in a circuit which is not illustrated, situated either between circuits 33 and 34 or between circuits 34 and 36. Said circuit supplies the readout means 36 with a signal which is a function of the values of the two logic signals supplied by the two measurement systems.

Alternatively, said function may be performed by the read-out means 36.

Claims

1. Apparatus for performing an antibiotic assay, said apparatus including means for receiving a transparent plastics tray which includes at least one row of identical cells each containing a culture of a micro-organism together with an antibiotic; said apparatus comprising: means for producing a parallel light beam which is directed perpendicularly to the surface of the contents of the cells, said light beam having a cross-section which is substantially equal to, but less than, the base area of each cell to avoid touching the side wall thereof; a photodiode support situated on the opposite side of the cell with respect to the means for producing the parallel light beam, said support supporting at least two photodiodes, a first photodiode being disposed to measure the intensity of the light after passing in a first direction directly through the cell, and a second photodiode being disposed to measure the intensity of the light beam propagating in a direction oblique to the first after diffusion by the culture, the centre of the light beam being aligned with the centre of the first photodiode; means for supporting said strip of cells and moving it in translation so that the cells cross the light beam; synchronizing means which start measurement by the photodiodes each time the centre of a cell coincides with the centre of the light beam; and electronic means for calculating the ratio between the two measured light intensities, said electronic means including a threshold circuit and being associated with read-out means which, for each cell, supply data which varies according to whether the threshold is reached or not, said data indicating the effectiveness or ineffectiveness of the corresponding antibiotic.

2. Apparatus according to claim 1, wherein the photodiode support supports at least one second photodiode disposed to measure the intensity of light propagation in a direction perpendicular to the direction of maximum inherent diffusion of the bottom of the cell.

3. Apparatus according to claim 1, wherein the photo-diode support supports at least two second photodiodes disposed to measure light intensity in two different directions which are oblique with respect to the central ray, and which are symmetrical about the central ray, the output signals of said second photodiodes being electrically combined.

4. Apparatus according to claim 1, 2 or 3, wherein said synchronizing means include a photo-electric arrangement which co-operates with opaque marks on the strip and aligned with the centres of the cells.

5. Apparatus according to claim 1,2 or 3, wherein said synchronizing means include a photo-electric arrangement which co-operates with orifices in the support of the strip.

6. Apparatus for performing an antibiotic assay, substantially as herein described with reference to and as illustrated in the accompanying drawings.