PROCESSES AND MATERIALS FOR CARRYING OUT MICROCHEMICAL AND MICROBIOLOGICAL TESTS

Abstract: A microbiological culture test process for detecting the presence of tryptophan-deaminase-positive microorganisms, which comprises (a) culturing a microorganism to be tested in the presence of L-tryptophan in an effective amount and for a period sufficient to allow production of indolepyruvic acid by the organism if it is of tryptophan-deaminase-positive character, (b) exposing the culture to a chromogenic amount of metal ions which are chromogenic with indolepyruvate, and to a fluorophor, and (c) assessing the fluoroscence of the culture after steps (a) and (b), thereby to show a reduction in fluoroscence in the presence of a tryptophan-deaminase-positive microorganism.

Description: Processes and materials for carrying out microchemical and microbiological tests This invention relates to processes and materials for carrying out microchemical and microbiological tests. In particular, it relates to chemical tests to detect the presence of small quantities of the enzyme activity tryptophan deaminase, and/or the product of its activity on tryptophan, i.e. indolepyruvic acid, and the application of such tests to detecting the production of this enzyme activity by microorganisms in culture, i.e. to identifying tryptophan-deaminase-positive microorganisms. A well established microbiological tryptophan deaminase test relies on the addition of acid ferric chloride solution to a microorganism culture grown on solid medium containing tryptophan for 18-24 hours, and observing intense red-brown coloration due to a ferric complex with indolepyruvic acid if tryptophan deaminase activity is present. As it stands, this test generally requires visual assessment of colour development, and it involves the use and handling of the somewhat noxious and inconvenient acid ferric chloride. The aim of this invention is to provide a tryptophan deaminase test that can be assessed by fluorescence in the resulting test material. It is also the aim of this invention to provide tests that can give a fluorescent result in a form suited to automatic fluorometric assessment, especially for example by fluorimetry that uses similar excitation and emission wavelengths as other and possibly hitherto chemically unrelated microbial tests, which can therefore be assessed simultaneously or in the same batch by similar or the same instrumentation. A further aim of the invention is to provide tryptophan deaminase detection tests adapted to microbiological use for assessment of the tryptophan deaminase activity of microbial cultures, e.g. to assist in their identification, in which the use of noxious reagents to develop a colour result is minimised or avoided. According to the present invention, a microorganism is cultured in the presence of enough L-tryptophan to allow formation of indolepyruvic acid if possible, and the fluorescence-quenching effect of the culture medium, after its exposure to reaction with ferric ions (or other chromogen-forming metal ions, especially other transition-metal ions), upon a fluorophor such as a fluorescent coumarin derivative is assessed fluoroscopically or fluorometrically. TDA-positive organisms then give substantial quenching of the coumarin fluorescence. The invention correspondingly provides a microbiological culture test process for detecting the presence of tryptophan-deaminase-positive microorgasms, which comprises (a) culturing a microorganism to be tested in the presence of L-tryptophan in an effective amount and for a period sufficient to allow production of indolepyruvic acid by the organism if it is of tryptophan-deaminasepositive character, (b) exposing the culture to a chromogenic amount of metal ions which are chromogenic with indolepyruvate, and to a fluorophor, and (c) assessing the fluorescence of the culture after steps (a) and (b), thereby to show a reduction in fluorescence in the presence of a tryptophan-deaminasepositive microorganism. Preferred coumarin derivatives are 4-methyl-umbelliferone, and 7-amino-4-methyl coumarin, more preferably the latter. The quantity of L-tryptophan present is generally more than the traces usually present in for example yeast extracts alone. Amounts of L-tryptophan of more than about 0.1%, e.g. 0.5%, and up to about 2%, are useful for example. The known TDA tests also employ such larger amounts than usual of L-tryptophan. We find advantageously that with this system it is not necessary to use strongly acid ferric chloride. A chromogenic amount of the metal ions is that which is effective to generate a fluorescence-quenching quantity of coloured product in the presence of indolepyruvate. We prefer to include the metal ions at low but effective concentration in the culture medium itself, in the form of a ferric chloride or ferric nitrate or ferric citrate ingredient at for example above about 1 millimolar and up to about 15 millimolar concentration. Most preferred is ferric citrate of the order of about 3 millimolar. When the fluorophor is also included in the culture medium it can be seen that this tryptophan deaminase test avoids the need for secondary reagents altogether, which simplifies manipulation to a very useful extent. Example: An example of a suitable culture broth formulation to carry out the test is as follows: Yeast extract 0.10% Sodium chloride 0.35% NaNH4HPO4. 4H2 0 0.20% K2HPO4.3H2O 0.20% L-tryptophan 0.50% Ferric citrate 0.072% 7-methyl-4-aminocoumarin 10 micromolar A microorganism can be most suitably inoculated into this liquid medium at an inoculum level of about 107 cells/ml, incubated for preferably about 4-6 hours, and the fluorescence measured by excitation at about 363nm and detection at about 450nm, e.g. using a Perkin-Elmer 1000 commercially available fluorometer. It is found that marked quenching of fluorescence results from the growth of at least the vast majority of tryptophan-deaminase-positive microorganisms but not from the growth of at least the vast majority of tryptrophan-deaminase-negative organisms. It can be seen from the above description that the invention provides a method of tryptophan deaminase activity detection or estimation in microorganisms: the invention also comprises materials for carrying out such tests. In particular, the material can comprise a microbiological broth containing tryptophan, and a source of ferric or other chromogen-forming metal ions, as well as a source of fluorophor, optionally and preferably all combined together, in suitable quantities (e.g. those given in detail above). If desired, a convenient embodiment can be used in which the indicated broth constituents, (or a selection of them including the essential tryptophan, and optionally also the ion source and/or the fluorophor), can be presented in dry form in one or more wells of a prepared (usually sterile) microtitre tray for use in carrying out microbial culture tests (usually protected by a removable adhesive sealing cover). An alternative dry form presents some or all of the materials in or associated with (usually sterile) containers for preparing a microbial inoculum suspension, e.g. a bottle or other container with a dry preparation of the materials contained therein; or a closure for such a bottle, such as a screw cap, with a dry preparation of the materials carried on a surface which in use is to contact the inoculum so that the materials may be dispersed therethrough, such as the inner surface of the screw cap or the surface of a wad placed within the screw cap; or in a dry form on a carrier material suitable to be added to such an inoculum or to a culture well, so that the materials can be dispersed through the inoculum or culture suspension, such as strips or discs of paper or other cellulosic or non-cellulosic, fibrous or non-fibrous carrier sheet material. All such preparations are desirably maintained dry and sterile before use, e.g. sealed within sterile foil sealing or packaging material. One of the special advantages of the processes described herein is that the tryptophan deaminase test gives a fluoroscopic/fluorometric result that can be measured optically with the same excitation/emission wavelengths as a number of other (known in themselves) microbial culture tests (e.g. in the presence of various quantities or types of antimicrobial) giving a variably fluorescent result: e.g. culture tests in which a fluorogenic substrate such as 4-methyl umbelliferone phosphate or other ester and/or an aminoacyl or peptidyl 7-amino-4-methyl coumarin is hydrolysed to fluorophor during growth of the microorganism if this growth occurs. Accordingly, combination tests including cultures for tryptophan deaminase testing and such antimicrobial tests are also included within the scope of the invention. Other modifications and variations within the scope of the described invention will be apparent to the skilled reader as a result of reading the above disclosure, and the several features described by way of example can be presented in any desired combinations.

Claims: Claims: 1. A microbiological culture test process for detecting the presence of tryptophan-deaminase-positive microorgasms, which comprises (a) culturing a microorganism to be tested in the presence of L-tryptophan in an effective amount and for a period sufficient to allow production of indolepyruvic acid by the organism if it is of tryptophan-deaminasepositive character, (b) exposing the culture to a chromogenic amount of metal ions which are chromogenic with indolepyruvate, and to a fluorophor, and (c) assessing the fluorescence of the culture after steps (a) and (b), thereby to show a reduction in fluorescence in the presence of a tryptophan-deaminasepositive microorganism. 2. A process according to claim 1, in which the ions and the fluorophor are present throughout the culture period. 3. A process according to claim 1 or 2 in which the fluorophor is a fluorescent coumarin derivative. 4. A process according to claim 3 in which the fluorophor is 7-amino-4-methyl coumarin or 4-methylumbelliferone. 5. A process according to claim 1, 2, 3 or 4, in which the chromogenic metal ions are ferric ions. 6. A process according to any preceding claims, in which the metal ion concentration is up to about 15 millimolar. 7. A microbial culture test medium suitable for carrying out a culture test process according to claim 1, comprising besides usual microbial culture medium ingredients, L-tryptophan, a fluorophor, and a chromogenic metal ion capable of forming with indolepyruvate a coloured product that quenches the fluorescence of the fluorophor. 8. A microbial culture test medium according to claim 7, wherein said fluorophor is a fluorescent coumarin, and the chromogenic metal ions are ferric ions. 9. A microbial culture test medium according to claim 7 or 8, wherein the concentration of L-tryptophan is of the order of 0.5% w/v, and the concentration of the metal ions is up to about 15 millimolar. 10. A test kit comprising prepared dry reagents for carrying out a microbial culture test according to claim 1, comprising a prepared with a plurality of receptacles test tray and/or a container for preparing microbial inoculum suspension, containing in dry form in one or more receptacles or containers thereof reagents for carrying our said culture test, said reagents comprising L-tryptophan and a salt of a chromogenic metal ion which is chromogenic with indolepyruvate, and a fluorophor capable of being quenched by the chromogenic product of the metal ion and indolepyruvate. 11. Microbiological test processes and materials therefor, substantially as described with reference to the joint and several features of the foregoing specification and examples.

Inventors: BEGGS, THOMAS STEWART, SANDS, THOMAS JOHN
Applicants: RADIOMETER CORPORATE DEV LTD
Application year: 1985
Main classification: C12Q1
IPC codes: C12Q1