Culture medium for detection of bacterium belonging to genus <i>Listeria</i>

Abstract

A medium containing (A) a chromogenic substance which develops a color when decomposed by β-glucosidase or a fluorogenic substance which develops a fluorescence when decomposed by β-glucosidase; (B) a chromogenic substance which develops a color when decomposed by phosphatidyl inositol-specific phospholipase C or a fluorogenic substance which develops a fluorescence when decomposed by phosphatidyl inositol-specific phospholipase C; and (C) a sugar that is useful for the detection of Listeria monocytogenes and/or Listeria ivanovii.

Claims

1. A medium suitable to detect and differentiate Listeria monocytogenes and/or Listeria ivanovii from other bacteria of the genus Listeria, comprising: a first chromogenic substance which develops a color when decomposed by β-glucosidase, or a first fluorogenic substance which develops a fluorescence when decomposed by β-glucosidase; a second chromogenic substance which develops a color when decomposed by phosphatidyl inositol-specific phospholipase C, or a second fluorogenic substance which develops a fluorescence when decomposed by phosphatidyl inositol-specific phospholipase C; and a sugar at a concentration of 5 g/L to 30 g/L at the time of detecting and differentiating Listeria monocytogenes and/or Listeria ivanovii from other bacteria of the genus Listeria.

2. The medium according to claim 1, wherein the sugar is glucose.

3. The medium according to claim 1, wherein the first chromogenic substance or the first fluorogenic substance is 5-bromo-4-chloro-3-indoxyl-β-D-glucopyranoside.

4. The medium according to claim 1, wherein the first chromogenic substance or the first fluorogenic substance is 4-methylumbelliferyl-β-D-glucopyranoside.

5. The medium according to claim 1, further comprising a gelling agent and a nutritional component for a bacterial cell other than the sugar, wherein said medium is supported or layered on a fibrous water-absorbing sheet.

6. A method for detecting and differentiating Listeria monocytogenes and/or Listeria ivanovi from other bacteria of the genus Listeria, the method comprising: seeding and culturing a specimen on the medium according to claim 1 and detecting a color or fluorescence of a colony which appeared on the medium thereby differentiating Listeria monocytogenes and/or Listeria ivanovi from other bacteria of the genus Listeria.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The culture medium for detection of L. monocytogenes and/or L. ivanovii of the present invention comprises the following components (A), (B), and (C):

(2) (A) a chromogenic substance which develops a color when decomposed by β-glucosidase, or a fluorogenic substance which develops a fluorescence when decomposed by β-glucosidase;

(3) (B) a chromogenic substance which develops a color when decomposed by phosphatidyl inositol-specific phospholipase C, or a fluorogenic substance which develops a fluorescence when decomposed by phosphatidyl inositol-specific phospholipase C; and

(4) (C) a sugar.

(5) Component (A) is for detecting a colony of Listeria bacteria, wherein the action of β-glucosidase from the Listeria bacteria leads to release of a detectable releasable group, resulting in color development. Examples of such a releasable group include a chromogenic compound and a fluorogenic compound. Specific examples of a substrate for β-glucosidase include 5-bromo-4-chloro-3-indoxyl-β-D-glucopyranoside (X-β-glucoside, blue), 5-bromo-6-chloro-3-indoxyl-β-D-glucopyranoside (MAGENTA-β-glucopyranoside, reddish violet), 5-bromo-6-chloro-3-indolyl-β-D-glucopyranoside (reddish violet), 6-chloro-3-indolyl-β-D-glucopyranoside (pink), 5-bromo-3-indolyl-s-D-glucopyranoside, 4-methyl umbelliferyl-β-D-glucopyranoside, o-nitrophenyl-β-D-glucopyranoside, phenyl-β-D-glucopyranoside, 3-nitrophenyl-β-D-glucopyranoside, 4-nitrophenyl-β-D-glucopyranoside, 3-indoxyl-β-glucopyranoside trihydrate, n-heptyl-f-D-glucopyranoside, and 5-bromo-4-chloro-3-indoxyl-2-acetamido-2-deoxy-β-D-glucopyranoside. Among these, 5-bromo-4-chloro-3-indoxyl-β-D-glucopyranoside (X-β-glucoside) and 4-methyl umbelliferyl-β-D-glcopyranoside are preferred in terms of detectability of L. monocytogenes. The concentration of the β-glucosidase substrate having the detectable releasable group is, as a concentration at the time of detection, preferably 0.001 g/L or more and 5 g/L or less, especially preferably 0.01 g/L or more and 0.25 g/L or less, and more preferably 0.02 g/L or more and 0.15 g/L or less.

(6) Component (B) is for detecting a colony of L. monocytogenes and/or L. ivanovii, wherein the action of PIPL C from the Listeria bacteria leads to release of a detectable releasable group, resulting in color development. Examples of such a releasable group include a chromogenic compound and a fluorogenic compound. Examples of such a substrate for PIPL C include a compound described in JP-B-5711218, and inositol phosphate which has a chromophore such as a 5-bromo-4-chloro-3-indoxyl group, a 5-bromo-6-chloro-3-indoxyl group, and a 6-chloro-3-indoxyl group is preferred. These compounds are preferably used in the form of a salt such as a sodium salt, a potassium salt, and an ammonium salt. Examples of a preferable commercial product include Aldol 514® inositol-phosphate, ammonium salt (available from Biosynth) and Aldol 495® inositol-phosphate, ammonium salt (available from Biosynth). The concentration of compound (B) is, as a concentration at the time of detection, preferably 0.001 g/L or more and 5 g/L or less, more preferably 0.05 g/L or more and 2 g/L or less, and even more preferably 0.1 g/L or more and 1 g/L or less.

(7) Component (C), the sugar, is not particularly limited as long as the Listeria bacteria can use the sugar. For example, glucose, fructose, galactose, mannose, mannite, sorbitol, dulcite, inositol, arabinose, xylose, rhamnose, and adonite are preferable, and glucose is more preferable in terms of the glucose metabolism ability of L. monocytogenes and L. ivanovii. Component (C) has the effect of making the color or fluorescence of the colony of L. monocytogenes and L. ivanovii clear and making detection of L. monocytogenes and L. ivanovii easy and accurate. The content of component (C) is, as a concentration at the time of detection, preferably 2 g/L or more and 30 g/L or less in terms of the ability to make the colony color clear and in terms of the growth of the Listeria bacteria. The concentration of component (C) is more preferably 5 g/L or more and 30 g/L or less, and even more preferably 5 g/L or more and 20 g/L or less in terms of the ability to make the colony color clear.

(8) For further enhancing the selectivity for the Listeria bacteria, the culture medium of the present invention preferably comprises one or more selected from the group consisting of moxalactam and colistin sulfate, and more preferably comprises moxalactam and colistin sulfate. The content of moxalactam is, as a concentration at the time of detection, preferably 0.001 g/L or more and 0.5 g/L or less, more preferably 0.005 g/L or more and 0.3 g/L or less, and even more preferably 0.01 g/L or more and 0.2 g/L or less, in terms of the selectivity for the Listeria bacteria. The content of the colistin sulfate is preferably 0.001 g/L or more and 0.5 g/L or less, more preferably 0.005 g/L or more and 0.2 g/L or less, and even more preferably 0.005 g/L or more and 0.1 g/L or less.

(9) In addition to the components described above, the culture medium of the present invention may comprise a nutritional component for a bacterial cell other than component (C), an inorganic salt, a saccharide, and/or a pH adjustor. Examples of the nutritional component for a bacterial cell include peptone, yeast extract, meat extract, and fish extract.

(10) Examples of the inorganic salt include a metal salt of an inorganic acid such as sodium chloride and sodium thiosulfate; and a metal salt of an organic acid such as ferric ammonium citrate and sodium citrate. Furthermore, examples of other inorganic salts include bile powder, sodium cholate, and sodium deoxycholate. A monosaccharide and an oligosaccharide can be used as a saccharide. Examples thereof include lactose, sucrose (white soft sugar), xylose, cellobiose, and maltose.

(11) The form of the culture medium of the present invention is not particularly limited. In addition to an ordinary agar culture medium, the culture medium of the present invention may be in the form of a sheet-shaped simple culture medium (JP-A-57-502200 and JP-A-6-181741) or may have a structure in which the culture medium is supported on, for example, a fibriform water-absorbing sheet having a mesh (JP-A-9-19282 and JP-A-2000-325072). Among these, a preferable simple culture medium is the one in which a composition is supported or layered on a fibrous water-absorbing sheet, the composition comprising, in addition to the above components (A) to (C), and as necessary moxalactam and colistin sulfate, (D) a gelling agent and (E) a nutritional component for a bacterial cell other than component (C).

(12) In this context, examples of the gelling agent include a water-soluble gelling agent, that is, a macromolecule which readily disperses in water and gelates. Specific examples thereof include pectin, guar gum, xanthan gum, tamarind gum, locust bean gum, gellan gum, gum arabic, tara gum, carrageenan, and carboxymethylcellulose. Among these, pectin, guar gum, and xanthan gum are more preferable and xanthan gum is especially preferable in terms of dispersibility in water, stability of the gel, and the like.

(13) The fibrous water-absorbing sheet used in the present invention needs to be a sheet in which a liquid specimen seeded thereon diffuses readily by a capillary phenomenon and examples thereof include a nonwoven fabric made from synthetic fibers, typically such as a rayon nonwoven fabric and a nonwoven fabric made from natural fibers, typically such as a cotton nonwoven fabric. The shape of these sheets is not particularly limited and may be any of a square, a rectangle, a circle, and other shapes. Although the size of these sheets is not particularly limited either, the sheet preferably has a longest diameter of 1 cm or more and 15 cm or less in view of the fact that the sheet is used for simple detection. Furthermore, the sheet preferably has a mesh size of 15 mesh or more and 100 mesh or less, and particularly 20 mesh or more and 50 mesh or less and has a thickness of 10 μm or more and 1,000 μm or less, and particularly 50 μm or more and 600 μm or less.

(14) Such a fibrous water-absorbing sheet is preferably placed on a waterproof flat plate. Although such a waterproof flat plate may be made of any material such as plastic and glass as long as the material is waterproof, the plate is preferably transparent for observation from the outside.

(15) Examples of specimens to be measured using the culture medium of the present invention include a dairy product, a meat product, a salad, a ready-to-eat food product, an environmental specimen (water, soil, and the like), and a clinical specimen (a human, a cow, a sheep, a pig, a goat, and the like). A culture solution obtained by preculturing these specimens in a Trypto-Soya broth culture medium and a culture solution obtained by culturing these specimens in a culture medium for enrichment can also be used as a sample.

(16) Detection of L. monocytogenes and/or L. ivanovii by using the culture medium of the present invention is performed by seeding and culturing the specimen on the culture medium and observing the property thereof such as color development and fluorescence of a colony. Usually, this culturing is preferably performed at 25 to 35° C. for 24 to 72 hours.

(17) For detection of L. monocytogenes and/or L. ivanovii by using the simple culture medium of the present invention, the liquid specimen is seeded on the surface of the culture medium. Then, the liquid specimen diffuses readily by the capillary phenomenon through the three-dimensional cavities of the meshes, and subsequently swells and gelates. Consequently, L. monocytogenes and L. ivanovii in the liquid specimen are entrapped, inhibited from free migration, and cultured to form a colony. Therefore, the formed colony can be found readily by observing the surface which defines the culture medium. A bacterial count can be calculated readily by seeding a sample on the simple culture medium quantitatively and counting colonies which appeared after culture.

(18) A method of seeding an aliquot with for example a pipette is usually used for seeding liquid which comprises bacterial cells, but a stamp method, that is, a method of pressing the culture medium against an specimen with a high water content and a method of immersing the culture medium in a liquid specimen may also be used. Culture after seeding of the liquid specimen may be performed in a stationary manner or may be performed during transportation.

EXAMPLES

(19) Hereinbelow, the present invention will be described in detail by way of examples, but the present invention is not limited by these examples at all.

Example 1 (Agar Culture Medium Comprising Color-Developing Substrate)

(20) A culture medium formulation is shown in Table 1. Components of this culture medium formulation were added to 1 liter of purified water, autoclaved at 121° C. for 15 minutes, and cooled to about 50° C. Then, 20 mL aliquots were dispensed aseptically in plastic petri dishes (90 mm in diameter) and allowed to stand until the culture medium set, thereby producing the culture medium of the present invention.

(21) Culture media which comprised glucose in an amount of 0, 2, 5, 20, or 30 g/L instead of the amount shown in Table 1 were also produced.

(22) TABLE-US-00001 TABLE 1 Composition g/L Meat peptone 18.00 g Casein peptone 6.00 g Yeast extract 10.00 g Magnesium sulfate (anhydrous) 0.50 g Magnesium glycerophosphate 1.0 g Sodium chloride 5.0 g Lithium chloride 10.0 g Disodium hydrogenphosphate 2.50 g Colistin sulfate 0.01 g Moxalactam 0.02 g Agar 15.00 g Silicon dioxide 10.00 g Aldol514 ™ inositol-phosphate, 0.30 g ammonium salt(Biosynth) 5-Brom-4-Chloro-3-indoxyl-beta-D- 0.05 g glucopyranoside(Biosynth) Glucose 10.00 g

(23) A strain to be tested was precultured in a sheep blood agar culture medium for 24 hours and the resulting colony was suspended in sterile saline to prepare a bacterial suspension. Each bacterial suspension was used for inoculation by streaking with a platinum loop. Color developed by the colony after culture at 37° C. for 44 hours is shown in Table 2.

(24) TABLE-US-00002 TABLE 2 Strain 0 g/L 2 g/L 5 g/L 10 g/L 20 g/L 30 g/L Listeria monocytogenes Red Red Red Red Red Red ATCC15313 Surrounding Surrounding Surrounding Surrounding Surrounding Surrounding area: Blue area: Blue area: Blue area: Blue area: Blue area: Blue Listeria monocytogenes NS5168 Red Red Red Red Red Red Surrounding Surrounding Surrounding Surrounding Surrounding Surrounding area: Blue area: Blue area: Blue area: Blue area: Blue area: Blue Listeria monocytogenes NS5169 Red Red Red Red Red Red Surrounding Surrounding Surrounding Surrounding Surrounding Surrounding area: Blue area: Blue area: Blue area: Blue area: Blue area: Blue Listeria monocytogenes NS5170 Red Red Red Red Red Red Surrounding Surrounding Surrounding Surrounding Surrounding Surrounding area: Blue area: Blue area: Blue area: Blue area: Blue area: Blue Listeria ivanovii JCM7681 Red Red Red Red Red Red Surrounding Surrounding area: Blue area: Pale blue Listeria ivanovii NS5167 Red Red Red Red Red Red Surrounding Surrounding area: Blue area: Pale blue Listeria innocua NS5166 Blue Blue Blue Blue Blue Blue Listeria welshimeri NS5172 Blue Blue Blue Blue Blue Blue

(25) Table 2 indicates that using the agar culture medium which comprised 2 g/L to 30 g/L glucose as component (C) in addition to component (A) and component (B) allowed for clearly differentiating the colonies of L. monocytogenes and L. ivanovii from the colonies of other Listeria bacteria.

Example 2 (Agar Culture Medium Comprising Fluorogenic Substrate)

(26) A culture medium formulation is shown in Table 3. Components of this culture medium formulation were added to 1 liter of purified water, autoclaved at 121° C. for 15 minutes, and cooled to about 50° C. Then, 20 mL aliquots were dispensed aseptically in plastic petri dishes (90 mm in diameter) and allowed to stand until the culture medium set, thereby producing the culture medium of the present invention.

(27) TABLE-US-00003 TABLE 3 Composition g/L Meat peptone 18.00 g Casein peptone 6.00 g Yeast extract 10.00 g Magnesium sulfate (anhydrous) 0.50 g Magnesium glycerophosphate 1.0 g Sodium chloride 5.0 g Lithium chloride 10.0 g Disodium hydrogenphosphate 2.50 g Colistin sulfate 0.01 g Moxalactam 0.02 g Agar 15.00 g Silicon dioxide 10.00 g Aldol514 ™ inositol-phosphate, 0.30 g ammonium salt(Biosynth) 4-Methylumbelliferyl-beta-D- 0.1 g glucopyranoside(Biosynth) Glucose 10.00 g

(28) A strain to be tested was precultured in a sheep blood agar culture medium for 24 hours and the resulting colony was suspended in sterile saline to prepare a bacterial suspension. Each bacterial suspension was used for inoculation by streaking with a platinum loop. Color developed by the colony and the presence or absence of fluorescence after culture at 37° C. for 44 hours are shown in Table 4.

(29) TABLE-US-00004 TABLE 4 Presence Color developed or absence of Strain by colony fluorescence Listeria monocytogenes ATCC15313 Red + Listeria monocytogenes NS5168 Red + Listeria monocytogenes NS5169 Red + Listeria monocytogenes NS5170 Red + Listeria ivanovii JCM7681 Red − Listeria ivanovii NS5167 Red − Listeria innocua NS5166 White + Listeria welshimeri NS5172 White +

(30) Table 4 indicates that using the agar culture medium which comprised 10 g/L glucose as component (C) in addition to component (A) and component (B) allowed for clearly differentiating the colonies of L. monocytogenes and L. ivanovii from the colonies of other Listeria bacteria.

Example 3 (Simple Culture Medium Comprising Color-Developing Substrate)

(31) A culture medium formulation is shown in Table 5. Components of this culture medium formulation were added to 1,000 mL of ethanol solution to prepare a suspension thereof. 1 mL aliquots of this suspension in ethanol were dispensed aseptically in containers (50 mm in diameter) holding a fibrous water-absorbing sheet (a cotton nonwoven fabric, 50 mm in diameter), and then were allowed to stand overnight to dry in a closed space in such a manner that the containers did not overlap. Then, a lid was put on the container, thereby producing the simple culture medium of the present invention.

(32) TABLE-US-00005 TABLE 5 Composition g/L Meat peptone 18.00 g Casein peptone 6.00 g Yeast extract 10.00 g Magnesium sulfate (anhydrous) 0.50 g Magnesium glycerophosphate 7.5 g Sodium chloride 5.0 g Lithium chloride 10.0 g Disodium hydrogenphosphate 2.50 g Colistin sulfate 0.01 g Moxalactam 0.02 g Xanthan gum 20.00 g Silicon dioxide 10.00 g Aldol514 ™ inositol-phosphate, 0.30 g ammonium salt(Biosynth) 5-Brom-4-Chloro-3-indoxyl-beta-D- 0.05 g glucopyranoside(Biosynth) Glucose 10.00 g

(33) A strain to be tested was precultured in a sheep blood agar culture medium for 24 hours and the resulting colony was suspended in sterile saline and diluted as appropriate to prepare a bacterial suspension. 1 mL aliquots of each bacterial suspension were seeded on the produced simple culture medium. Color developed by the colony after culture at 37° C. for 44 hours is shown in Table 6.

(34) TABLE-US-00006 TABLE 6 Strain Color developed by colony Listeria monocytogenes ATCC15313 Red (Surrounding area: Blue) Listeria monocytogenes NS5168 Red (Surrounding area: Blue) Listeria monocytogenes NS5169 Red (Surrounding area: Blue) Listeria monocytogenes NS5170 Red (Surrounding area: Blue) Listeria ivanovii JCM7681 Red Listeria ivanovii NS5167 Red Listeria innocua NS5166 Blue Listeria welshimeri NS5172 Blue

(35) Table 6 indicates that using the simple culture medium which comprised 10 g/L glucose as component (C) in addition to component (A) and component (B) allowed for clearly differentiating the colonies of L. monocytogenes and L. ivanovii from the colonies of other Listeria bacteria.