Culture device and methods for enumerating mold colonies
A thin film culture device for enumerating mold colonies is provided. The device comprises water-resistant first and second substrates with a growth region disposed therebetween, a dry, cold water-soluble gelling agent disposed in the growth region, and an effective amount of a calcium-chelating compound disposed in the growth region. The effective amount of calcium-chelating compound is capable of reducing a rate of lateral enlargement of the colony-forming unit growing in the culture device relative to the rate of lateral enlargement of a colony of the same mold species growing in an otherwise identical culture device that does not contain the effective amount disposed in the growth region, wherein reducing the rate of lateral enlargement of the colony-forming unit does not substantially delay detection of the colony. A corresponding method is also provided.
1. A method for enumerating mold microorganisms, comprising:
contacting an aqueous fluid with a culture medium in a growth region of a thin film culture device to hydrate the growth region,
exposing aqueous fluid to a dry coating in the growth region;
incubating the inoculated culture medium for a period of time sufficient to form a macroscopically-detectable colony of a mold microorganism; and
counting a number of macroscopically-detectable colonies of mold microorganisms in the growth region; wherein
the growth region comprises
(a) an effective amount of calcium-chelating compound,
(b) a nutrient medium capable of supporting the growth of a mild microorganism, and
(c) the dry coating, the dry coating, wherein the dry coating comprises ethylenediamine tetraacetic acid;
and wherein, when the step of contacting the growth region with aqueous fluid comprises inoculating the growth region with a colony-forming unit of a mold species, the calcium-chelating compound reduces a rate of lateral enlargement of the colony-forming unit growing in the culture device relative to the rate of lateral enlargement of a colony of the same mold species growing in an otherwise identical culture device that does not contain the effective amount disposed in the growth region;
without substantially delaying detection of the colony.
2. The method of claim 1 :
wherein forming an inoculated culture medium comprises depositing an aqueous sample into the thin film culture device;
wherein, prior to depositing the aqueous sample, the thin film culture device comprises a nutrient medium, an indicator reagent, and/or the calcium-chelating compound.
3. The method of claim 1 , wherein incubating the inoculated culture medium for a predetermined period of time comprises incubating the inoculated culture medium between about 40 hours and about 96 hours.
4. The method of claim 1 , wherein counting a number of macroscopically-detectable colonies comprises obtaining an image of a growth region of the thin film culture device.
5. The method of claim 1 , wherein the predetermined volume of aqueous liquid comprises a high calcium food product.
6. The method of claim 1 , wherein the predetermined volume of aqueous liquid comprises milk.
7. The method of claim 1 , wherein the ethylenediamine tetraacetic acid is sufficient to reduce an average colony diameter of the mold microorganism by at least 50%, relative to an average colony diameter of the mold microorganism grown in a substantially identical thin film culture device that lacks the calcium-chelating compound, after 48 hours of incubation at a temperature suitable for growing the mold microorganism.
8. The method of claim 7 , wherein the ethylenediamine tetraacetic acid is sufficient to reduce the average colony diameter of the mold microorganism by at least 90%, relative to an average colony diameter of the mold microorganism grown in a substantially identical thin film culture device that lacks the calcium-chelating compound, after 48 hours of incubation at a temperature suitable for growing the mold microorganism.
9. The method of claim 1 , the dry coating comprises a calcium salt.
10. The method of claim 1 , wherein the ethylenediamine tetraacetic acid comprises disodium ethylenediamine tetraacetic acid dihydrate.
11. The method of claim 1 , the dry coating comprising disodium ethylenediamine tetraacetic acid dihydrate at a coating density of about 0.5 mg/cm 2 to about 12 mg/cm 2 .