Differential Clostridial Agar (DCA) acc. to WEENK

Differential Clostridial Agar (DCA)

EMD Cat. No. 1.10259.0500

500 g

For the enumeration of sulfite-reducing clostridia in dried foods.

Additives

EMD Cat. No.	Product	Pack Size
1.10641.0007	Anaerocult^® A mini	1 x 25
13677	Anaerocult^® A	1 x 10

Experimental Procedure and Evaluation	Quality Control
Typical Composition (g/liter)	Literature

Mode of Action
The medium consists of a nutritionally rich base medium, including starch to promote spore germination. Resazurin is added as redox-indicator, turning red at high redox-potential, indicating aerobic conditions. Sulfite and an iron source are added as indicators. Sulfite redicing clostridia produce sulfide from sulfite, which gives a black precipitate with the iron present in the medium. Sulfite reducing clostridia are enumerated as black colonies.

Typical Composition (g/liter)
Peptone from casein 5.0; peptone from meat 5.0; meat extract 8.0; starch 1.0; D(+)glucose 1.0; yeast extract 1.0; cysteinium chloride 0.5; resazurin 0.002; agar-agar 20.0.

Preparation
Suspend 41.5 g in 1 liter of purified water and autoclave (15 min at 121°C).

Cool to about 48°C and aseptically add, just before use, 5 ml/liter medium freshly prepared ferric(III) ammonium citrate solution (1 g in 5 ml purified water, heat sterilized: 15 min. at 121 °C) and 7.5 ml/liter sodium sulfite x 7 H2O solution (0.75 g in 7.5 ml purified water, filter-sterilized).
The complete medium is yellowish to reddish-brown. The medium is to be used immediately. Do not store. The base medium can be stored for at least 2 weeks at 4°C.
pH: 7.6 ± 0.2 at 25°C.

Experimental Procedure and Evaluation
1 ml sample per plate, pour-plate method. After solidification the plates are overlaid with sterile DCA.
Incubation: At 30°C for 3 days anaerobically (e.g. with Anaerocult^®, Anaerocult^® A mini)

Reading of results and interpretation:
Discrete black colonies of 1-5 mm in diameter are considered to be presumptive sulfite-reducing clostridia.
Note: In order to facilitate spore germination, a heat treatment of the spores/sample of 10 minutes at 30 °C before inoculation of the agar is recommended.

Quality control

Test strains	Recovery rate (%)	Growth	Black colonies
Clostridium perfringens ATCC 10⁵43	70	good / very good	+
Clostridium sporogenes ATCC 19404	70	good / very good	+
Clostridium bifermentans ATCC 19299	70	good / very good	+
Clostridium perfringens ATCC 13124	70	good / very good	+
Bacillus licheniformis ATCC 14580		poor / fair	-

Literature

WEENK, G., FITZMAURICE, E., MOSSEL, D.A.A.: Selective enumeration of spores of Clostridium species in dried foods. - J. Appl. Bact., 70; 135-143 (1991).