Deoxycholate Citrate Agar (DCA) is a selective and differential medium, recommended for the isolation of enteric pathogens particularly Salmonella and Shigella species. Deoxycholate Citrate Agar is a modification of Deoxycholate Agar formulated by Leifson. DCA is selective for enteric pathogens owing to increased concentrations of both citrate and deoxycholate salts. Sodium deoxycholate at pH 7.3 to 7.5 is inhibitory for gram positive bacteria. Citrate salts, in the concentration included in the formulation, are inhibitory to gram-positive bacteria and most other normal flora present in
DCA is best prepared from ready to use dehydrated powder, available from most suppliers of culture media.
Composition of Deoxycholate Citrate Agar (DCA):
|Ferric Ammonium Citrate||1.0|
|pH 7.3 ± 0.2 @ 25°C|
Peptone provides carbon, nitrogen, vitamins and minerals.
Lab-Lemco is a meat extract made from specially selected raw materials, adjusted to neutrality and dried to a finepowder. It will enhance the growth of many bacteria and it is incorporated intoa wide range of culture media as a solid foundation material as it enhance the growth of many bacteria.
Dipotassium phosphate buffers the medium.
Lactose helps in differentiating enteric bacilli (lactose fermenters produce red/pink colonies while lactose non-fermenters produce colourless colonies). The fermentation of lactose causes acidification and the pH indicator neutral red changes its colour to red. Lactose-fermenting colonies may have a turbid zone of precipitation around them caused by the precipitation of deoxycholate in the acidic environment.
The reduction of ferric ammonium citrate to iron sulfide is indicated by the formation of black iron sulfide. If the bacteria produce H2S, the colonies will have black centers.
Coliform bacteria and gram-positive bacteria are inhibited or greatly suppressed due to sodiumdeoxycholate, sodium citrate and ferric ammonium citrate.
Preparation of Deoxycholate Citrate Agar (DCA)
Prepare the medium as instructed by the manufacturer.
- Suspend 52g of dehydrated medium (supplied by the manufacturer) in 1 litre of distilled water.
- Heat with great care to dissolve the medium completely. Do not boil or autoclave the medium. Avoid excessive or prolonged heating. DO NOT AUTOCLAVE. (If autoclaved the agar becomes soft and almost impossible to streak)
- As soon as the medium has cooled to 50°C -55°C mix well and dispense aseptically in sterile
- Label the plates “DCA”. Date the medium and give it a batch number. Store the plates at 2°C – 8°C, preferably in sealed plastic bags to prevent loss of moisture.
Storage conditions and Shelf life
- Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
- Store the prepared agar plates at 2-8°C. Shelf life is up to 6 weeks providing there is no change in the appearance of the medium to suggest contamination or an alteration of pH.
- Dehydrated medium: Straw/pink coloured, free-flowing powder
- Prepared medium: Pink coloured gel
Inoculation of the prepared medium
- Dry the agar surface before use.
- Inoculate the medium heavily with faeces or rectal swabs, spreading part of the original inoculum in order to obtain
well separatedcolonies on some portion of the plate.
- Incubate for 18-24 hours at 35°C.
- If organisms are late developers or if no non-lactose fermenters are observed, incubate for a further 24 hours.
Colony characteristics in Deoxycholate Citrate Agar (DCA)
Salmonella and Shigella species do not ferment lactose but Salmonella may produce H2S, forming colorless colonies with or without black centers.
Colony morphology of common enteric bacteria on Deoxycholate Citrate Agar (DCA) is summarized below:
|Escherichia coli||Most strains are inhibited, but the few strain which grow produces pink umbilicated colonies, encircled by zone of precipitate|
|Shigella sonnei||Colonies are smooth and initially colourless, becoming pale pink on further incubation due to late lactose fermentation|
|Shigella flexneri||Colonies are colourless and similar in appearance to those of Shigella sonnei|
|Salmonella spp||Salmonellae produce non-lactose fermenting |
|Enterobacter/Klebsiella spp.||Large, pale mucoid colonies with pink centre|
|Proteus/Providencia spp.||Large, colourless to tan, with or without black centre. Proteus colonies are often glossy (more translucent than those of the pathogens), with a large central black dot and a `fishy’ odour.|
|Enterococci||No growth to slight growth|
References and Further Reading:
(You can get product specification and details from the following manufacturers of DCA media)