DCA Agar: Composition, Colony Characteristics

By Acharya Tankeshwar •  Updated: 05/24/22 •  4 min read

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 the intestine.

DCA is best prepared from ready to use dehydrated powder, available from most suppliers of culture media. 

Composition of Deoxycholate Citrate Agar (DCA)

Ingredients Gm/litre
Peptone 5.0
Lab-Lemco powder 5.0
Lactose 10.0
Sodium citrate 8.5
Sodium thiosulphate 5.4
Ferric ammonium citrate 1.0
Sodium deoxycholate 5.0
Neutral red 0.02
Agar 12.0
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 fine powder. It will enhance the growth of many bacteria and it is incorporated into a wide range of culture media as a solid foundation material as it enhances 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 colorless colonies). The fermentation of lactose causes acidification and the pH indicator neutral red changes color 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.

  1. Suspend 52g of dehydrated medium (supplied by the manufacturer) in 1 liter of distilled water.
  2. 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)
  3. As soon as the medium has cooled to 50°C -55°C mix well and dispense aseptically in sterile Petri dishes
  4. 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

Appearance

Inoculation of the prepared medium

  1. Dry the agar surface before use.
  2. Inoculate the medium heavily with feces or rectal swabs, spreading part of the original inoculum in order to obtain well-separated colonies on some portion of the plate.
  3. Incubate for 18-24 hours at 35°C.
  4. 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)

Colony of Salmonella (with black centers) in Deoxycholate Citrate Agar

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: 

Species Colony Morphology
Escherichia coli Most strains are inhibited, but the few strains which grow produce pink umbilicated colonies, encircled by a zone of precipitate.
Shigella sonnei Colonies are smooth and initially colorless, becoming pale pink on further incubation due to late lactose fermentation
Shigella flexneri Colonies are colorless and similar in appearance to those of Shigella sonnei
Salmonella spp Salmonellae produce non-lactose fermenting colorless colonies with (for H2S producing salmonellae) or without black centers
Enterobacter/Klebsiella spp. Large, pale mucoid colonies with a pink center.
Proteus/Providencia spp. Large, colorless to tan, with or without a black center. Proteus colonies are often glossy (more translucent than those of the pathogens), with a large central black dot and a `fishy’ odor.
Yersinia enterocolitica Colorless
Enterococci No growth to slight growth

References and Further Reading:
(You can get product specifications and details from the following manufacturers of DCA media)

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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45 responses to “Blood Agar and Types of Hemolysis”

  1. dilz says:

    Excellent..

  2. Dr Shripad Taklikar says:

    Hello Mr. Acharya,
    Your blog is useful. I saw it for preparation of Manuals for NABL accreditation of laboratory.

    Could you please help me:
    1. You mentioned about shelf life of blood agar as 4 weeks. Can you give a standard reference for this
    2. Do we have to pack this blood agar in single packing…..
    3. Sterility Check of blood agar should be done for how many percent of plates prepared.

    If the same principles can be applied for other media like MHA

  3. Abdulwasiu says:

    I need your help sir, am writing a project on hydrocarbon degrading bacteria, what are the procedure sir.

    • Tankeshwar Acharya says:

      Dear Abdul Wasiu
      Thank you for your query, but my areas of expertise is only on pathogenic bacteria, So i may not be the right person to guide on the project you are planning to begin.

  4. younas says:

    sir plz tell me that in how much time blood agar will change to choclate agar?

  5. Anil says:

    Sir why they named delta hemolysis as one of the type of hemolysis. They are not mentioned properly in any books . Could you help me??

    • tankeshwar says:

      In some papers, it is mentioned that Staphylococcus has delta hemolysin, but we do not use delta hemolysis much often so i do not have much idea about this type of hemolysis. Where you find about delta hemolysis?

  6. Mekhala says:

    Thank you sir..can you help me out comparion between sheep blood agar, horse blood agar , human blood agar

  7. truong says:

    VERY CLEAR and helpful info and tech for every one who want to learn from; Sincerely. Thanks

  8. Ahmed says:

    What anticoagulant should be used while collecting sheep blood please ?
    Does any can be harmful to the media ?

  9. pd daps says:

    If blood agar was to be used to culture S. aureus and P. aeruginosa will there be a possibility that another type of bacteria will grow???

    • Acharya Tankeshwar says:

      Yes, if the sample contains other bacteria, they will also grow on it. Blood agar is an enriched culture medium and supports growth of most of the pathogenic bacteria except few fastidious bacteria like Haemophilus influenzae and obligate intracellular bacteria.

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