Cystine Tryptic Agar- Composition, Principle, Preparation, Results, Uses
Cystine Tryptic Agar (CTA) is a semi-solid medium that contains cystine and peptone as the main sources of nutrients for the growth of fastidious microorganisms. CTA can be supplemented with different carbohydrates and a pH indicator to detect fermentation reactions. CTA can also be used to test for bacterial motility by observing the extension of growth from the stab line of inoculation.
The following table lists the ingredients and measurements for preparing 1000 ml of Cystine Tryptic Agar base:
Cystine Tryptic Agar (CTA) is a differential medium that allows the detection of carbohydrate fermentation and bacterial motility by fastidious microorganisms, such as Neisseria and Haemophilus species. The medium contains cystine and peptone as sources of nitrogen, amino acids, vitamins, and minerals that support the growth of these organisms. CTA also contains agar, which solidifies the medium and enables the observation of motility along the stab line of inoculation.
To prepare Cystine Tryptic Agar, follow these steps:
- Suspend 28.51 grams of the dehydrated medium in 1000 ml of distilled water. This will make a 2.85% solution.
- Heat the solution to boiling and stir until the medium is completely dissolved.
- Dispense the medium in tubes in 8-10 ml amounts. The tubes should be slanted to create a deep butt and a shallow slant.
- Sterilize the tubes by autoclaving at 121°C for 15 minutes at 15 psi pressure.
- Cool the tubes to 50°C and add the appropriate carbohydrate solution. The carbohydrate solution should be sterile and have a concentration of 1% (w/v). The amount of carbohydrate solution to be added depends on the volume of the medium in the tube. For example, if the tube contains 10 ml of medium, add 0.1 ml of carbohydrate solution.
- Mix well and allow the medium to solidify in an upright position. The medium should have a red-pink color before inoculation.
The results of Cystine Tryptic Agar (CTA) can be interpreted by observing the color change and the growth pattern of the inoculated bacteria. The color change is due to the pH indicator phenol red, which turns yellow in acidic conditions and red-pink in alkaline conditions. The growth pattern indicates the motility of the bacteria along the stab line.
Fermentation reactions are detected by the color change of the medium in the inoculated area. A positive reaction means that the bacteria can ferment the specific carbohydrate present in the medium and produce acid as a by-product. A negative reaction means that the bacteria cannot ferment the carbohydrate and either use peptone as an alternative source of energy or do not grow at all.
Motility reactions are detected by the growth pattern of the bacteria along and away from the stab line. A positive reaction means that the bacteria are motile and can move through the agar medium. A negative reaction means that the bacteria are non-motile and remain confined to the stab line.
Cystine Tryptic Agar (CTA) is a versatile medium that has several applications in microbiology. Some of the uses of CTA are:
- Identification and maintenance of the gonococcus and other bacteria
- Determination of carbohydrate fermentation by fastidious microorganisms
- Detection of bacterial motility
- Holding medium for the maintenance of fastidious microorganisms
Cystine Tryptic Agar (CTA) has some limitations that should be considered when using it for identification and maintenance of microorganisms:
- CTA is not a definitive identification medium
- CTA may not detect all carbohydrate fermentation reactions
- CTA may give false-negative or false-positive results for motility
- CTA requires a heavy inoculum and proper inoculation technique
- CTA may show reversion reactions due to peptone utilization
- CTA requires aerobic incubation
- CTA does not support the growth of all fastidious microorganisms
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