PLET Medium: Principle, Procedure & 2 Results

PLET medium is recommended for the selective isolation of Bacillus anthracis. PLET medium consisting of polymyxin, lysozyme, ethylene diamine tetra acetic acid, and thallons acetate added to heart infusion agar has been isolated to bacillus anthracis from other spores containing bacilli. Bacillus anthracis is cautive agent of Anthrax. Anthrax is a disease that commonly affects animals and humans.

Principle Of PLET Medium

PLET Agar originally formulated by Knisley is the best selective medium for the cultivation of Bacillus anthracis from environmental specimens, inhibiting Bacillus cereus. Beef heart infusion and tryptose are a source of carbon, nitrogen, and other compounds necessary for the growth of bacteria. Sodium chloride maintains the osmotic pressure. EDTA, thallous acetate, and polymixin (present in the supplement are selective agents and inhibit most microorganisms. Lysozyme (present in the supplement) attacks the cell walls of gram-negative bacteria and inhibits them.

It is possible to streak directly from a suspect colony but also heat treated or alcohol-treated specimens can be used for streaking. Colonies of Bacillus anthracis appear 36-40 hours after incubation at 37°C. The colonies are circular, creamy-white with a ground glass texture. For confirmation, the suspect colonies should be subcultured on Blood Agar plates for identification. Capsule building can be seen directly or in Blood Agar plates.

Composition Of PLET Medium

IngredientsGrams/Litre
Beef heart infusion (extract from 500 g)10.0
Tryptose10.0
Sodium chloride5.0
EDTA0.3
Thallous acetate0.04
Agar15.0
Composition Of PLET Medium

Preparation Of PLET Medium

  1. Suspend 4.04 grams of PLET agar, in 99 mL distilled water.
  2. Heat properly to dissolve the medium completely.
  3. Sterilize by autoclaving at 15 lbs pressure (121°C) for 10 minutes.
  4. After autoclaving,  leave for cooling to 50°C and aseptically add rehydrated contents of 1 vial of Anthracis Selective Supplement.
  5. Mix well before dispensing.
  6. Pour PLET agar into each plate and leave plates on the sterile surface until the agar has solidified.
  7. Store the plates in a refrigerator at 2-8°C.

PLET Sample Collection

  • Pus and Swab from malignant pustule.
  • Sputam in pulmonary anthrax.
  • Blood
  • CSF
  • EAR lobe from a dead animal

Procedure Of PLET Medium

  • Allow the plates to warm at 37°C or to room temperature, and the agar surface to dry before inoculating.
  • Take a plate for one specimen.
  • Inoculate and streak the specimen as soon as possible after collection.
  • If the specimen to be cultured is on a swab, roll the swab over a small area of the agar surface.
  • Streak for isolation with a sterile loop.
  • Incubate the plate aerobically at 35-37°C for 36-40 hours.
  • Examine colony characteristics.

Colony Of PLET Medium

  • Gram-positive, non-acid-fast, non-motile. The large rectangular capsule comprises a polypeptide, the polymer of D glutamic acid. This capsule is plasmid-coded. It inhibits complement-mediated phagocytosis.

Animal Anthrax

Anthrax is primarily a zoonotic disease. Animals affected with Herbivores – cattle, sheep, and less often horses and pegs. Acquired by ingestion of spores present in the soil. Direct spread from animal to animal is rare. Infected material is discharged from the mouth, nose, and rectum. Bacilli sporulate in soil.

Human Anthrax

Human anthrax is highly prevalent in Africa and central and southern Asia. Affected by infected animal products such as hides, hair, bristles, and wool.

PLET Medium Storage

Store prepared media below 8°C, protected from direct light. Store dehydrated powder, in a dry place, in tightly sealed containers at 2-25°C.

Symptoms Of Anthrax

  • Fever And Chills
  • Chest Discomfort
  • Shortness of breath
  • Dizziness
  • Cough
  • Nausea and vomiting
  • Headache
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