XLE Media and Bacterial Identification

XLE Media Overview

• XLE: A selective culture medium used for growing specific bacteria, including E. coli, Salmonella, and Shigella.
• Components:
  • Sugars: Contains three types of sugars:
  • Thylose
  • Lactose
  • Sucrose
  • Amino Acid: Lysine (helps in fermentation and decarboxylation processes)
  • Sodium Thiosulfate: Provides sulfur for bacterial growth and differentiation.

Bacterial Fermentation and Identification

• E. coli Growth:

• Ferments lactose and sucrose.

• Result: Lowers pH of the medium, producing a yellow color in the colonies.

• Shigella Growth:

• Unable to ferment any sugars present.

• Deaminates lysine, resulting in a pH increase (basic environment).

• Result: Pink discoloration in the colonies.

• Salmonella Growth:

• Initially ferments sugars, lowering pH.

• Deaminates lysine, which increases pH (pink colonies).

• Reduced sulfur reacts with iron in the medium resulting in a black precipitate.

• Result: Pink with black centers, helping differentiate from other bacteria:

  • Pink only: Regular Salmonella.
  • Pink with black center: Specific identification of Salmonella.

Media Preparation and Handling

• Preparation:

• Mix components in water, boil, cool, and pour into plates.

• No autoclave required due to the toxic nature of components preventing unwanted bacterial growth.

• Inoculation Techniques:

• Use a flat inoculating loop to minimize damage to the media.

• Divide plates into quadrants for simultaneous testing of different bacteria.

Free Type Lipolytic Media

• Purpose: To determine the organism's tolerance to different oxygen levels.

• Media shows a distinct coloration at the top due to oxygen exposure.

• Preparation:

• Autoclave to ensure anaerobic conditions.

• Use caution not to disrupt the top layer when inoculating with bacteria.

• Inoculation Method:

• Use a loop to retrieve bacteria from the bottom without swirling to minimize oxygen exposure.

Growth Observation

• Over 48 hours:
• Bacterial growth at different depths indicates oxygen sensitivity:
  • Top Growth: High oxygen tolerance.
  • Bottom Growth: Anaerobic or low oxygen tolerance.

Final Steps and Lab Management

• Assign bacteria for inoculation without repeating among table members.
• Perform proper labeling and rack management during the experiment.
• Store all tubes appropriately after use and ensure proper incubation.