lab practical

Media Types in Microbiology

Complex Media

• Definition: Media with undefined components, where exact chemical makeup is unknown.

• Characteristics: Contains ingredients like yeast extract or peptones, but the specific molecules aren't identified.

• Usage: Most common in labs, supports a wide variety of microorganisms.

Defined Media

• Definition: Media where all components are known and precisely defined by their chemical identity.

• Agar: An inert substance often excluded from the definition of the ingredients.

• Examples: Used when controlling specific variables, such as the carbon source.

• Notable Usage: Citrate media is a defined media to control the carbon source exclusively.

Selective and Differential Media

Selective Media

• Definition: Allows the growth of certain types of bacteria while inhibiting others through the incorporation of inhibitors.

• Examples: MSA (Mannitol Salt Agar): Contains 7.5% sodium chloride, inhibiting most bacteria, allowing halotolerant (halophiles) organisms to thrive.

• Inhibitor: Sodium chloride.

Differential Media

• Definition: Contains reagents that produce a visible change to help differentiate between organisms based on metabolic activity.

• Examples: MSA can be both selective and differential. Positive fermentation of mannitol is indicated by color change due to phenol red pH indicator; yellow indicates successful fermentation.

Halophiles vs. Halotolerant

• Halophiles: Require high salt concentrations for growth.

• Halotolerant: Can survive in high salt but do not require it for growth.

• Identification: If bacteria grow on both MSA and NA (Nutrient Agar) plates, they are determined to be halotolerant.

PEA Plate (Phenyl Ethyl Alcohol Agar)

• Type: Selective only, inhibiting Gram-positive bacteria.

• Inhibitor: Phenyl ethyl alcohol.

• Usage: Supports the growth of Gram-negative bacteria but not all. Growth can be poor or good due to varying sensitivity to the inhibitor.

NA Plate (Nutrient Agar)

• Type: Neither selective nor differential.

• Purpose: Control media to determine the normal growth of bacteria for comparison against selective media.

EMV (Eosin Methylene Blue) Test

• Differential Importance: Distinguishes fecal coliforms (Gram-negative bacteria that ferment lactose) from other types.

• Lactose Fermentation: Produces a color change; typically dark purple or a green metallic sheen for coliforms.

• Non-fermenters may lose color, while fermenters generate acid resulting in color changes ranging from pink to purple.

Biochemical Tests Overview

• Key Components: Understand the substrate, enzyme, product, and detecting reagents for tests.

• Examples:

  • Gelatinase Test: Determines gelatinase presence based on whether gelatin becomes liquid.

  • Motility Test: Indicates motility by growth diffusion.

  • Nitrate Reduction Test: Determines the bacterial ability to reduce nitrate. Positive results indicate either nitrate reduction or denitrification, confirmed with reagents A and B, zinc added as a further step when no color change occurs.

General Tips for Unknown Project

• Identifying Bacteria: Know key tests for identification and what results indicate.

• Troubleshooting: Ask questions for clarification; lab strains may change or lose expected functions.

• Spirit: Approach as a fun learning experience rather than a source of anxiety.