Microbiology Culture Media Overview

Culture Media

• General Purpose: Nutrient Agar is used to cultivate a wide range of microorganisms.
• Enriched: Thioglycollate Broth supports the growth of fastidious organisms, such as those requiring specific growth factors.
• Enriched, Differential: Blood Agar is used to cultivate non-fastidious organisms and differentiate between hemolytic reactions (alpha, beta, or gamma).
• Selective, Differential: MacConkey Agar selects for Gram-negative bacteria and distinguishes between lactose fermenters (pink colonies) and non-fermenters (colorless colonies).
• Selective: Lowenstein-Jensen (LJ) Agar is used for the selective isolation of Mycobacterium species, particularly M. tuberculosis.
• Selective, Differential: Bile Esculin Agar (BEA) is used for the selective isolation of Group D Streptococci; hydrolysis of esculin turns the medium black.
• Enriched, Selective: Thayer-Martin Agar is used to isolate Neisseria gonorrhoeae and Neisseria meningitidis.
• Selective: Sabouraud Dextrose Agar (SDA) is selective for fungi, inhibiting bacterial growth due to its low pH and high glucose concentration.
• Selective, Differential: Tinsdale Agar is selective for Corynebacterium diphtheriae; it differentiates based on the production of black colonies due to tellurite reduction.
• Selective, Differential: Xylose Lysine Deoxycholate (XLD) Agar selects for Gram-negative enteric pathogens and differentiates between Salmonella (black colonies) and Shigella (red colonies).
• Differential: Triple Sugar Iron (TSI) Agar differentiates bacteria based on their ability to ferment glucose, lactose, or sucrose and produce hydrogen sulfide (H₂S).
• Selective: Cetrimide Agar is selective for Pseudomonas aeruginosa; it enhances pigment production in Pseudomonas species.
• Selective, Differential: Hektoen Enteric Agar (HEA) is selective for Salmonella (black colonies) and Shigella (green colonies) from other enteric organisms.

General Characteristics of Culture Media

• Essential characteristics of culture media include:
  • Satisfactory and rapid growth from a single culture.
  • Maintenance of pH during storage.
  • Reasonable cost and availability
  • Sterility throughout experimentation.

Uses of Culture Media

• Cultivation and isolation of microorganisms.
• Identification of different microorganisms.
• Assessing sterility of equipment and procedures.
• Assessing effectiveness of preservatives.
• Evaluating disinfectants.
• Assessing antimicrobial susceptibility.

Colony

• A macroscopically visible collection of millions of bacteria originating from a single bacterial cell.

Solid Media

• Generally contains 2% agar.
• Allows for visualization of colony morphology, pigmentation, and hemolysis.

Selective Media

• Contains one or more agents that inhibit the growth of certain microbes, encouraging the growth of a specific microbe.
• Examples:
  • Mannitol Salt Agar (MSA): encourages the growth of Staphylococcus aureus.
  • MacConkey’s medium: used for Gram-negative bacteria.
  • TCBS: used for Vibrio cholerae.
  • Lowenstein Jensen medium: used for Mycobacterium tuberculosis.
  • Wilson and Blair medium: used for Salmonella Typhi.
  • Potassium tellurite medium: used for diphtheria bacilli.

Indicator Media

• Contains an indicator that changes color when a bacterium grows in it.
• Examples:
  • Blood agar
  • MacConkey’s medium
  • Christensen’s urease medium

Differential Media

• Contains substances that allow for the differentiation of bacteria based on specific characteristics.
• Examples:
  • MacConkey’s medium: differentiates between lactose fermenters and non-lactose fermenters.

Sugar Media

• Media containing a fermentable substance.
• Examples:
  • Glucose, arabinose, lactose, starch, etc.

Transport Media

• Media used for transporting samples.
• Examples:
  • Stuart’s medium: used for transporting non-nutritive soft agar gel containing a reducing agent.
  • Buffered glycerol saline: used for transporting enteric bacilli.

Culture Methods

• Streak Culture: used for isolating bacteria in pure culture from clinical specimens.
• Lawn Culture: provides a uniform surface growth of a bacterium, used for purposes like bacteriophage typing and antibiotic sensitivity testing.
• Stroke Culture: prepared in tubes containing agar slope/slant, used for obtaining pure cultures for slide agglutination.
• Stab Culture: prepared by puncturing a suitable medium with a charged wire, used for demonstrating gelatin liquefaction and determining oxygen requirements.
• Pour Plate Method: involves mixing inoculum with molten agar, used for estimating viable bacterial counts.
• Liquid Culture: inoculated by touching with a charged loop or adding an inoculum, used for blood culture and sterility testing.
• Anaerobic Culture Methods: used to grow anaerobic bacteria.