Media

Culture media provide the nutrients and environmental conditions necessary for the growth and isolation of microorganisms from clinical specimens. Proper selection and use of media are essential for accurate pathogen identification and antimicrobial susceptibility testing

Components of Culture Media

• Water: Serves as the solvent and transport medium for nutrients
• Carbon Sources: Provide energy and building blocks (e.g., glucose, lactose, sucrose, glycerol)
• Nitrogen Sources: Supply amino acids and peptides (e.g., peptone, tryptone, soytone, beef extract, yeast extract)
• Minerals: Act as cofactors for enzymes and maintain osmotic balance (e.g., NaCl, K2HPO4, MgSO4, CaCl2, FeCl3)
• Growth Factors: Organic compounds required by fastidious organisms (e.g., vitamins, amino acids, purines, pyrimidines, hemin, NAD)
• Selective Agents: Inhibit the growth of unwanted microorganisms (e.g., antibiotics, dyes, bile salts, high salt concentrations)
• Differential Agents: Allow differentiation based on metabolic activities (e.g., pH indicators, sugars, red blood cells, dyes)
• Buffers: Maintain stable pH (e.g., phosphate buffers, Tris buffers, HEPES buffers)
• Solidifying Agents: Provide a solid surface for growth (e.g., agar, gelatin, silica gel)

Applications and Uses of Culture Media

• General-Purpose Growth: Supports a wide range of non-fastidious microorganisms (e.g., Nutrient Agar, Tryptic Soy Agar)
• Selective Isolation: Inhibits growth of some organisms while allowing others to grow (e.g., MacConkey Agar, Mannitol Salt Agar, Hektoen Enteric Agar)
• Differential Identification: Distinguishes between microorganisms based on appearance or growth characteristics (e.g., MacConkey Agar, Blood Agar, Eosin Methylene Blue Agar)
• Enrichment: Provides specific nutrients for fastidious organisms (e.g., Blood Agar, Chocolate Agar, Thioglycollate Broth)
• Antimicrobial Susceptibility Testing: Determines the susceptibility of microorganisms to antimicrobial agents
• Stock Culture Maintenance: Preserves pure cultures for future use

Selection for Specific Specimen Types and Organisms

• Blood Cultures: Nutrient broth (e.g., TSB, BHI) with anticoagulants and specialized blood culture bottles
• Urine Cultures: Blood Agar, MacConkey Agar, Cystine Lactose Electrolyte Deficient (CLED) Agar
• Stool Cultures: Blood Agar, MacConkey Agar, Hektoen Enteric Agar, Xylose Lysine Deoxycholate Agar, Campylobacter Agar, Cefsulodin-Irgasan-Novobiocin Agar
• Respiratory Cultures: Blood Agar, Chocolate Agar, MacConkey Agar
• Wound Cultures: Blood Agar, MacConkey Agar, Columbia CNA Agar, Thioglycollate Broth, Sabouraud Dextrose Agar
• Cerebrospinal Fluid (CSF) Cultures: Blood Agar, Chocolate Agar, Thioglycollate Broth, Sabouraud Dextrose Agar

Specialized Media for Fastidious Bacterial Species

• Chocolate Agar: Enriched with lysed red blood cells for Haemophilus and Neisseria species
• Modified Thayer-Martin (MTM) Agar: Selective for Neisseria gonorrhoeae and Neisseria meningitidis
• Loeffler’s Serum Slant: Enriched with coagulated serum for Corynebacterium diphtheriae
• Buffered Charcoal Yeast Extract (BCYE) Agar: Enriched with yeast extract and charcoal for Legionella species
• Skirrow’s Agar: Selective for Campylobacter jejuni and Campylobacter coli
• Cefsulodin-Irgasan-Novobiocin (CIN) Agar: Selective for Yersinia enterocolitica
• Bordet-Gengou (BG) Agar: Enriched with potato infusion and glycerol for Bordetella pertussis and Bordetella parapertussis

Media for Recovery of Mycobacteria and Fungi

• Mycobacteria
  • Egg-Based Media: Lowenstein-Jensen (LJ) Medium, Petragnani Medium
  • Agar-Based Media: Middlebrook 7H10 Agar, Middlebrook 7H11 Agar
  • Liquid Media: Middlebrook 7H9 Broth, Bactec MGIT
  • Selective Media: LJ Medium with Antimicrobials, Middlebrook 7H11 Agar with Antimicrobials
• Fungi
  • Sabouraud Dextrose Agar (SDA): High dextrose concentration and low pH
  • Brain Heart Infusion (BHI) Agar: Nutrient-rich for a wide range of fungi
  • Inhibitory Mold Agar (IMA): Contains chloramphenicol to inhibit bacteria
  • Dermatophyte Test Medium (DTM): Contains cycloheximide and gentamicin, with a pH indicator for dermatophytes
  • Potato Dextrose Agar (PDA): Promotes sporulation and pigment production