Types of Media Table

Key Terms -​ -​ -​ -​ -​ Selective media → inhibits some organisms while allowing others to grow. Differential media → distinguishes organisms based on colour change, hemolysis, fermentation, or biochemical reactions. Enrichment media → supports growth of fastidious organisms. Supportive media → supports growth of many non-fastidious organisms. Fermentation → bacteria metabolize a sugar (commonly lactose or mannitol), causing colour changes. Types of Media — Consistency ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Media Type Solid / Liquid / Both Description Main Use Solid Media Solid Prepared by adding agar to the nutrient liquid media Isolation of colonies, observation of colony morphology, hemolysis, biochemical studies Broth Media Liquid Meat extract or protein-based liquid media Enrichment, wound cultures, anaerobic cultures, and recovery of low numbers of organisms Agar Slants Solid Agar solidified at an angle in a tube Storage, transport, biochemical testing Semisolid Media Semisolid Lower agar concentration Motility testing. Functional Types of Media ​ ​ ​ ​ ​ Functional Type Solid / Liquid / Both Enrichment Media Usually solid but can be liquid Supportive Media ​ ​ ​ ​ ​ ​ ​ ​ ​ Purpose ​ ​ ​ Notes Supports growth of fastidious organisms Provides special nutrients and growth factors Usually solid Supports growth of most non-fastidious bacteria Does not another Selective Media Usually solid Inhibits some organisms while allowing others to grow Uses dyes, salts, antibiotics, or chemicals Differential Media Usually solic Differentiates organisms based on colour or biochemical reactions Uses indicators and dyes favour one organism over Media Solid / Liquid / Both Type of Media Fermentation / Reaction Organisms That Grow or Are Targeted Organisms Inhibited Appearance / Interpretation Purpose / Diseases / Tests Blood Agar Plate (BAP) / Solid Differential, Supportive Hemolysis reaction Supports the growth of most bacteria, especially Streptococcus spp. None Alpha hemolysis – green partial hemolysis; Beta hemolysis – clear complete hemolysis Gamma hemolysis – no hemolysis Primary plating medium; differentiates hemolytic bacteria; useful for Streptococcus identification Chocolate Agar (CHOC) Solid Enrichment No fermentation Fastidious organisms, Haemophilus spp., Neisseria spp. None Brown colous from lysed RBCs Used for gonorrhea, meningitis, fastidious bacteria; provides X factor (hemin) and V factor (NAD); requires 3–7% CO2 atmosphere Buffered–Charcoal –Yeast Extract Agar Solid Enrichment Not described Legionella pneumophila None Not described Used for Legionnaires’ disease organism MacConkey Agar (MAC) Solid Selective, Differential Lactose fermentation Gram-negative bacilli Gram-positive bacteria inhibited by crystal violet and bile salts Lactose fermenters – pink/red; Non-lactose fermenters – clear or colourless Isolation and differentiation of Gram-negative bacilli; commonly used for urine cultures and primary plating Mannitol Salt Agar (MSA) Solid Selective, Differential Mannitol fermentation Staphylococcus aureus Many other bacteria due to 10% NaCl Mannitol fermentation – yellow Used to recover and identify S. aureus Collistin-Nalidixic Acid Agar with Blood (CNA) Solid Selective, Differential Hemolysis reaction Gram-positive bacteria Gram-negative bacteria inhibited by colistin and nalidixic acid Hemolysis patterns sheep blood on Recovery and differentiation of Gram-positive organisms Hektoen Enteric Agar (HE) Solid Selective, Differential Lactose fermentation and H2S production Salmonella spp., Shigella spp., enteric pathogens Gram-positive organisms and many nonpathogenic enteric Gram-negative bacilli Lactose fermenters– yellow; Salmonella – green/transparent with black centres (H2S positive); Shigella – green/transparent (H2S negative) Isolation of enteric pathogens from stool specimens Thayer-Martin Agar Solid Selective, Enriched Not described Neisseria gonorrhoeae, Neisseria meningitidis Normal flora and fungi such as Candida albicans Not described Used for gonorrhea meningitis organisms Sabouraud Dextrose Agar (SAB) Solid Selective Not fermentation-based Fungi Bacteria inhibited by low pH and antibiotics Not described Isolation of fungi from nonsterile sites; fungal cultures Sheep Blood Agar with / without antibiotics and Salmonella Shigella Agar (SS Agar) Solid Selective, Differential Lactose fermentation and H2S production Salmonella spp., Shigella spp. Gram-positive organisms and coliforms inhibited by bile salts, sodium citrate, brilliant green Lactose fermentation turns colonies red due to acidic pH; non-lactose fermenters appear transparent/colourless; H2S producers form black precipitate Isolation and differentiation of Salmonella and Shigella from stool and food specimens Eosin Methylene Blue Agar (EMB) Solid Selective, Differential Lactose fermentation Coliforms and Gram-negative bacteria Gram-positive bacteria inhibited by eosin and methylene blue dyes Colour changes based on lactose fermentation Used for coliform isolation and differentiation Phenylthyl Alcohol Agar (PEA) Solid Selective Not fermentation-based Staphylococci, Streptococci, Gram-positive organisms Gram-negative bacteria inhibited due to interference with DNA synthesis Can contain sheep blood for anaerobic growth enhancement Isolation of Gram-positive organisms and anaerobes Mueller Hinton Agar Solid Supportive Media foe susceptibility testing Not fermentation-based Many bacterial pathogens None Zone of measured antibiotic discs inhibition around Used for Antibiotic Susceptibility Testing (AST), Kirby-Bauer disc diffusion test Urea Agar Solid slant Differential biochemical media Urea breakdown by urease enzyme Organisms capable of producing urease None Pink-red or magenta colour indicates ammonia production and positive urease activity Biochemical identification test for bacteria Triple Sugar Iron (TSI) Agar Solid slant Differential biochemical media Fermentation of sugars and H2S production Enteric bacteria None K/K (red/red) – no sugar fermentation; H2S production may produce black precipitate Differentiates enteric organisms based on sugar fermentation and H2S production Blood Culture Bottles (Aerobic and Anaerobic Bottles) Liquid Enrichment broth Not fermentation-based Bacteria and fungi in blood None Positive bottles show colour change due to CO2 production Blood cultures for septicemia, endocarditis, unexplained fever, bloodstream infections Broth Media Liquid Enrichment / Supportive Turbidity indicates growth Organisms present in low numbers, wound cultures, anaerobes None Clear broth becomes turbid when bacteria grow Backup medium enrichment medium Cary-Blair Transport Media Semisolid / transport media Transport media Not fermentation-based Intestinal pathogens in stool None Preserves viability Used for stool culture transport Charcoal Transport Media Transport media Transport media Not fermentation-based Certain bacterial specimens None Charcoal toxins and substances Used for specimen transport and preservation Amies Transport Media Transport media Transport media Not fermentation-based Clinical swab specimens None Preserves organisms during transport neutralizes inhibitory Transport specimens of and microbiology Fermentation Summary ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Media Fermentation Type Positive Result Negative Result MacConkey Agar Lactose fermentation Pink / red colonies Clear / colourless colonies Mannitol Salt Agar Mannitol fermentation Yellow colour No yellow colour Hektoen Enteric Agar Lactose fermentation Yellow colonies Green / transparent colonies SS Agar Lactose fermentation Red colonies in acidic pH Transparent / colourless colonies EMB Agar Lactose fermentation Colour change depending on fermenter Lack of significant colour change Triple Sugar Iron Agar Sugar fermentation Acid reactions / colour changes Red slant and butt (K/K) indicates no fermentation H2S-Produing Media Summary ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Media Detects H2S? Positive Apperance Organisms Mentioned Hektoen Enteric Agar Yes Black precipitate / Black centres Salmonella spp. SS Agar Yes Black precipitate Salmonella spp. Triple Sugar Iron Agar Yes Blackening of medium Enteric pathogens Media Commonly Used for Specific Diseases or Organisms ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Disease / Organism Media Disease / Organism Media Gonorrhea (Neisseria gonorrhoeae) Chocolate agar, Thayer-Martin agar Streptococcus spp. Blood agar Meningitis (Neisseria meningitidis) Chocolate agar, Thayer-Martin agar Fungal infections Sabouraud Dextrose agar Legionnaires’ disease Buffered-charcoal-yeast extract agar Urinary Tract Infections (UTIs) Blood agar, MacConkey agar Salmonella infection Hektoen Enteric agar, SS agar Bloodstream infections (septicemia) Blood Culture bottles Shigella infection Hektoen Enteric agar, SS agar Antibiotic susceptibility testing Mueller Hinton agar Staphylococcus aureus Mannitol Salt agar Important Notes ​ ​ ​ ​ ​ ​ → Some media can belong to more than one category ○​ MAC – selective, differential ○​ CNA – selective, differential ○​ Hektoen – selective, differential ​ ​ ​ → Selective media often contain: ○​ antibiotics ○​ dyes ○​ bile salts ○​ high salt concentrations → Differential media commonly use: ○​ pH indicators ○​ dyes ○​ hemolysis reactions ○​ sugar fermentation reactions → Broths are especially useful when organisms are present in low numbers → Blood culture bottles contain: ○​ growth media ○​ anticoagulant (SPS) ○​ resin to absorb antibiotics → Chocolate agar is created by heating agar so RBCs lyse and release growth factors → Sabouraud agar should be incubated at 30°C → Mueller Hinton agar is specifically associated with the Kirby-Bauer antibiotic susceptibility test ​ ​ ​ ​ ​ ​ ​ ​ ​ ​