Culture media

Bacterial

• Primary media (i.e. those onto which samples are directly inoculated) vary by specimen
  • Most commonly include blood, chocolate, and MacConkey agars
  • For anaerobic cultures, pre-reduced Brucella agar

Media	Abbr	Description
5% sheep's blood	SBA	A general, non-selective media; allows for identification of hemolysis patterns
Chocolate	CHOC	Enriched media for difficult-to-grow bacteria
MacConkey	MAC	Selective for Gram-negative bacteria, with an indicator for lactose fermentation
Sorbitol-MacConkey	SMAC	Used to identify sorbitol-negative strains of E. coli such as O157:H7
Mueller-Hinton		Used for antibiotic susceptibility testing
BCSA	BCSA	For isolation of Burkholderia cepacia, incubated at 30ºC
Columbia naladixic acid	CNA	Selective for Gram-positive bacteria
Bile esculin		Selective for Enterococcus
Brucella blood	BRUC	General anaerobic medium
Neomycin	NEO	Selective plate for anaerobes
CHROMagar Staph. aureus	CSA	Selective and chromogenic for Staphylococcus aureus
Bacteroides bile esculin	BBE	Increases sensitivity for Bacteroides (which grow black)
Egg yolk		Enriched media for Clostridium

Blood Agar

• Most common medium used
• Allows for differentiation of hemolysis pattern:
  • Partial hemolysis (α): Streptococcus pneumoniae, Enterococcus, Streptococcus bovis, Viridans group streptococci
  • Full hemolysis (β): Streptococcus pyogenes, Group B, C, and G streptococci, some Streptococcus anginosus, and rare Enterococcus
  • No hemolysis (γ): Viridans-group streptococci

Mueller-Hinton Agar

• Standardized pH and solute content that are used for antimicrobial susceptibility testing

Bile Esculin Agar

• Selects for bacteria that can grow in 40% bile and can hydrolyze esculin
• E.g. Enterococcus and Streptococcus bovis

Chocolate Agar

• Sheep's blood that has been lysed
• Used most commonly to identify Haemophilus influenzae and Neisseria

MacConkey Agar

• Uses bile salts and crystal violet to inhibit Gram-positive and some fastidious Gram-negative bacteria
• Uses an indicator to identify lactose fermentation
  • Lactose fermenters turn pink or red
  • Non-lactose fermenters stay yellow

Hektoen Agar

• Contains lactose, sucrose, salicin, and a high concentration of bile salts (intended to inhibit Gram-positive and coliform Gram-negative bacteria)
• Black colonies suggests Salmonella, from hydrogen sulfide production
• Bright orange to pink colonies suggests rapid lactose fermenters like Escherichia coli
• Green colonies include Salmonella and Shigella

Sorbitol-MacConkey Agar

• The lactose in MacConkey is replaced with sorbitol
• Common strains of Escherichia coli ferment sorbitol
• Escherichia coli O157:H7 is a sorbitol non-fermenter
• However, many other enterohemorrhagic strains are fermenters

Columbia Naladixic Acid Agar

• Uses colistin and naladixic acid to inhibit the growth of Gram-negative bacteria

Chromogenic Urine Plates

• In the HRLMP:
• Pink colonies are definitive for Escherichia coli
• Blue colonies suggest Klebsiella, Enterobacter, or Serratia

Thioglycolate Broth

• Thio broth is much more sensitive for both aerobes and anaerobes
• Useful only with specimens from sterile sites
• If an organism grows from broth only, it may be a fastidious organism, a very small inoculum, or a contaminant

Fungal

• Sabouraud with and without antibacterials: originally made for dermatophyte growth, also useful for yeasts. Not good for dimorphics.
  • Chloramphenicol and gentamicin added to inhibit growth of bacteria.
  • Cycloheximide added to inhibit growth of rapidly-growing contaminating molds.
    • However, will inhibit Mucorales, so always need media with and without cycloheximide.
  • Inhibitory antimicrobials not needed if collected from a sterile site.
• BHI (blood-heart infusion) with blood: enriched medium. Good for yeasts, including Cryptococcus, and dimorphic fungi. Non-selective medium. Used for saprophytic and dimorphic fungi (cycloheximide inhibits saprophytes).
• Sabouraud’s BHI (SABHI): also exists, good for fastidious fungi.
• Littman: selective medium for fungi. Crystal violet and streptomycin inhibit bacterial growth.
• Inhibitory Mould Agar (IMA): used to grow dimorphic fungi. Saprophytic and dermatophytic fungi will not grow. Can be used instead of SAB.
• Potato flake agar: used for fastidious and slow-growing strains.
• Cycloheximide in primary media: prevents growth of rapidly-growing molds that are frequently contaminants. However, it does inhibit some pathogenic fungi including Mucorales, Cryptococcus, Candida krusei, other Candida spp., Trichosporon spp., P. boydii, and Aspergillus spp.