Types of culture media
- Enrichment culture media: provides optimal conditions for general bacterial growth
- Selective culture media
- Used to grow only select bacteria and thus to isolate specific pathogens
- Contain substances (e.g., antibiotics) that prevent the growth of other organisms
- Example: Thayer-Martin agar
- Indicator media (differential media)
- Contain indicator substances that undergo a change in color when coming in contact with metabolic products of certain organisms
- Example: MacConkey agar
Most common bacterial cultures
- MacConkey agar
- Contains lactose, bile salts, sodium chloride, and a pH indicator
- Lactose fermenters (e.g., E. coli and Klebsiella)
- Pink colonies (indicator turns pink when pH is lowered due to fermentation of lactose to acidic hydrogen sulfide)
Mnemonic
- Fermentation of lactose → pink colonies on MacConkey agar. Examples include Citrobacter, E coli, Enterobacter, Klebsiella, Serratia.
- McCowkey CEEKS milk.
- Hektoen enteric agar
- Composed of proteose peptone, various sugars (e.g., lactose, sucrose, salicin), sodium thiosulfate, and iron (III) ammonium citrate
- Enteric bacteria (e.g., Salmonella and Shigella)
- Salmonella: black colonies
- Salmonella produces hydrogen sulfide (H2S) using thiosulfate, which results in black colony formation, that can be differentiated from Shigella colonies that do not produce H2S.
- Shigella: green colonies
- Salmonella: black colonies
- Thayer-Martin agar
- Contains the following antibiotics to inhibit growth of other organisms:
- Vancomycin: inhibits growth of gram‑positive bacteria
- Trimethoprim and colistin: inhibit growth of other gram‑negative bacteria
- Nystatin: inhibits growth of fungi
- Neisseria spp.
- Contains the following antibiotics to inhibit growth of other organisms:
- Chocolate agar
- Contains X factor (hematin) and V factor (NAD+) to promote growth of fastidious organisms (organisms with complex or stringent nutritional requirements that are difficult to culture and only grow with specific nutritional supplementation and precise environmental control)
- Made by heating sheep/horse blood to 80 ℃ and lysing RBCs. This releases hemoglobin which turns brown when heated - this is called “chocolate-ing” of the blood.
- Haemophilus influenzae
- Contains X factor (hematin) and V factor (NAD+) to promote growth of fastidious organisms (organisms with complex or stringent nutritional requirements that are difficult to culture and only grow with specific nutritional supplementation and precise environmental control)