Bacterial cultures
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)!Pasted image 20250112172159.png
- 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
- Eosin-methylene blue agar
- A growth medium used to culture and identify Escherichia coli. The medium turns dark purple when colonized by most lactose fermenters and is colorless for lactose nonfermenters. E. coli colonies cultured on the medium have a green metallic sheen.