Culture media
A medium is a question put to an organism. General purpose media ask whether it will grow at all; selective and differential media ask what it is.
Lab 8. The categories overlap on purpose — MacConkey and mannitol salt are both selective and differential, and being able to say why is usually the question.
How four common plates read
The media
Grouped by what the medium is for. The contains column is what does the work.
| Medium | Kind | Contains | How you read it |
|---|---|---|---|
| Nutrient agar | General purpose | Beef extract and peptone | Grows most non-fastidious bacteria. A general purpose plate. |
| Tryptic soy agar | General purpose | Digested casein and soybean | A richer general purpose medium. |
| MacConkey agar | Selective | Bile salts, crystal violet, lactose, neutral red | Selective for Gram-negatives; also differential — pink colonies ferment lactose, colourless ones do not. |
| Mannitol salt agar | Selective | 7.5% NaCl, mannitol, phenol red | Selective for staphylococci; differential too — yellow means mannitol was fermented, which points to S. aureus. |
| Eosin methylene blue | Selective | Eosin Y and methylene blue | Selective for Gram-negatives; E. coli gives a green metallic sheen. |
| Blood agar | Enriched | 5% sheep blood in a base agar | Enriched and differential — beta is complete clearing, alpha is green partial, gamma is none. |
| Chocolate agar | Enriched | Heated blood, which releases NAD and haemin | Grows fastidious organisms such as Haemophilus and Neisseria. |
| Thioglycollate broth | Transport / special | A reducing agent that sets up an oxygen gradient | Where growth sits in the tube shows the oxygen requirement. |
The four words
These get confused constantly, and a question can turn entirely on which one is meant.
Selective
Something in the medium stops unwanted organisms growing. Bile salts, high salt, dyes. Fewer things grow, and that is the point.
Differential
Everything grows, but they look different. Usually a fermentable sugar plus a pH indicator, so acid production changes the colour.
Enriched
Extra nutrients added so fastidious organisms will grow at all. Blood and chocolate agar are the examples.
Both at once
MacConkey selects for Gram-negatives with bile salts and crystal violet, then differentiates lactose fermenters with neutral red. One plate, two jobs.
Oxygen requirements
Thioglycollate broth sets up an oxygen gradient — high at the surface, absent at the bottom — and where the growth sits is the answer.
Reading haemolysis on blood agar
Blood agar is enriched and differential at once, and the three patterns have names you will be asked for.
| Pattern | On the plate | Means | Typical organism |
|---|---|---|---|
| Beta (β) | A clear, colourless zone around the colony | Red cells completely lysed | Streptococcus pyogenes |
| Alpha (α) | A green or brownish zone | Partial lysis; haemoglobin reduced rather than destroyed | Streptococcus pneumoniae |
| Gamma (γ) | No change at all | No haemolysis | Enterococcus faecalis |