4.2 Types of media
The choice of media and incubation conditions are guided by the specific requests in the submission form and the type of organisms expected, depending on the range of pathogens circulating in the region.
Bacteriological media are classified according to physical state (Figure 5) and nutrient characteristics.
Liquid media (broths) provide a fertile environment for the amplification of the bacteria present in the specimen, and are usually used to ‘enrich’ the target organism so that it surpasses the LoD of culture.
What are the potential disadvantages of using a liquid rather than a solid medium as the primary growth medium?
Answer
The disadvantages are:
- Unlike on solid media, no discrete bacterial colonies are formed, so it is not possible to obtain individual colonies for testing. This may present difficulties if there is a mixed infection.
- The relative abundance of the bacterial species initially present changes during culture and, in the case of a mixed flora, the target organism might be overgrown by a faster competitor.
All bacteriological media contain a balanced blend of energy sources and micro and macronutrients necessary for in vitro bacterial growth. Supplements may be added to encourage or suppress the growth of particular microorganisms, or to help identify potential pathogens. Table 3 describes the main media types used in microbiology laboratories.
| Media | Characteristics/example | Purpose |
|---|---|---|
| Basic | Contain a balanced mix of nutrients to support bacterial growth. Example: nutrient agar/broth | For |
| Enriched | Supplemented with extra nutrients (blood, serum, vitamins etc). Example: blood agar | For |
| Selective | Contain substances (antibiotics, salts, dyes) which suppress certain organisms while favouring the growth of others. Example: Mannitol Salt agar used to recover Staphylococcus aureus from Selective broths are often called ‘selective enrichment’ broths, as they enrich the target organism while inhibiting others. |
For non-sterile specimens where competing |
| Differential | Contain a substance, such as a pH indicator, that changes colour in response to bacterial activity, for example, when carbohydrate fermentation leads to acid production by certain organisms. Differential media are sometimes referred to as indicator media. | For basic differentiation of target organisms from other organisms potentially present in the inoculum. |
| Selective-differential | Combine the properties of selective and differential media. Example: MacConkey agar – see Figure 6. Also referred to as selective-indicator media. | To select for and differentiate target organisms from others. |
4 Bacterial isolation
