General purpose media
These contain all the essential requirements described in the previous section to enable the growth of most bacterial species. Material such as sterile blood (horse or sheep) may be added to improve the growth through addition of essential vitamins and minerals, required by more delicate organisms.
Selective culture media
This is a general purpose media with additives, such as chemicals, dyes, or antibiotics. These additional substances will promote the growth of certain specific organisms whilst inhibiting the growth of other bacterial species. For example, adding 10% sodium chloride (salt) to a general purpose medium makes it specific for the growth of Staphylococcus spp. If meticillin is also added this would make the medium specific for meticillin-resistant Staphylococcus aureus (MRSA). Using a culture medium like this improves the likelihood and speed of isolating MRSA from clinical wound samples.
The choice of culture medium used for cultivation of bacteria from wound samples will depend on the specific preferences of the processing laboratory, and on supplied clinical details. However, they will include the following:
- ? Blood agar (incubated both aerobically and anaerobically) which is a non-selective medium and will allow the growth of all bacterial species. This gives a general picture of the total bacterial load of the specimen.
- ? MacConkey or CLED (cysteine lactose electrolyte deficient) agar. Designed to isolate faecal organisms such as Escherichia coli, Pseudomonas spp., enteric streptococci, Proteus spp., and so on. May indicate the level to which the wound is contaminated or colonized by the patient’s own gut flora.
- ? Anaerobic selective medium designed to isolate anaerobic bacteria such as Clostridium spp. and Bacteroides spp.
- ? A variety of other selective medium, dependent upon the clinical information provided.