M.mic- Case on WFH

Case: Surgical wound infection
Specimen: Wound swab

There are 2 main types of culture for wound infection. For external wounds (on the surface of the skin), swabs are sent in dry tube and stuart transport medium. The dry swab is used for gram staining to prelimarily identify organisms present on the wound. We normally look out for gram positive or negative organisms, bacilli or cocci. Afterwhich, the swab sent in stuart transport medium is used for culturing on blood agar (BA), macconkey agar (MAC) and cooked meat agar (CM).

BA is used for general growth of organisms and identifying streptococcus strains based on the hemolytic activities (alpha, beta or gamma hemolysis).

MAC is used as an inhibitory and differential medium used to distinguish lactose-fermenting enteric gram-negative bacilli from nonfermenters.

CM is used for isolation of Clostridium and to evaluate proteolysis.


For internal wounds (below the skin surface area), swabs are sent in dry tube, PRAS transport medium and stuart transport medium. The procedures is the same as above for the dry and stuart swabs. For the PRAS swab, we use it to culture on another BA for incubation in CO2 and ANO2

Since a dry swab will be provided, the external culture will be done.

The procedure:

1. Culture on blood agar
2. Culture on Macconkey agar
3. Culture on cooked meat agar
4. Do a gram stain

Gram stain is done after plating because it will contaminate the swab. (the slide is not sterile)

The most common causative organisms isolated for wound infection are:

1. Styphylococcus aureus.
2.Streptococcus pyogenes
3.Clostridium
4. Pseudomonas
5. Enterococcus
6. Proteus mirabilis
7. Enterobacteriaceae


References

Lehmann (1998) Clinical Bacteriology. Saunders Manual of Clinical Laboratory science pages 589 to 667. WB Saunders, Philadelphia

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