Microbiologists want W.O.U.N.D.S. to become colonised with bacteria!

Bacteria like to colonise warm moist sites with poor blood supply and hence a poor immune response. They especially like broken areas of skin such as ulcers and lacerations.

Broken areas of skin are not sterile. Wounds should be colonised with relatively harmless bacteria like the Coagulase-negative Staphylococci, Coryneform bacteria (Diptheroids) and Propionibacterium spp. This normal process prevents more dangerous bacteria from getting in and causing infections such as Staphylococcus aureus and the Beta-haemolytic Streptococci. It is therefore really important that we only treat breaks in the skin when infections have occurred, rather than trying to prevent them occurring.

Consider a patient who has a bad pretibial laceration having fallen in the street. She attends the hospital to have the wound cleaned and dressed. The wound is large so the Doctors decide to try and prevent an infection occurring by starting PO Flucloxacillin, thinking this will stop the pathogenic S. aureus or Streptococci. One week later the wound looks a bit sloughy so samples are sent to the microbiology laboratory. The laboratory reports the presence of Meticillin-Resistant Staphylococcus aureus (MRSA). The patient is then admitted for IV Teicoplanin to treat the MRSA that has grown, and because the wound is large, the patient is referred to the surgeons for consideration of skin grafting. The surgeons decide that skin grafting is a good idea and ask for the patient to remain on Teicoplanin until the skin grafting is done. 

Does this sound familiar? 
A further week down the line the microbiologist is called because the wound is still sloughy and now has a green/blue discolouration which the surgeons think is a Pseudomonas  aeruginosa infection and they want to know what antibiotics to give. The surgeons are right, the green/blue colour probably is Pseudomonas aeruginosa (these bacteria produce chemicals called pyoverdin and pyocyanin which are green and blue respectively) but they are probably wrong about the bacterium causing infection. As a result of the presence of Pseudomonas aeruginosa the surgeons now won’t do the skin graft because of the high risk of the graft not taking and the wound not healing. This will result in the patient being left with the laceration healing by secondary intention and hence they will have a large scar and defect. The microbiologist makes the point that the best treatment of the patient would have been to not start the antibiotics in the first place, and that a better clinical assessment of the patient is required.

Let’s look at this in more detail.
The diagnosis of a wound infection is clinical not microbiological. Just because bacteria grow doesn’t necessarily mean they are causing infection, they may well be normal flora. Infected wounds (see image below - A) show signs of acute inflammation (pain, erythema and swelling) as well as increased purulent discharge. These signs are detected by asking questions and examining the patient. Wounds that look “mucky” in the middle (see image below - B) tend to contain lots of slough which is dead and detached tissue and is not a sign of infection, debrided these wounds can be returned to clean (see image below - C).

Click for larger image

Microbiologists want wounds to become colonised with bacteria! But they want healthy normal bacteria. Giving the antibiotics Flucloxacillin and Teicoplanin, in the example above, has ensured that no normal bacteria can grow and colonise the wound. These antibiotics are very good at killing the normal Gram-positive bacteria, and leave behind a warm moist environment for nasty Gram-negative bacteria such as Pseudomonas aeruginosa, to grow in. As this patient did not originally have an infected wound, giving antibiotics has led to the colonisation with Gram-negative bacteria. The doctors have created the contraindication to the patient’s best possible treatment (skin grafting) as the growth of Pseudomonas aeruginosa was an inevitable consequence of the original use of Flucloxacillin as prophylaxis for the laceration. This is clearly not good medical practice! Think
very carefully about the microbiological consequences of antibiotic prophylaxis.

When dealing with possible wound infections try to remember W.O.U.N.D.S…

Wound infection is a clinical diagnosis based upon the presence of inflammation (pain, erythema, swelling and
purulent discharge)
Only take microbiological samples from infected looking wounds, broken areas of skin are not sterile...even “clean wounds” will grow something
Usual causes of wound infections are Staphylococcus aureus and the Beta-haemolytic Streptococci
Normal flora is normal, just because bacteria grow doesn’t necessarily mean the wound is infected
Do not routinely give antibiotic prophylaxis for wounds, all it does is prevent normal flora colonising the wound and leads to the presence of microbial nasties!
Slough is a sign of dead and detached tissue, not infection; it should be removed before microbiological specimens are taken from the remaining healthy margin