Why does hydrogen peroxide bubble on a wound?

because it’s really flesh eating bacteria

Hydrogen peroxide is H2O2, effectively water plus one oxygen atom. When it comes in contact with catalase, an iron carrying enzyme found in most living organisms, the enzyme uses the iron atom to liberate an two oxygen atoms from two hydrogen peroxide molecules to produce water plus free oxygen (O2). The bubbles we see are the released oxygen.

Excellent answer Harp. To elaborate further, this effect of catalase is actually used in a clinical test to differentate staphylococcus (which has catalase) and streptococcus (one version of which is the so-called flesh eating bacteria and is catalase negative). You can put a smear of the bacteria on a glass slide, then add 3% hydrogen peroxide. If the unknown bacteria is staphylococcus, the slide will bubble. If its streptococcus or enterococcus, it won’t.

So is catalase found in my blood or in the bacteria festering in my wound?

Edit: Both, actually, I would imagine.

Yep, both.

But not on my skin, right?

Skin cells contain catalase, but my guess is that it wouldn’t be available for reaction unless the cells were ruptured.

Most skin wound infections are due to staphylococcus (aureus or epidermidus), streptococcus pyogenes (so called Group A strep) or a combination of both. The catalase in the wound is mostly from blood and neutrophils (a type of leukocyte that kills bacteria).

With respect to your skin, there are many bacteria living there normally. However, the numbers are on the low side, so putting some hydrogen peroxide there won’t yield much in the way of catalase activity. Try a little experiment at home and see what happens.