The skin microbiome influences skin barrier function by controlling factors such as temperature, pH, hydration, and lipid content. Changes in these factors can weaken the skin barrier as can deficiencies in structural proteins like filaggrin.  A "leaky" skin barrier, when penetrated by bacteria, can lead to inflammation and itchy flares. 

Staphylococcus bacterium is gram-positive spherically shaped bacterium that is a normal part of the human skin and nose microbiomes, where it is normally benign.  About 10% of healthy people become colonized by S. aureus, but 80% of those with eczema are disproportionally colonized. 

When S. aureus populations increase, they can form biofilms that are hard to kill and can lead to severe infections.  If the skin barrier is compromised and leaky, the bacterium can enter the bloodstream where it can become dangerous.  Even worse are strains that are resistant to antibiotics, like MRSA (methicillin-resistant S. aureus).

A staph infection typically starts as red bumps or red streaks that can evolve into sores, pus-filled blisters or yellow-colored crusts.  For those with eczema, these infections may lead to severe itch, which leads to scratching, bleeding and a higher risk of spreading the infection into the bloodstream.