Staph: a weakness for iron

I don't know why I did that, but I had to get it out of the way; write a post about red wine and the symmetry of itching. Now, here's what I intended to write about: a story that appeared in today's New York Times about new research on Staphylococcus aureus. Scientists at Vanderbilt University have found that S. aureus has evolved to plague humans, and that it's specialized to extract iron from our hemoglobin.

Now I expect that the problem that eczema patients have with S. aureus is more to do with cracks in the skin barrier and surface pH (S. aureus likes it alkaline, and eczematous skin obliges; normal skin is acidic). But I am intrigued by the role iron seems to play. Pathogenic microbes, it appears, need iron to live, and they don't get it by eating spinach or red meat or drinking red wine. They get it from us. They get it from our blood.

In another recent paper I covered, the authors mention that the skin is an iron-poor environment, and S. aureus actually has an iron sensor that, when there isn't any iron, helps the bacteria adhere to skin. Maybe this is to help it infiltrate the skin and get into the blood.

What the scientists find in this latest paper is that S. aureus is much better at binding hemoglobin from humans than hemoglobin from mice. The bacteria grow better when they feed on the human version, rather than the mouse version. And they have a receptor for hemoglobin, called IsdB, which is essential to hemoglobin capture. Without IsdB, S. aureus is crippled.

This is interesting, but for you and me possibly the most important result of the paper is that the authors show how useful a transgenic mouse model--an animal that has one copy of the gene for mouse hemoglobin, and one copy of the gene for human hemoglobin-- is in the study of S. aureus. If you study S. aureus using "normal" mice, the pathogen doesn't have access to iron like it does in humans, so it may be behaving differently in other ways too. Scientists using this new transgenic mouse will be more confident that their results are relevant for humans.

In particular, I wouldn't be surprised if a number of studies in the Atopic Dermatitis Vaccine Network (which is focused on MRSA) adopt this transgenic mouse as their model animal.