Also, as a microbiologist, the smell is microbes growing.

Washing isn't absolutely necessary to kill them. Certain types of gas (ozone or N2 works as well). Sunlight does as well (E. coli only survives 15 seconds in direct sunlight). As does freezing. I'm not a materials scientist, so I'm not going to tell you what works best on your helmet material.

I found that throwing my jeans in the -80C/-112F freezer at work for a few days cleans them as well (smell out) and is much easier on the raw denim.