A major/concentration in Microbiology (I had 3 courses Gen Micro/Lab, Med Micro, and Virology along with TAing the Micro lab twice) will give you an edge when it comes to second year. Trust me. It's the only class I've honored (Top 15 percentile) so far in med school. I've been close many times but was off by a question or two each time.
I guess a large part of it is that I enjoy it, but it definitely helps to know all of the lab techniques, medical microbiology concepts, and a very very important thing being which are gram +, -, and acid fast. (Especially for choosing an antibiotic or which ones have LPS or some other toxins). Plus, it allowed me to study less since I have a good memory with things I am interested in so that was nice.
Other than micro, it would probably help having basic knowledge in genetics, biochemistry, and phys. You certainly don't have to major in biology but if you enjoy it then definitely take it. You can easily minor in biology to take these types of courses and major in something else. You can stand out by doing some interesting theses in areas not related to biology. For example, something in literature, arts, or humanities will be highly regarded when it comes to applying to medical school. They want to see that you are first and foremost human, are able to observe things beneath the surface, and are able to think rationally and in an abstract sense (tested by verbal reasoning). Though what I wrote about was MAP Kinase Phosphatases so yeah do what you like.