Should I mention in a PhD application statement of purpose that I earn a large income from a business I created? I am UK national applying for a PhD (microbiology) in the USA. The application process requires a statement of purpose which includes, among other things, a section on recent / current work activities that are relevant to the application. 

I am a Master's student and have plenty of relevant details to talk about from an academic perspective (there is a separate section for this), but I also run a successful IT business and the transferable skills that I have learned from this form a core part of my relevant work experience for this section. 

To illustrate my motivation and potential for success at graduate level, I was going to mention that I essentially started my business from scratch a few years ago and now earn a six-figure income. However, I am worried that the selection committee might view this unfavorably (talking about income is quite a taboo subject here in the UK), or worse they might question my commitment to a graduate program if they think I have a comfortable job to fall back on.

I should also mention that the PhD program I am applying to is fully-funded, but the cost of living in the area is notoriously high and most grad students have to supplement their income by teaching. I wouldn't need to do this as I have enough savings to be financially secure for the duration of the program. 

Should I mention my income to demonstrate my aptitude, or will it do more harm than good?
No, don't mention income here. This just looks smug. But ofc you should mention that you are running a successfull buisness and explain how this can contribute to your Phd (experience, skills,...). The problem I see is that you want to do a microbiology PhD while running your own buisness? This could be a big problem because they don't want to fund people who are not 100% focused on their work here.