(Bloomberg) — When it comes to taxes, corporate America is getting a bit less corporate. And a bit less American.

Fueled by a wave of inversions, a record 54 companies in the Standard & Poor's 500 Index of leading U.S. firms are now at least partially exempt from the corporate income tax. That's more than twice the number four years ago.

The biggest factor is the recent wave of companies, such as Medtronic and Mylan, that have completed what's known as an inversion, in which they move their tax address overseas. Other companies have declared themselves to be real estate investment trusts, or REITs, which the Internal Revenue Service doesn't treat as corporations. Just this year, Equinix Inc., a California company that operates data centers, became a REIT to lower its effective tax rate to as little as 10 percent. At 35 percent, the U.S. corporate rate is the highest in the developed world.