Scott Bradlee, center, leads a rotating cast of characters in his genre-crossing covers band Postmodern Jukebox. (Postmodern Jukebox)

Meghan Trainor merely sings “All About That Bass.” Jazz musician Kate Davis actually plays a bopping bass while cooing “I’m bringing bootie back” in Postmodern Jukebox’s cover of the 2014 hit.

Credit for the makeover goes to bandleader Scott Bradlee, who disliked piano lessons as a kid but is now a keyboard maestro, turning pop and rap hits into ragtime, old-timey jazz and gutbucket soul. (Bradlee and company come to the Birchmere on Monday, just days after releasing the new Postmodern Jukebox album “Selfies on Kodachrome.”)

Bradlee, 33, says he owes his success to Gershwin’s “Rhapsody in Blue.” When he heard it at age 12, he was determined to learn to play that way. Cassettes and CDs from the library were his teachers.

His first attempt at genre mixing came in high school with ragtime gangsta rap. Years later, as a jazz musician trying to make it in New York, he decided to post one of his offbeat clips to YouTube: a medley of ’80s songs done ragtime style. Noted author Neil Gaiman tweeted about it; viral popularity ensued.

By 2011, Bradlee had formed Postmodern Jukebox and began putting popular music in a time machine. Most recently, he’s filled Taylor Swift’s “Blank Space” with sultry cabaret flavor and asked “American Idol” alum Casey Abrams to give a swing vibe to the Fountains of Wayne earworm “Stacy’s Mom.”

If a song already has a soulful or jazzy hue, “it’s hard to say something new,” Bradlee says, of how he chooses covers. Instead, he looks for chords that can transition to swing jazz or ragtime and key changes that have an “epic” feel.

He also relishes a challenge. “The silliest song is the most difficult to cover,” Bradlee says, because he wants to make good music, not jokes.

Applying a bluegrass brush to Nicki Minaj’s “Anaconda” (and eliminating the swears, as he always does, for YouTube friendliness) made it bounce with joy and also turned it into “a song just about snakes.”

Bradlee acknowledges that it’s easy to view what his band does as gimmicky. “But we take it seriously,” he says. “We don’t do it as a parody. It’s about doing what feels authentic to me. It’s not the coolest stuff. But it’s stuff I would want to listen to if it was on the radio.”

Superstars like it, too. During a Reddit AMA, Lorde said Bradlee’s “Royals” — sung in the booming voice of seven-foot-tall clown/performance artist Puddles — is her favorite cover.

And Bradlee’s choice of vocalist isn’t all that far out: A giant clown is as much an outsider as an angsty teen.

Birchmere, 3701 Mount Vernon Ave., Alexandria; Mon., 7:30 p.m., sold out.

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