[1] Stanford Univ, Dept Bioengn, Stanford, CA 94305 USA
[2] Univ Copenhagen, Sect Microbiol, Copenhagen, Denmark
[3] Univ North Carolina Chapel Hill, Howard Hughes Med Inst, Chapel Hill, NC 27599 USA
[4] Univ North Carolina Chapel Hill, Dept Biol, Chapel Hill, NC 27599 USA
[5] Princeton Univ, Dept Chem & Biol Engn, Princeton, NJ 08544 USA