ETH Zurich, Institute of Microbiology, 8093 Zurich, Switzerland. kiefer@micro.biol.ethz.ch|ETH Zurich, Institute of Microbiology, 8093 Zurich, Switzerland. kiefer@micro.biol.ethz.ch|ETH Zurich, Institute of Microbiology, 8093 Zurich, Switzerland. kiefer@micro.biol.ethz.ch