1. Physiopathology Research Unit, National Animal Disease Center, US Department of Agriculture-Agricultural Research Service, Ames, Iowa, USA
2. Lehrstuhl für Ökologische Mikrobiologie, BITöK, Universität Bayreuth, D-95440, Bayreuth, Germany
3. Department of Microbiology, Immunology and Preventive Medicine, Iowa State University, Ames, Iowa, USA