Journal of Microbiology and Antimicrobials Vol. 2(4), pp. 36-46, July 2010 Available online http://www.academicjournals.org/JMA ISSN 1996-0875 ©2010 Academic Journals Full Length Research Paper Comparative analysis of antibacterial activities of Xenorhabdus species on related and non-related bacteria in vivo András Fodor 1,2,4 *, Andrea M. Fodor 2,7 , Steven Forst 3 , Joseph S. Hogan 4 , Michael G. Klein 5 , Katalin Lengyel 2 , Gyula Sáringer 1 , Erko Stackebrandt 6 , R. A. J. Taylor 5 and Éva Lehoczky 1 1 Institute of Plant Protection, Georgikon Faculty, University of Pannonia, H-8360 Keszthely, Deák Ferenc utca 17, Hungary. 2 Institute of Biology, Departments Genetics and Biochemistry, Faculty of Natural Sciences, Eötvös University, Pázmány Péter sétány 1C, Budapest, Hungary, H-1117; Hungary. 3 Department of Biology, Lapham Hall, Milwaukee, WI; University of Wisconsin-Milwaukee, Arizona USA. 4 Department of Animal Sciences, The Ohio State University (OSU), OARDC, 1680 Madison Avenue Wooster, Ohio 44691; USA. 5 Department of Entomology, OSU, OARDC, 1680 Madison Avenue Wooster, Ohio 44691; USA. 6 DSMZ, Deutsche Sammlung von Mikroorganizmen und Zellkulturen GmbH, Braunschweig, Germany. 7 MCIC, OSU, OARDC, 1680 Madison Avenue Wooster, Ohio 44691, USA. Accepted 25 June, 2010 Insect-nematode-bacterium mutualistic associations provide attractive systems for discovery of inter kingdom signal compounds and antibiotics. A better understanding of the biological meaning of the inter-specific diversity of compounds with antimicrobial activity of the Steinernema-symbiont Xenorhabdus bacteria may provide options for simultaneous applications in pathogen control. Anti- bacterial activities of representative strains of Xenorhabdus budapestensis, Xenorhabdus szentirmaii, Xenorhabdus innexi, Xenorhabdus ehlersii, Xenorhabdus nematophila, Xenorhabdus bovienii and Xenorhabdus cabanillassii were tested on non-related (Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus) bacteria and on each other by previously published bioassays. All active compounds were adsorbed by Amberlite R XAD1180. Chemical and thermal stability of antibacterial factors were determined. Antibiotic factors produced by different Xenorhabdus species against each other differ from those used against other competing bacterial genera. Anti-Xenorhabdus activity of the cell-free medium and sensitivity of the cells of other Xenorhabdus strains negatively correlated in X. innexi and X. bovienii. Some activity remained unchanged during high pressure and 121°C for 10 min. The first comparative analysis of the intraspecific antibacterial activities of Xenorhabdus species demonstrated that some Xenorhabdus species with strong antibacterial activity could be co-cultured and they might be used simultaneously for pathogen control. Key words: Xenorhabdus, autoclaveable antimicrobials, intra-generic, cross-tolerance. INTRODUCTION The entomopathogenic nematode / bacterium (EPN/EPB, *Corresponding author. E-mail: fodorandras@yahoo.com. Tel: +36-83-545-219. Steinernema / Xenorhabdus and Heterorhabditis / Photorhabdus) symbiotic associations (Goodrich-Blair and Clarke, 2007) are potential tools for biological control of insect pests (Gaugler, 2002; ffrench-Constant et al., 2007) and microbial pathogens (Böszörményi et al., 2009) of agricultural importance. Mechanistic details of