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Applied and Environmental Microbiology, February 2004, p. 693-703, Vol. 70, No. 2
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.2.693-703.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

NorM, an Erwinia amylovora Multidrug Efflux Pump Involved in In Vitro Competition with Other Epiphytic Bacteria

Antje Burse, Helge Weingart, and Matthias S. Ullrich*

School of Engineering and Sciences, International University of Bremen, 28759 Bremen, Germany

Received 21 August 2003/ Accepted 7 November 2003

Blossoms are important sites of infection for Erwinia amylovora, the causal agent of fire blight of rosaceous plants. Before entering the tissue, the pathogen colonizes the stigmatic surface and has to compete for space and nutrient resources within the epiphytic community. Several epiphytes are capable of synthesizing antibiotics with which they antagonize phytopathogenic bacteria. Here, we report that a multidrug efflux transporter, designated NorM, of E. amylovora confers tolerance to the toxin(s) produced by epiphytic bacteria cocolonizing plant blossoms. According to sequence comparisons, the single-component efflux pump NorM is a member of the multidrug and toxic compound extrusion protein family. The corresponding gene is widely distributed among E. amylovora strains and related plant-associated bacteria. NorM mediated resistance to the hydrophobic cationic compounds norfloxacin, ethidium bromide, and berberine. A norM mutant was constructed and exhibited full virulence on apple rootstock MM 106. However, it was susceptible to antibiotics produced by epiphytes isolated from apple and quince blossoms. The epiphytes were identified as Pantoea agglomerans by 16S rRNA analysis and were isolated from one-third of all trees examined. The promoter activity of norM was twofold greater at 18°C than at 28°C. The lower temperature seems to be beneficial for host infection because of the availability of moisture necessary for movement of the pathogen to the infection sites. Thus, E. amylovora might employ NorM for successful competition with other epiphytic microbes to reach high population densities, particularly at a lower temperature.

* Corresponding author. Mailing address: School of Engineering and Science, International University of Bremen, Research II, Room 60, Campusring 1, 28759 Bremen, Germany. Phone: 49 421 200 3245. Fax: 49 421 200 3249. E-mail: m.ullrich{at}iu-bremen.de.

Applied and Environmental Microbiology, February 2004, p. 693-703, Vol. 70, No. 2
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.2.693-703.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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