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Applied and Environmental Microbiology, March 2004, p. 1356-1359, Vol. 70, No. 3
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.3.1356-1359.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Inoculum Size Effect in Dimorphic Fungi: Extracellular Control of Yeast-Mycelium Dimorphism in Ceratocystis ulmi

Jacob M. Hornby,{dagger} Sarah M. Jacobitz-Kizzier,{ddagger} Donna J. McNeel,§ Ellen C. Jensen, David S. Treves,|| and Kenneth W. Nickerson*

School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588-0666

Received 25 June 2003/ Accepted 10 December 2003

We studied the inoculum size effect in Ceratocystis ulmi, the dimorphic fungus that causes Dutch elm disease. In a defined glucose-proline-salts medium, cells develop as budding yeasts when inoculated at >=106 spores per ml and as mycelia when inoculated at <106 spores per ml. The inoculum size effect was not influenced by inoculum spore type, age of the spores, temperature, pH, oxygen availability, trace metals, sulfur source, phosphorous source, or the concentration of glucose or proline. Similarly, it was not influenced by added adenosine, reducing agents, methyl donors, amino sugars, fatty acids, or carbon dioxide. Instead, growing cells excreted an unknown quorum-sensing factor that caused a morphological shift from mycelia to budding yeasts. This yeast-promoting effect is abolished if it is extracted with an organic solvent such as ethyl acetate. The quorum-sensing activity acquired by the organic solvent could be added back to fresh medium in a dose-dependent fashion. The quorum-sensing activity in C. ulmi spent medium was specific for C. ulmi and had no effect on the dimorphic fungus Candida albicans or the photomorphogenic fungus Penicillium isariaeforme. In addition, farnesol, the quorum-sensing molecule produced by C. albicans, did not inhibit mycelial development of C. ulmi when present at concentrations of up to 100 µM. We conclude that the inoculum size effect is a manifestation of a quorum-sensing system that is mediated by an excreted extracellular molecule, and we suggest that quorum sensing is a general phenomenon in dimorphic fungi.

* Corresponding author. Mailing address: School of Biological Sciences, University of Nebraska, Lincoln, NE 68588-0666. Phone: (402) 472-2253. Fax: (402) 472-8722. E-mail: Knickerson1{at}unl.edu.

{dagger} Present address: Division of Natural Sciences, Lewis-Clark State College, Lewiston, ID 83501.

{ddagger} Present address: Department of Bacteriology, University of Wisconsin, Madison, WI 53706.

§ Deceased 27 November 2000.

Permanent address: Biological Sciences, College of St. Benedict's and St. John's University, Collegeville, MN 56321.

|| Present address: Department of Biology, Indiana University Southeast, New Albany, IN 47150.

Applied and Environmental Microbiology, March 2004, p. 1356-1359, Vol. 70, No. 3
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.3.1356-1359.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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