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Applied and Environmental Microbiology, July 2009, p. 4720-4726, Vol. 75, No. 14
0099-2240/09/$08.00+0     doi:10.1128/AEM.00251-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Involvement of Cell Surface Structures in Size-Independent Grazing Resistance of Freshwater Actinobacteria{triangledown} ,{dagger}

Mitsunori Tarao,1,2* Jan Jezbera,3 and Martin W. Hahn1

Institute for Limnology, Austrian Academy of Sciences, Mondseestrasse 9, A-5310 Mondsee, Austria,1 Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183-8509, Japan,2 Biology Centre ASCR, v.v.i., Institute of Hydrobiology CZ-37005, Ceské Budejovice, Czech Republic3

Received 2 February 2009/ Accepted 27 May 2009

We compared the influences of grazing by the bacterivorous nanoflagellate Poterioochromonas sp. strain DS on ultramicrobacterial Actinobacteria affiliated with the Luna-2 cluster and ultramicrobacterial Betaproteobacteria of the species Polynucleobacter cosmopolitanus. These bacteria were almost identical in size (<0.1 µm3) and shape. Predation on a Polynucleobacter strain resulted in a reduction of >86% relative to the initial bacterial cell numbers within 20 days, while in comparable predation experiments with nine actinobacterial strains, no significant decrease of cell numbers by predation was observed over the period of ≥39 days. The differences in predation mortality between the actinobacterial strains and the Polynucleobacter strain clearly demonstrated size-independent grazing resistance for the investigated Actinobacteria. Importantly, this size-independent grazing resistance is shared by all nine investigated Luna-2 strains and thus represents a group-specific trait. We investigated if an S-layer, previously observed in an ultrastructure study, was responsible for the grazing resistance of these strains. Experiments aiming for removal of the S-layer or modification of cell surface proteins of one of the grazing-resistant strains by treatment with lithium chloride, EDTA, or formaldehyde resulted in 4.2- to 5.2-fold higher grazing rates in comparison to the levels for untreated cells. These results indicate the protective role of a proteinaceous cell surface structure in the size-independent grazing resistance of the actinobacterial Luna-2 strains, which can be regarded as a group-specific trait.

* Corresponding author. Mailing address: Laboratory of Environmental Microbiology, Faculty of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan. Phone and fax: 81-(0)42-367-5852. E-mail: tarao{at}cc.tuat.ac.jp

{triangledown} Published ahead of print on 5 June 2009.

{dagger} Supplemental material for this article may be found at http://aem.asm.org/.

Applied and Environmental Microbiology, July 2009, p. 4720-4726, Vol. 75, No. 14
0099-2240/09/$08.00+0     doi:10.1128/AEM.00251-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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