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Applied and Environmental Microbiology, March 2005, p. 1562-1569, Vol. 71, No. 3
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.3.1562-1569.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Enhanced Survival of Salmonella enterica in Vesicles Released by a Soilborne Tetrahymena Species

M. T. Brandl,1* B. M. Rosenthal,2 A. F. Haxo,1 and S. G. Berk3

Produce Safety and Microbiology Research Unit, Agricultural Research Service, U.S. Department of Agriculture, Albany, California,1 Animal Parasitic Diseases Laboratory, Henry A. Wallace Beltsville Agricultural Research Center—East, Beltsville, Maryland,2 Center for the Management, Utilization and Protection of Water Resources, Tennessee Technological University, Cookeville, Tennessee3

Received 14 July 2004/ Accepted 6 October 2004

Nondestructive ingestion by soilborne protozoa may enhance the environmental resiliency of important bacterial pathogens and may model how such bacteria evade destruction in human macrophages. Here, the interaction of Salmonella enterica serovar Thompson with a soilborne Tetrahymena sp. isolate was examined using serovar Thompson cells labeled with the green fluorescent protein. The bacteria were mixed in solution with cells of Tetrahymena at several ratios. During incubation with serovar Thompson, Tetrahymena cells released a large number of vesicles containing green fluorescent serovar Thompson cells. In comparison, grazing on Listeria monocytogenes cells resulted in their digestion and thus the infrequent release of this pathogen in vesicles. The number of serovar Thompson cells per vesicle increased significantly as the initial ratio of serovar Thompson to Tetrahymena cells increased from 500:1 to 5,000:1. The density of serovar Thompson was as high as 50 cells per vesicle. Staining with propidium iodide revealed that a significantly higher proportion of serovar Thompson cells remained viable when enclosed in vesicles than when free in solution. Enhanced survival rates were observed in vesicles that were secreted by both starved (F = 28.3, P < 0.001) and unstarved (F = 14.09, P < 0.005) Tetrahymena cells. Sequestration in vesicles also provided greater protection from low concentrations of calcium hypochlorite. Thus, the release of this human pathogen from Tetrahymena cells in high-density clusters enclosed in a membrane may have important implications for public health.

* Corresponding author. Mailing address: USDA-ARS, Produce Safety and Microbiology Research Unit, 800 Buchanan St., Albany, CA 94710. Phone: (510) 559-5885. Fax: (510) 559-6162. E-mail: mbrandl{at}pw.usda.gov.

Applied and Environmental Microbiology, March 2005, p. 1562-1569, Vol. 71, No. 3
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.3.1562-1569.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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