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Journal of Bacteriology, April 2005, p. 2768-2773, Vol. 187, No. 8
0021-9193/05/$08.00+0     doi:10.1128/JB.187.8.2768-2773.2005

Surprising Dependence on Postsegregational Killing of Host Cells for Maintenance of the Large Virulence Plasmid of Shigella flexneri

Sameera Sayeed,^1, Therese Brendler,¹ Michael Davis,^2, Lucretia Reaves,¹ and Stuart Austin^1*

Gene Regulation and Chromosome Biology Laboratory, National Cancer Institute—Frederick, Frederick, Maryland,¹ Department of Biological Sciences, Central Connecticut State University, New Britain, Connecticut²

Received 24 October 2004/ Accepted 10 January 2005

Low-copy-number plasmids all encode multiple systems to ensure their propagation, including replication, partition (active segregation), and postsegregational killing (PSK) systems. PSK systems kill those rare cells that lose the plasmid due to replication or segregation errors. PSK systems should not be used as the principle means of maintaining the plasmid. The metabolic cost of killing the many cured cells that would arise from random plasmid segregation is far too high. Here we describe an interesting exception to this rule. Maintenance of the large virulence plasmid of Shigella flexneri is highly dependent on one of its PSK systems, mvp, at 37°C, the temperature experienced during pathogenesis. At 37°C, the plasmid is very unstable and mvp efficiently kills the resulting cured bacterial cells. This imposes a major growth disadvantage on the virulent bacterial population. The systems that normally ensure accurate plasmid replication and segregation are attenuated or overridden at 37°C. At 30°C, a temperature encountered by Shigella in the outside environment, the maintenance systems function normally and the plasmid is no longer dependent on mvp. We discuss why the virulent pathogen tolerates this self-destructive method of propagation at the temperature of infection.

Present address: Center for Genomic Sciences, Allegheny General Hospital, Pittsburgh, PA 15212.

Present address: Department of Biomolecular Sciences, Central Connecticut State University, New Britain, Conn.

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