A Single Mechanosensitive Channel Protects Francisella tularensis subsp. holarctica from Hypoosmotic Shock and Promotes Survival in the Aquatic Environment

Abstract

ABSTRACT Francisella tularensis subsp. holarctica is found in North America and much of Europe and causes the disease tularemia in humans and animals. An aquatic cycle has been described for this subspecies, which has caused waterborne outbreaks of tularemia in at least 10 countries. In this study, we sought to identify the mechanosensitive channel(s) required for the bacterium to survive the transition from mammalian hosts to freshwater, which is likely essential for the transmission of the bacterium between susceptible hosts. A single 165-amino-acid MscS-type mechanosensitive channel ( Ft MscS) was found to protect F. tularensis subsp. holarctica from hypoosmotic shock, despite lacking much of the cytoplasmic vestibule domain found in well-characterized MscS proteins from other organisms. The deletion of this channel did not affect virulence within the mammalian host; however, Ft MscS was required to survive the transition from the host niche to freshwater. The deletion of Ft MscS did not alter the sensitivity of F. tularensis subsp. holarctica to detergents, H 2 O 2 , or antibiotics, suggesting that the role of Ft MscS is specific to protection from hypoosmotic shock. The deletion of Ft MscS also led to a reduced average cell size without altering gross cell morphology. The mechanosensitive channel identified and characterized in this study likely contributes to the transmission of tularemia between hosts by allowing the bacterium to survive the transition from mammalian hosts to freshwater. IMPORTANCE The contamination of freshwater by Francisella tularensis subsp. holarctica has resulted in a number of outbreaks of tularemia. Invariably, the contamination originates from the carcasses or excreta of infected animals and thus involves an abrupt osmotic downshock as the bacteria enter freshwater. How F. tularensis survives this drastic change in osmolarity has not been clear, but here we report that a single mechanosensitive channel protects the bacterium from osmotic downshock. This channel is functional despite lacking much of the cytoplasmic vestibule domain that is present in better-studied organisms such as Escherichia coli ; this report builds on previous studies that have suggested that parts of this domain are dispensable for downshock protection. These findings extend our understanding of the aquatic cycle and ecological persistence of F. tularensis , with further implications for mechanosensitive channel biology.