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Francisella tularensis , a gram–negative facultative intracellular bacterial pathogen, is the causative agent of tularemia and able to infect many mammalian species, including humans. Because of its ability to cause a lethal infection, low infectious dose, and aerosolizable nature,  F .  tularensis  subspecies  tularensis  is considered a potential biowarfare agent. Due to its  in vitro  efficacy, ciprofloxacin is one of the antibiotics recommended for post-exposure prophylaxis of tularemia. In order to identify therapeutics that will be efficacious against infections caused by drug resistant select-agents and to better understand the threat, we sought to characterize an existing ciprofloxacin resistant (CipR) mutant in the Schu S4 strain of  F .  tularensis  by determining its phenotypic characteristics and sequencing the chromosome to identify additional genetic alterations that may have occurred during the selection process. In addition to the previously described genetic alterations, the sequence of the CipR mutant strain revealed several additional mutations. Of particular interest was a frameshift mutation within  kdsD  which encodes for an enzyme necessary for the production of 3-Deoxy- D - manno -Octulosonic Acid (KDO), an integral component of the lipopolysaccharide (LPS). A  kdsD  mutant was constructed in the Schu S4 strain. Although it was not resistant to ciprofloxacin, the  kdsD  mutant shared many phenotypic characteristics with the CipR mutant, including growth defects under different conditions, sensitivity to hydrophobic agents, altered LPS profiles, and attenuation in multiple models of murine tularemia. This study demonstrates that the KdsD enzyme is essential for  Francisella  virulence and may be an attractive therapeutic target for developing novel medical countermeasures.

plos one

A spontaneous mutation in kdsD, a biosynthesis gene for 3 Deoxy-D-manno-Octulosonic Acid, occurred in a ciprofloxacin resistant strain of Francisella tularensis and caused a high level of attenuation in murine models of tularemia