Virulence differences among Francisella tularensis subsp. tularensis clades in mice

Francisella tularensis subspecies tularensis (type A) and holarctica (type B) are of clinical importance in causing tularemia. Molecular typing methods have further separated type A strains into three genetically distinct clades, A1a, A1b and A2. Epidemiological analyses of human infections in the United States suggest that A1b infections are associated with a significantly higher mortality rate as compared to infections caused by A1a, A2 and type B. To determine if genetic differences as defined by molecular typing directly correlate with differences in virulence, A1a, A1b, A2 and type B strains were compared in C57BL/6 mice. Here we demonstrate significant differences between survival curves for infections caused by A1b versus A1a, A2 and type B, with A1b infected mice dying earlier than mice infected with A1a, A2 or type B; these results were conserved among multiple strains. Differences were also detected among type A clades as well as between type A clades and type B with respect to bacterial burdens, and gross anatomy in infected mice. Our results indicate that clades defined within F. tularensis subsp. tularensis by molecular typing methods correlate with virulence differences, with A1b strains more virulent than A1a, A2 and type B strains. These findings indicate type A strains are not equivalent with respect to virulence and have important implications for public health as well as basic research programs

Successful Treatment of Human Plague with Oral Ciprofloxacin

The US Food and Drug Administration recently approved ciprofloxacin for treatment of plague (Yersina pestis infection) based on animal studies. Published evidence of efficacy in humans is sparse. We report 5 cases of culture-confirmed human plague treated successfully with oral ciprofloxacin, including 1 case of pneumonic plague

PFGE analysis of <i>F. tularensis</i> strains.

<p>A diagram depicting the eight <i>F. tularensis</i> strains used to infect mice (two strains per each of the four <i>F. tularensis</i> groups, A1a, A1b, A2 and type B). <i>F. tularensis</i> subsp. <i>tularensis</i> strain Schu S4 (A1a) and LVS (type B) were included in the dendogram as reference strains.</p

Weight loss and spleen, liver and lung weights for mice infected with <i>F. tularensis</i> A1a, A1b, A2, and type B.

<p>C57BL/6J mice (n = 14/group) were challenged intradermally with 10-20 CFU of <i>F. tularensis</i> A1a, A1b, A2, and type B. Mice were weighed prior to infection and at time of death. Organs were harvested and weighed at time of death. <b>A</b>) Mouse weight (grams) for each infection group and both control groups. No statistical difference was observed between round 1 and round 2. <b>B</b>) Spleen weight (grams) for each infection group and both control groups. No statistical difference was observed between round 1 and round 2. <b>C</b>) Liver weight (grams) for each infection group and both control groups. No statistical difference was observed between round 1 and round 2. <b>D</b>) Lung weight (grams) for each infection group and both control groups. A statistical difference was observed between round 1 and round 2. For all graphs, significant differences (p<0.05) in Log₁₀ CFU within round 1 infections and within round 2 infections are shown by different letters (A, B, etc.). If two strains have the same letter, no significant differences were identified between them.</p

Bacterial burden of <i>F. tularensis</i> within the blood, spleen, liver and lungs of infected mice.

<p>C57BL/6J mice (n = 14/group) were challenged intradermally with 10-20 CFU of <i>F. tularensis</i> A1a, A1b, A2, and type B. At time of death, organs were harvested and blood was taken from each mouse. <b>A</b>) Bacterial burden (Log₁₀ CFU/ml) within blood samples of all infection groups. No statistical difference was observed between round 1 and round 2. <b>B</b>) Bacterial burden (Log₁₀ CFU/spleen) within spleens of all infection groups. A statistical difference was observed between round 1 and round 2. <b>C</b>) Bacterial burden (Log₁₀ CFU/liver) within livers of all infection groups. No statistical difference was observed between round 1 and round 2. <b>D</b>) Bacterial burden (Log₁₀ CFU/lung) within lungs of all infection groups. A statistical difference was observed between round 1 and round 2. For all graphs, significant differences (p<0.05) in Log₁₀ CFU within round 1 infections and within round 2 infections are shown by different letters (A, B, etc.). If two strains have the same letter, no significant differences were identified between them.</p

<i>Francisella tularensis</i> strains used in this study.

<p><i>Francisella tularensis</i> strains used in this study.</p

Survival curves of naïve mice following challenge with <i>F. tularensis</i> A1a, A1b, A2 and type B.

<p>C57BL/6J mice (n = 14/group; n = 7/strain) were challenged intradermally with 10–20 CFU of <i>F. tularensis</i> A1a (strains OK01-2528 and MO02-4195), A1b (strains MD00-2970 and MA00-2987), A2 (strains WY96-3418 and NM99-1823) and type B (strains KY99-3387 and MI00-1730), and survival was monitored over time. <b>A</b>) Time of death for n = 14 mice per <i>F. tularensis</i> group (A1a, A1b, A2 and type B). Circles represent mice infected in round 1(n = 7) and triangles represent mice infected in round 2 (n = 7). <b>B</b>) Step-down survival curves for proportion of mice surviving over time (hours). Data shown is from round 1 and round 2 for A1a, A1b, A2 and type B data. Median times of death are noted as a bold cross. <b>C</b>) Fitted survival curves for proportion of mice surviving over time (hours) using a Weibull distribution for combined (n = 14 mice/group) round 1 and round 2 A1a, A1b, A2 and type B data.</p