Francisella tularensis subsp. novicida isolated from a human in Arizona

Abstract

Background: Francisella tularensis is the etiologic agent of tularemia and is classified as a select agent by the Centers for Disease Control and Prevention. Currently four known subspecies of F. tularensis that differ in virulence and geographical distribution are recognized:tularensis (type A), holarctica (type B), mediasiatica, and novicida. Because of the Select Agent status and differences in virulence and geographical location, the molecular analysis of any clinical case of tularemia is of particular interest. We analyzed an unusual Francisella clinical isolate from a human infection in Arizona using multiple DNA-based approaches.

Findings: We report that the isolate is F. tularensis subsp. novicida, a subspecies that is rarely isolated.

Conclusion: The rarity of this novicida subspecies in clinical settings makes each case study important for our understanding of its role in disease and its genetic relationship with other F. tularensis subspecies.

Figure 1

Neighbor-joining phylogeny constructed from 16S rDNA gene sequences. Sequences were obtained from the clinical human isolate, other F. tularensis isolates, and isolates from related species. The tree, which was created using MEGA2 software [23], is midpoint rooted. Bootstrap values ≥ 60 are presented and are based upon 1,000 replicates. The human clinical isolate clusters with the F. tularensis strains; right vertical bar indicates F. tularensis clade. Sequences with an asterisk (*) were generated as part of this study. All other sequences were obtained from GenBank. Accession numbers for all sequences are shown in parentheses. The scale bar, expressing mean character difference, corresponds to 0.02 substitutions per nucleotide position.

Figure 2

Neighbor-joining phylogeny constructed from concatenated MLST gene fragment sequences. Sequences were obtained from the clinical human isolate and other strains representing the four subspecies of F. tularensis. The tree, which was created using MEGA2 software [23], is midpoint rooted. Bootstrap values ≥ 60 are presented and are based upon 1,000 replicates. The human clinical isolate clusters with the F. tularensis subsp. novicida clade, which is indicated with the right vertical bar. Sequences with an asterisk (*) were generated as part of this study. All other sequences were obtained from GenBank and originated from a study by Svensson et al. [12]. The scale bar, expressing mean character difference, corresponds to 0.002 substitutions per nucleotide position.