Open Access Research article

Rapid differentiation of Francisella species and subspecies by fluorescent in situ hybridization targeting the 23S rRNA

Wolf D Splettstoesser1*, Erik Seibold1, Ella Zeman1, Karlheinz Trebesius2 and Andreas Podbielski3

Author Affiliations

1 Bundeswehr Institute of Microbiology, German Reference Laboratory for Tularemia, Neuherbergstr 11, 80937 Munich, Germany

2 University of Applied Sciences Munich, Lothstr. 34, 80335 Munich, Germany

3 Department of Microbiology, Virology & Hygiene, University Hospital Rostock, Schillingallee 70, 18057 Rostock, Germany

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BMC Microbiology 2010, 10:72  doi:10.1186/1471-2180-10-72

Published: 8 March 2010



Francisella (F.) tularensis is the causative agent of tularemia. Due to its low infectious dose, ease of dissemination and high case fatality rate, F. tularensis was the subject in diverse biological weapons programs and is among the top six agents with high potential if misused in bioterrorism. Microbiological diagnosis is cumbersome and time-consuming. Methods for the direct detection of the pathogen (immunofluorescence, PCR) have been developed but are restricted to reference laboratories.


The complete 23S rRNA genes of representative strains of F. philomiragia and all subspecies of F. tularensis were sequenced. Single nucleotide polymorphisms on species and subspecies level were confirmed by partial amplification and sequencing of 24 additional strains. Fluorescent In Situ Hybridization (FISH) assays were established using species- and subspecies-specific probes.

Different FISH protocols allowed the positive identification of all 4 F. philomiragia strains, and more than 40 F. tularensis strains tested. By combination of different probes, it was possible to differentiate the F. tularensis subspecies holarctica, tularensis, mediasiatica and novicida. No cross reactivity with strains of 71 clinically relevant bacterial species was observed. FISH was also successfully applied to detect different F. tularensis strains in infected cells or tissue samples. In blood culture systems spiked with F. tularensis, bacterial cells of different subspecies could be separated within single samples.


We could show that FISH targeting the 23S rRNA gene is a rapid and versatile method for the identification and differentiation of F. tularensis isolates from both laboratory cultures and clinical samples.