Emerg Infect DisEmerging Infect. DisEIDEmerging Infectious Diseases1080-60401080-6059Centers for Disease Control and Prevention17283646337235206-062910.3201/eid1211.060629Letters to the EditorReal-time PCR for Francisella tularensis Types A and BPCR for Francisella tularensis Types A and BKugelerKiersten J.*PappertRyan*ZhouYan*PetersenJeannine M.*Centers for Disease Control and Prevention, Fort Collins, Colorado, USAAddress for correspondence: Jeannine M. Petersen, Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention, Foothills Campus, PO Box 2087, Fort Collins, CO 80522, USA; email: nzp0@cdc.gov112006121117991801Keywords: Francisella tularensistularemiareal-time PCRtype Atype Bletter

To the Editor: Francisella tularensis, the etiologic agent of tularemia, is highly infectious and considered a potential bioweapon (13). Although 4 subspecies of F. tularensis are recognized, most cases of tularemia are due to infection by subsp. tularensis (type A) or holarctica (type B). North America is the only region where both type A and type B cause human disease. Subspecies novicida is also found in North America, but it is of reduced virulence. Disease incidence attributable to either type A or type B is essentially unknown because the traditional method for classification of these subspecies is glycerol fermentation, which requires culture recovery (4). F. tularensis is fastidious and slow growing, with isolates recovered in a small percentage of cases.

We developed real-time TaqMan PCR assays for classification of F. tularensis type A and type B after F. tularensis is identified by culture or, in the absence of culture, by a PCR method such as the F. tularensis multitarget TaqMan assay (5). The type A TaqMan assay targets pdpD, which is present in type A, almost entirely absent from type B, and contains a 144-bp insert in novicida (6,7) (F: 5´-GAGACATCAATTAAAAGAAGCAATACCTT-3´; R: 5´-CCAAGAGTACTATTTCCGGTTGGT-3´; probe: 5´-AAAATTCTGC"T"CAGCAGGATTTTGATTTGGTT-3´). The type B assay targets a junction between ISFtu2 and a flanking 3´ region (GenBank AY06) (F: 5´- CTTGTACTTTTATTTGGCTACTGAGAAACT-3´; R: 5´- CTTGCTTGGTTTGTAAATATAGTGGAA-3´; probe: 5´- ACCTAGTTCAACC"T"CAAGACTTTTAGTAATGGGAATGTCA-3´). In type A and novicida, ISFtu2 is absent from this position (8). Oligonucleotides were designed with Primer Express version 2.0 (Applied Biosystems, Foster City, CA, USA). Probes were synthesized with a 5´ 6-carboxy-fluorescein reporter and an internal quencher (either BHQ1 [type A] or QSY-7[type B]) at the nucleotide position indicated by the quotation marks.

Assays were optimized by using 1 ng of type A (strain SchuS4) or type B (strain LVS) DNA on the LightCycler 1.2 (Roche Applied Science, Indianapolis, IN, USA). Optimized concentrations (20 μL final volume) were 1× LightCycler Fast Start DNA Master Hybridization Probe mix (Roche), 750 nmol/L primers, 200 nmol/L probe, 5 mmol/L MgCl2 and 0.5 U uracil-DNA glycosylase. PCR conditions were 50°C for 2 min, 95°C for 10 min, 45 cycles of 95°C for 10 s and 65°C for 30 s, then 45°C for 5 min. Cycle threshold (Ct) values were calculated by using the second derivative maximum method with the y-axis at F1/F3 (LightCycler software version 3.5).

Sensitivity of each assay was assessed by using 10-fold serial dilutions (100,000 to 1 genomic equivalents [GE]) of SchuS4 or LVS DNA. Testing was performed in triplicate, with a reproducible detection limit of 10 GE for both assays. Specificity of each assay was tested with 1 ng of DNA from a panel of 62 Francisella isolates (Table A1) and 22 non-Francisella isolates (Acinetobacter, Bacillus, Brucella, Corynebacterium, Enterobacter, Enterococcus, Escherichia, Haemophilus, Klebsiella, Legionella, Proteus, Pseudomonas, Serratia, Staphylococcus, Streptococcus, and Yersinia species). Isolates were grown, DNA purified, and quantified as previously described (5). Specificity was also evaluated with DNA (2 μL) extracted as previously described from Francisella-like tick endosymbionts of Dermacentor variabilis and Francisella-like soil bacteria (Table A1) (9,10). The type A assay recognized all type A isolates with an average Ct value of 17.9 (n = 19). The type B assay detected all type B strains with an average Ct value of 17.1 (n = 21). Neither assay displayed cross-reactivity with F. tularensis subsp. novicida (n = 7), F. philomiragia (n = 15), Francisella-like tick endosymbionts (n = 3), Francisella-like soil bacteria (n = 7) (Table A1), or non-Francisella spp. (n = 22).

To evaluate the ability of the type A and type B TaqMan assays, in conjunction with the multitarget assay, to identify F. tularensis and classify subspecies in primary specimens, human, animal, and tick samples were tested (Table ). DNA was extracted from 200 μL fluid, 25 mg liver, and 10 mg spleen or lung by using the QIAamp DNA MiniKit (Qiagen, Valencia, CA, USA) and 1 μL tested. Multitarget PCR conditions were as described (5).

The multitarget and subspecies-specific PCR assays accurately identified and classified F. tularensis in all specimens positive by standard diagnostic methods (Table). In addition, the type A and type B assays provided subspecies information for positive specimens in which an isolate was not recovered for glycerol fermentation testing (Table ). All specimens negative by standard diagnostic methods tested negative by PCR. These preliminary results suggest that a F. tularensis PCR identification method, in combination with the type A and type B assays, provides the capability to identify F. tularensis and determine subspecies in the absence of culture.

We describe real-time PCR assays capable of classifying F. tularensis type A and type B and distinguishing these subspecies from the less virulent subsp. novicida. These assays are designed for use after F. tularensis has been identified by culture or by PCR. Supplemental use of these assays will allow laboratories to actively subtype F. tularensis isolates and primary specimens, thus providing subspecies information for a higher percentage of tularemia cases. Improved subspecies information will further understanding of the disease incidence and geographic distribution of F. tularensis type A and type B in North America.

Comparison of standard diagnostic methods with the multitarget <italic>Francisella tularensis</italic> TaqMan assay and type A and type B assays using primary specimens
SpecimenSourceF. tularensis identified*Subspecies identification†Multitarget F. tularensis TaqMan assay‡
Type A assay‡
(Ct value)§Type B assay‡
(Ct value)
ISFtu2IglCtul4
Lymph node aspirateHuman++++31
Bronchial washHuman+A+++34
Upper lungHuman+A+++20
Lower lungHuman+A+++26
LiverHuman+A+++29
SpleenHuman+A+++31
Pleural fluidHuman+B+++36
BloodHuman++++38
SpleenHuman
LiverHuman
Cerebrospinal fluidHuman
Blood
Human







Liver/spleenTamarin++++28
TissueTick¶+A+++26
TissueTick¶+A+++33
BloodPrairie dog+B+++30
BloodPrairie dog+B+++27
SpleenPrairie dog+B+++21
SpleenPrairie dog+B+++31
SpleenPrairie dog
LiverCat
LiverRat
SpleenRat
SpleenSquirrel

*F. tularensis infection identified by culture, direct fluorescent antibody testing, or serologic testing.
†Subspecies was determined by glycerol fermentation when an isolate was recovered.
‡+ = positive result, 17<Ct <38; – = negative result, no fluorescence detected after 45 cycles of amplification.
§Ct, cycle threshold.
¶Tick species tested were Haemaphysalis leporispalustris and Dermacentor andersoni.

Suggested citation for this article: Kugeler KJ, Pappert R, Zhou Y, Petersen JM. Real-time PCR for Francisella tularensis types A and B [letter]. Emerg Infect Dis [serial on the Internet]. 2006 Nov [date cited]. http://dx.doi.org/10.3201/eid1211.060629

Acknowledgments

We thank Francis Nano for sharing information regarding the pdpD gene; Cheryl Kuske and Susan Barns for sharing DNA from Francisella-like bacteria in soil; and Nikos Gurfield, Jean Creek, and Heidi Goethert for providing Francisella-like tick endosymbiont DNA samples.

Specificity evaluation with DNA from Francisella spp
OrganismSample IDSourceGeographic originType A assay
(Ct value)*Type B assay
(Ct value)*
F. tularensis subsp.
tularensis (type A)
SchuS4HumanOhio15.9
WY963418HumanWyoming15.9
MA002987HumanMassachusetts18.4
CO012364CatColorado18.3
CO013713RabbitColorado16.5
ND000952HumanNorth Dakota16.9
KS000948CatKansas16.7
OK002731HumanOklahoma18.4
NC993990RabbitNorth Carolina16.8
AR000028HumanArkansas18.8
UT983134HumanUtah16.9
NM990295RabbitNew Mexico17.2
NC973057RabbitNorth Carolina16.5
NC015379CatNorth Carolina18.6
AR982146RabbitArkansas18.0
OK004337HumanOklahoma18.4
AR011117HumanArkansas22.9
MO011907HumanMissouri20.3

CA020099
Human
California
18.2

F. tularensis subsp.
holarctica (type B)
LVSRatRussia16.0
KY993387HumanKentucky19.7
OR960246MonkeyOregon16.7
CN985979HumanCanada17.1
AZ001325RatArizona16.2
IL003633HumanIllinois18.4
MO011673HumanMissouri16.8
KY001708HumanKentucky15.8
OH013029Prairie dogOhio16.7
UT002098HumanMissouri18.2
SP986120RabbitSpain17.5
IN983055RatIndiana15.6
CO961243VoleColorado15.5
CA990837HumanCalifornia16.5
IN002758HumanIndiana15.8
AZ001324SquirrelArizona15.9
CA993992MonkeyCalifornia16.9
SP982108HumanSpain17.8
NM002642HumanNew Mexico17.9
JAP5-3-11HumanJapan18.6
KO971026HumanKorea19.0
F. tularensis subsp.
novicida
GA993548HumanLouisiana
GA993549HumanCalifornia
GA993550WaterUtah
UT014992HumanUtah
AS020814HumanAustralia
FX1HumanTexas

FX2
Human
Texas


F. philomiragia
GA012793HumanCalifornia
GA012794HumanColorado
GA012795HumanNew York
GA012796HumanCalifornia
GA012797HumanPennsylvania
GA012799HumanConnecticut
GA012800HumanConnecticut
GA012801HumanNew York
GA012802HumanCalifornia
GA012803HumanNew Mexico
GA012804HumanVirginia
GA012806HumanMassachusetts
GA012810WaterUtah
GA012811WaterUtah

ATCC 25015
Muskrat
Utah


Francisella-like
tick endosymbionts†
2040372TickCalifornia
2040460TickCalifornia

MV2
Tick
Massachusetts


Francisella-like
bacteria‡
005SoilTexas
013SoilTexas
015SoilTexas
027SoilTexas
045SoilTexas
034SoilTexas
039SoilTexas

*Ct, cycle threshold; –, negative result, no fluorescence detected after 45 cycles of amplification.
†See Kugeler et al. (10).
‡See Barns et al (9).

ReferencesEllis J, Oyston PC, Green M, Titball RW Tularemia. Clin Microbiol Rev. 2002;15:63146 10.1128/CMR.15.4.631-646.200212364373Dennis DT, Inglesby TV, Henderson DA, Bartlett JG, Ascher MS, Eitzen E, Tularemia as a biological weapon: medical and public health management. JAMA. 2001;285:276373 10.1001/jama.285.21.276311386933Sjostedt A Family XVII. Francisellaceae, genus I. Francisella In: Brenner DJ, editor. Bergey’s manual of systematic bacteriology. New York: Springer-Verlag; 2005Olsufjev NG, Meshcheryakova IS Infraspecific taxonomy of tularemia agent Francisella tularensis McCoy et Chapin. J Hyg Epidemiol Microbiol Immunol. 1982;26:29197142690Versage JL, Severin DD, Chu MC, Petersen JM Development of a multitarget real-time TaqMan PCR assay for enhanced detection of Francisella tularensis in complex specimens. J Clin Microbiol. 2003;41:54929 10.1128/JCM.41.12.5492-5499.200314662930Nano FE, Zhang N, Cowley SC, Klose KE, Cheung KK, Roberts MJ, A Francisella tularensis pathogenicity island required for intramacrophage growth. J Bacteriol. 2004;186:64306 10.1128/JB.186.19.6430-6436.200415375123Larsson P, Oyston PC, Chain P, Chu MC, Duffield M, Fuxelius HH, The complete genome sequence of Francisella tularensis, the causative agent of tularemia. Nat Genet. 2005;37:1539 10.1038/ng149915640799Petersen JM, Schriefer ME, Carter LG, Zhou Y, Sealy T, Bawiec D, Laboratory analysis of tularemia in wild-trapped, commercially traded prairie dogs, Texas, 2002. Emerg Infect Dis. 2004;10:4192515109407Barns SM, Grow CC, Okinaka RT, Keim P, Kuske CR Detection of diverse new Francisella-like bacteria in environmental samples. Appl Environ Microbiol. 2005;71:5494500 10.1128/AEM.71.9.5494-5500.200516151142Kugeler KJ, Gurfield N, Creek JG, Mahoney KS, Versage JL, Petersen JM Discrimination between Francisella tularensis and Francisella-like endosymbionts when screening ticks by PCR. Appl Environ Microbiol. 2005;71:75947 10.1128/AEM.71.11.7594-7597.200516269811