Johansson A, Göransson I, Larsson P, Sjöstedt A
Department of Clinical Microbiology, Infectious Diseases, Umeå University, Umeå, Sweden.
J Clin Microbiol. 2001 Sep;39(9):3140-6. doi: 10.1128/JCM.39.9.3140-3146.2001.
Members of the genus Francisella and the species F. tularensis appear to be genetically very similar despite pronounced differences in virulence and geographic localization, and currently used typing methods do not allow discrimination of individual strains. Here we show that a number of short-sequence tandem repeat (SSTR) loci are present in F. tularensis genomes and that two of these loci, SSTR9 and SSTR16, are together highly discriminatory. Labeled PCR amplification products from the loci were identified by an automated DNA sequencer for size determination, and each allelic variant was sequenced. Simpson's index of diversity was 0.97 based on an analysis of 39 nonrelated F. tularensis isolates. The locus showing the highest discrimination, SSTR9, gave an index of diversity of 0.95. Thirty-two strains isolated from humans during five outbreaks of tularemia showed much less variation. For example, 11 of 12 strains isolated in the Ljusdal area, Sweden in 1995 and 1998 had identical allelic variants. Phenotypic variants of strains and extensively cultured replicates within strains did not differ, and, for example, the same allelic combination was present in 55 isolates of the live-vaccine strain of F. tularensis and another one was present in all 13 isolates of a strain passaged in animals. The analysis of short-sequence repeats of F. tularensis strains appears to be a powerful tool for discrimination of individual strains and may be useful for a detailed analysis of the epidemiology of this potent pathogen.
弗朗西斯菌属的成员以及土拉弗朗西斯菌尽管在毒力和地理分布上存在显著差异，但在基因上似乎非常相似，目前使用的分型方法无法区分各个菌株。在此我们表明，土拉弗朗西斯菌基因组中存在多个短序列串联重复（SSTR）位点，其中两个位点SSTR9和SSTR16共同具有高度鉴别力。通过自动DNA测序仪对来自这些位点的标记PCR扩增产物进行大小测定，并对每个等位基因变体进行测序。基于对39株无关的土拉弗朗西斯菌分离株的分析，辛普森多样性指数为0.97。鉴别力最高的位点SSTR9的多样性指数为0.95。在五次兔热病暴发期间从人类分离出的32株菌株显示出的变异要少得多。例如，1995年和1998年在瑞典吕瑟达尔地区分离出的12株菌株中有11株具有相同的等位基因变体。菌株的表型变体以及菌株内广泛培养的重复菌株没有差异，例如，土拉弗朗西斯菌活疫苗株的55株分离株中存在相同的等位基因组合，而在一株经动物传代的菌株的所有13株分离株中存在另一种等位基因组合。对土拉弗朗西斯菌菌株短序列重复的分析似乎是区分各个菌株的有力工具，可能有助于对这种强效病原体的流行病学进行详细分析。
