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Analysis of an additional 9loci MIRU-VNTR confirms a large single clone of Mycobacterium tuberculosis in the West Midlands, United Kingdom

Abstract number: P2034

Gibson A., Evans J., Gardiner S., Smith G., Hawkey P.

Objective: The Mercian strain of Mycobacterium tuberculosis is the single most prevalent strain of tuberculosis in the West Midlands, UK. It was first identified in 2003 after the start of routine DNA fingerprinting and accounted for 171/1899 (9.0%) of all clustered cases between 2004–2008. Archive data suggests Mercian M. tuberculosis has been circulating for at least 14 years, however it appears to be geographically restricted as only 6 isolates have been identified outside the West Midlands. Routine 15-loci Mycobacterial Interspersed Repetitive Unit-Variable Number tandem Repeat (MIRU-VNTR) typing identified a common profile unique to Mercian strains. The aim of this study was to determine if the addition of the optimal 9-loci (P Supply et al, 2006. J Clin Microbiol 44:4498–510) to the 15-loci MIRU-VNTR typing panel would sub-divide this highly prevalent clade.

Methods: A total of 182/198 (91.9%) Mercian isolates identified between 2003–2008 were studied. In addition, isolates (n = 19) identified as the Mercian strain through previous epidemiological investigations from 1996–2002 were analysed. All isolates (n = 201) had previously undergone 15-loci MIRU-VNTR typing and were further typed for the additional 9 loci using a WAVE® System.

Results: A large single clone remained after the addition of 9-loci to the MIRU-VNTR typing panel. The most common Mercian strain type (type A) accounted for 170/201 (84.6%) isolates. The remaining 31 isolates were single locus variants (2 clusters of 23 and 5 isolates respectively [type B and C] and 2 unique isolates [type D and E]) along with a double locus variant (1 isolate, type F). Only 3 out of 9 loci were variable. Analysis according to year of isolation showed the most prevalent MIRU-VNTR type (type A or "True Mercian"), was present in isolates from all study years. In 2000 the first variant type was identified (type B), followed by type C in 2002, type D in 2005 and types E and F in 2006, suggesting that the Mercian strain is evolving slowly over time.

Conclusion: The addition of 9-loci confirmed the presence of a highly prevalent conserved clone (True Mercian) containing 170 isolates. To the best of our knowledge this is the largest single M. tuberculosis clone to date, based on the optimal 24-loci MIRU-VNTR typing method. True Mercian may have unique genetic properties that enable it to outcompete other more widely encountered strains in the UK.

Session Details

Date: 10/04/2010
Time: 00:00-00:00
Session name: Abstracts 20th European Congress of Clinical Microbiology and Infectious Diseases
Subject:
Location: Vienna, Austria, 10 - 13 April 2010
Presentation type:
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