European Society of Mycobacteriology - Instituto Nacional de Saúde ...

PP-32
DOES A MUTATION IN THE RPOB MEAN THAT THE
M.tuberculosis IS RESISTANT TO RIFAMPICIN?
Yates, Malcolm; Brown, Tim; Drobniewski, Francis
HPA National Mycobacterium Reference Unit
Testing for mutations in the “hot spot” region of the rpoB gene is gaining momentum for the diagnosis of rifampicin
resistant strains of M.tuberculosis and as a surrogate marker for MDRTB.
The use of PCR combined with hybridisation to commercial strips (Hain Lifesciences, Innolipa)has decreased time and
increased ease of use so that these investigations are being performed by more and more laboratories.
The strips consist of a series of overlapping probes that cover the whole region (S bands) and are all present in Wild Type
isolates. There are also a series of probes (R bands) covering the commonest mutations which are linked with rifampicin
resistance and with the deletion of the corresponding S band.
Occasionally an S band is deleted with no R band appearing. We report these as “unidentified mutations”. The question
is whether these isolates are resistant or sensitive to rifampicin.
Recently we identified three patients from Sao Tome, an island off the West Coast of Africa (population 55000),
with a strains of M.tuberculosis that had the same S band deleted. Sequencing data showed that the strains had the
same synonymous mutation, VNTR/MIRU profiles were identical, and all strains were fully sensitive to first line drugs.
On analysis of our database, 466 strains were found to have rpoB mutations, 103 (22%) of these were unidentified mutations
of which 13% were sensitive to rifampicin. A further 15 isolates gave a WT result but were rifampicin resistant.
All strains were sequenced to identify the mutation.
Before rpoB mutations can be used as a surrogate for MDRTB the rate of mono-resistance to rifampicin in the area must
be determined: e.g. in the London area 30% of rifampicin resistance is mono.
Care must, therefore, be taken when reporting these rpoB mutation results to Clinicians as
1) unidentified mutations do not always correspond to rifampicin resistance and 2) rpoB mutations may not always correspond
to MDRTB
104 ESM 2009