Methods: At the Intermountain Central Microbiology Laboratory CD is diagnosed using an enzyme immunoassay for GDH and Toxin A/B (Alere, Waltham, MA) with discordant results reflexed to the Xpert assay for additional testing. From March 2014 to January 1, 2015, toxin positive samples were also processed with Xpert. Forty-nine cryopreserved stool samples from 49 patients were cultured for CD and subjected to WGS (MiSeq, Illumina). Multilocus sequence type (MLST) was determined from WGS data. Samples were selected to represent both presumptive O27 and non-O27 as determined by Xpert assay.
Results: Between January 1, 2014 and December 31, 2014 1513 samples tested positive for CD; 373 (25%) by toxin EIA and 1140 by Xpert. Presumptive O27 was more frequently identified in toxin A/B positive samples compared to samples positive by Xpert only (23% vs 7.6%, P<0.001). Twenty presumptive O27 and 29 non-O27 samples were sequenced. MLST showed high discriminatory power (Simpsons Index of 0.943) with 15 known sequence types (ST) and 9 previously undescribed STs. Sequence types suggestive of hypervirulent strains were observed in 14 (29%) isolates: 11 ST1 (ribotype O27) and 3 ST11 (ribotype O78). Of the 20 presumptive O27 calls by Xpert assay, 10 were ST1, two were ST 11 and 8 represented different STs. Non-O27 calls by Xpert were more accurate with only one (3%) identified as ST1.
Conclusion: A greater proportion of toxin A/B positive CD cases were presumptive O27 by Xpert compared to isolates positive for tcdB gene only. Compared to MLST, the Xpert assay overestimated prevalence of O27 and results should be confirmed with alternative methods.
J. Burke, None
B. K. Lopansri, Cubist, Inc: Investigator , Research support
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