Background
Multidrug resistance of Tuberculosis strains (MDR-TB) are one of the major WHO health concerns. One of the challenges that hampers the effective response to MDR-TB is the long turnaround time of phenotypic Drug Susceptibility Testing (DST). To counter this, new fast and sensitive DNA-based methods were successfully introduced over the last years. However, these (a) are based on the knowledge on resistance mutations, (b) do not distinguish living from dead cells, (c) ignore all intrinsic resistance mechanisms, and (d) ignore the influence of compensatory mutations.
Objectives
We introduce a next-generation diagnostic test based on quantification of drug-specific RNA biomarkers. The basic principle is that a brief antibiotic exposure triggers specific transcriptional responses in susceptible, but not in resistant, microbes within a few hours. This has the advantage that long culture-dependent steps are avoided, yet the resistance phenotype is detected independent of the specific cause of resistance.
Materials & Methods
First, the global transcriptional response of two TB strains to 10 anti-TB drugs was determined using RNAtaq-Seq. A set of highly responsive genes was selected for each drug and RNA-targeting probes were designed.
Next, the RNA-based DST was developed in 96 well format. In short, 200 µl of a positively flagged MGITTM (BD) culture is spiked with a drug, while a replicate is incubated in absence of the drug. Multiplex mRNA quantification is performed directly on crude cell lysates using a combination of the bead-based MagPixTM (Luminex) and QuantigeneTM Plex (Thermo Fisher) technology. The normalized expression levels are combined to one numeric value which determines the drug susceptibility of the investigated strain.
Results
We successfully developed 8 primary sets of RNA biomarkers for ten 1st-line, 2nd-line and new drugs. Taking isoniazid as proof of principle, we present a biomarker set of 5 responsive genes and 3 normalizing genes, which enables to distinguish susceptible, low- and high resistant TB strains after 6 hours incubation. Next, preliminary results demonstrate that the biomarker sets can successfully discriminate between susceptible and resistance strains for the selected drugs.
Conclusion
We present a robust, RNA-based DST without the need for RNA extraction. The assay was proven to be efficient for isoniazid. With a total of 8 biomarker sets under optimization, the drug resistance profile of up to 14 drugs can be determined.
