Prediction of Susceptibility to First-Line Tuberculosis Drugs by DNA Sequencing.

BACKGROUND: The World Health Organization recommends drug-susceptibility testing of Mycobacterium tuberculosis complex for all patients with tuberculosis to guide treatment decisions and improve outcomes. Whether DNA sequencing can be used to accurately predict profiles of susceptibility to first-line antituberculosis drugs has not been clear. METHODS: We obtained whole-genome sequences and associated phenotypes of resistance or susceptibility to the first-line antituberculosis drugs isoniazid, rifampin, ethambutol, and pyrazinamide for isolates from 16 countries across six continents. For each isolate, mutations associated with drug resistance and drug susceptibility were identified across nine genes, and individual phenotypes were predicted unless mutations of unknown association were also present. To identify how whole-genome sequencing might direct first-line drug therapy, complete susceptibility profiles were predicted. These profiles were predicted to be susceptible to all four drugs (i.e., pansusceptible) if they were predicted to be susceptible to isoniazid and to the other drugs or if they contained mutations of unknown association in genes that affect susceptibility to the other drugs. We simulated the way in which the negative predictive value changed with the prevalence of drug resistance. RESULTS: A total of 10,209 isolates were analyzed. The largest proportion of phenotypes was predicted for rifampin (9660 [95.4%] of 10,130) and the smallest was predicted for ethambutol (8794 [89.8%] of 9794). Resistance to isoniazid, rifampin, ethambutol, and pyrazinamide was correctly predicted with 97.1%, 97.5%, 94.6%, and 91.3% sensitivity, respectively, and susceptibility to these drugs was correctly predicted with 99.0%, 98.8%, 93.6%, and 96.8% specificity. Of the 7516 isolates with complete phenotypic drug-susceptibility profiles, 5865 (78.0%) had complete genotypic predictions, among which 5250 profiles (89.5%) were correctly predicted. Among the 4037 phenotypic profiles that were predicted to be pansusceptible, 3952 (97.9%) were correctly predicted. CONCLUSIONS: Genotypic predictions of the susceptibility of M. tuberculosis to first-line drugs were found to be correlated with phenotypic susceptibility to these drugs. (Funded by the Bill and Melinda Gates Foundation and others.).

A multi-center blinded study on the efficiency of phenotypic screening methods to detect glycopeptide intermediately susceptible Staphylococcus aureus (GISA) and heterogeneous GISA (h-GISA).

BACKGROUND: To determine the true incidence of hGISA/GISA and its consequent clinical impact, methods must be defined that will reliably and reproducibly discriminate these resistant phenotypes from vancomycin susceptible S. aureus (VSSA). METHODS: This study assessed and compared the ability of eight Dutch laboratories under blinded conditions to discriminate VSSA from hGISA/GISA phenotypes and the intra- and inter-laboratory reproducibility of agar screening plates and the Etest method. A total of 25 blinded and unique strains (10 VSSA, 9 hGISA and 6 GISA) were categorized by the PAP-AUC method and PFGE typed to eliminate clonal duplication. All strains were deliberately added in quadruplets to evaluate intra-laboratory variability and reproducibility of the methods. Strains were tested using three agar screening methods, Brain Heart Infusion agar (BHI) + 6 microg/ml vancomycin, Mueller Hinton agar (MH) + 5 microg/ml vancomycin and MH + 5 microg/ml teicoplanin) and the Etest macromethod using a 2 McFarland inoculum. RESULTS AND DISCUSSION: The ability to detect the hGISA/GISA phenotypes varied significantly between methods and phenotypes. BHI vancomycin and MH vancomycin agar screens lacked the ability to detect hGISA. The MH teicoplanin agar screen was more sensitive but still inferior to Etest that had a sensitivity of 98.5% and 99.5%, for hGISA and GISA, respectively. Intra- and inter-laboratory reproducibility varied between methods with poorest performance seen with BHI vancomycin. CONCLUSION: This is the first multi-center blinded study to be undertaken evaluating various methods to detect GISA and hGISA. These data showed that the ability of clinical laboratories to detect GISA and hGISA varied considerably, and that screening plates with vancomycin have a poor performance in detecting hGISA.

Whole genome sequencing as the ultimate tool to diagnose tuberculosis.

In the past two decades, DNA techniques have been increasingly used in the laboratory diagnosis of tuberculosis (TB). The (sub) species of the Mycobacterium tuberculosis complex are usually identified using reverse line blot techniques. The resistance is predicted by the detection of mutations in genes associated with resistance. Nevertheless, all cases are still subjected to cumbersome phenotypic resistance testing. The production of a strain-characteristic DNA fingerprint, to investigate the epidemiology of TB, is done by the 24-locus variable number tandem repeat (VNTR) typing. However, most of the molecular techniques in the diagnosis of TB can eventually be replaced by whole genome sequencing (WGS). Many international TB reference laboratories are currently working on the introduction of WGS; however, standardization in the international context is lacking. The European Centre for Infectious Disease Prevention and Control in Stockholm, Sweden organizes a yearly round of quality control on VNTR typing and in 2015 for the first time also WGS. In this first proficiency study, only three out of eight international TB laboratories produced WGS results in line with those of the reference laboratory. The whole process of DNA isolation, purification, quantification, sequencing, and analysis/interpretation of data is still under development. In this presentation, many aspects will be covered that influence the quality and interpretation of WGS results. The turn-around-time, analysis, and utility of WGS will be discussed. Moreover, the experiences in the use of WGS in the molecular epidemiology of TB in The Netherlands are detailed. It can be concluded that many difficulties still have to be conquered. The state of the art is that bacteria still have to be cultured to have sufficient quality and quantity of DNA for succesful WGS. The quality of sequencing has improved significantly over the past 7years, and the detection of mutations has, therefore, become more reliable. The resistance mutations detected in WGS are in line with the ones visualized in reverse line blot techniques. The turnover in the genome of M. tuberculosis is very low, ∼0.3-0.5 mutations per genome per year. However, there is a wide variation in the occurrence of mutations per strain and genotype. Still, the resolution of WGS in epidemiological typing is higher than that in VNTR typing; previously suggested epidemiological links by VNTR typing are sometimes refuted on the basis of WGS. Although WGS offers the highest resolution in typing, in a country like The Netherlands, there are many strains with a limited genetic distance up to 100 mutations, without an apparent epidemiological link between the respective cases. These lookalikes are presumably even more prevalent in settings where predominant genotypes of M. tuberculosis are circulating. In summary, WGS seems to yield a more reliable prediction of resistance by the (lack of) detection of mutations in all 25 genes ever associated with resistance. This may within a short while prevent the need for many phenotypic resistance tests. Although more robust algorithms need to be developed, the recognition of the (sub) species in the M. tuberculosis complex seems possible. The first detailed studies on the population structure of M. tuberculosis strains in The Netherlands provide more resolution in typing but also an interesting observation that a part of the strains are genetically so conserved that they are separated by less than 100 mutations. This demands a more extended and accurate validation and understanding of the utility of WGS in the epidemiology of TB.

Methicillin-resistant Staphylococcus aureus in pigs with exudative epidermitis.

Despite a strict control program for methicillin-resistant Staphylococcus aureus (MRSA) in human medicine in the Netherlands, MRSA was cultured from exudative epidermitis lesions of 4 piglets on a breeding farm, 20 pigs on a supplier farm, and 2 workers on these farms. The MRSA strains were indistinguishable, suggesting direct transmission.