Applications of Microfluidics for Diagnostics and Detecting Drug-Resistant Strains
Employing microfluidics is a promising approach for rapid and cost-effective diagnostics for
M. tuberculosis. Detecting the pathogen with robust and reproducible fluidic models offers capabilities for clinical procedures and scientific exploration. Interestingly, a bacteria enrichment microfluidic chip and a microfluidic immunoassay chip have detected airborne
M. tuberculosis. Jing and colleagues (2014) validated a method whereby a micro-pump draws air containing bacteria into the enrichment fluidic chip and then a full immunoassay reaction is performed on a separate chip. The method offers the potential to accurately screen
M. tuberculosis in the aerosol
[42]. Airborne
M. tuberculosis currently requires long cultivation due to the low concentration in air samples. Capturing and directly detecting airborne
M. tuberculosis will aid effective disease prevention and control as there is a requirement to detect samples directly from patients for quicker analysis. The small volume sizes in microfluidic chip cultivation provides rapid detection at lower sample concentrations. Diagnosing TB, especially in developing countries, requires low-cost point-of-care technologies. A paper-based microfluidics system detected sputum samples containing mycobacteria. The system used enabled the decontamination of non-mycobacteria and storage of the sputum sample
[43]. A laser-etched indium tin oxide glass and PDMS microfluidic chip were used to rapidly detect and quantitate
M. tuberculosis with high sensitivity within forty-five minutes. By creating an eight-chamber microfluidic electrochemical system with real-time loop-mediated isothermal amplification (LAMP), amplification of three respiratory related infections including
M. tuberculosis could be monitored by measuring the electrochemical signal of methylene blue
[44]. Here, a microfluidic chip, with different sample chambers, provides cost- and time-efficient detection which would benefit clinicians to decide on optimal antibiotic treatments.
Six species of mycobacteria, including nontuberculous species and members of the
M. tuberculosis complex, were detected by combining a closed system of bead-beating, droplet fluidics, and surface-enhanced Raman spectroscopy. The spectral information obtained from the vibrational signals of the mycobacterial cell wall component, mycolic acid, effectively identified the different species. This is a promising step forward for ensuring the correct treatments are administered for the correct infections
[45]. Small channel dimensions enable the manipulation of cell environments and thus can represent improved biological investigation. A potential method for quantitively detecting
M. tuberculosis in droplet microfluidics was developed by detecting cells that express the endogenous β-lactamase, BlaC—an enzyme marker naturally expressed by
M. tuberculosis. By encapsulating a specific fluorescent probe of BlaC and samples of bacterial strains that express BlaC in droplets, the researchers could calculate the initial concentration of cells based on fluorescence
[46].
Other researchers have combined PCR techniques with microfluidics. Ip et al. (2018) used a single chip comprising positive and negative reaction chambers, as well as small liquid handling chambers. They performed isolation of
M. tuberculosis H37Ra with magnetic beads and differentiation of live/dead bacteria with propidium monoazide dye, followed by RT-PCR and optical detection within two hours. By measuring the threshold cycle number, a low detection limit of 14 colony-forming units per reaction was achieved
[47]. Besides the above new PCR microfluidic approaches, genetically detecting
M. tuberculosis without the laborious need for PCR amplification has been achieved. For example, Domínguez et al. (2015) created a micro-cantilever platform, where hydration-induced stress could identify
M. tuberculosis and RIF resistance within 1.5 h
[48].
Previously, label-free DNA of
M. tuberculosis from clinical isolates was detected by an integrated system of microfluidics and electrochemical biosensing. The platform consisting of a monolithic chip and multiwall carbon nanotubes detects
M. tuberculosis without the need for DNA amplification
[49]. Another biosensing device was developed to detect MPT64—an antigen secreted by
M. tuberculosis. The protein is a biomarker for actively dividing mycobacteria, detected by electrochemical impedance spectroscopy and synthetic aptamers integrated with a microfluidic chamber
[50].
Detecting drug-resistant strains early in the infection will aid clinical decision making and shorten the time for optimal drug treatment. Sophisticated detection of resistant strains will also transform drug discovery and innovation within the laboratory. Researchers detected single-nucleotide polymorphisms between RIF-resistant
M. tuberculosis isolates and susceptible isolates by combining a microfluidic chip with post-PCR high-resolution melting analysis (HRMA). The authors’ “Light Forge” microfluidic DNA melting-based TB test showed better performance of melting temperature differences compared to conventional Sanger sequencing, as well as a HRMA device on its own and phenotypic drug susceptibility testing
[51]. Additionally, evidence shows that by incorporating open-chip microfluidics with padlock probe (PLP) ligation and rolling circle amplification (RCA), a two-hour assay is achievable for detecting an INH resistance caused by mutations in the gene (katG) in
M. tuberculosis. The lab-on-a-disc platform utilised separate fluidic chambers for ligation and amplification steps, which provided temperature control
[52]. Law et al. (2018) combined a lab-on-a-disk and recombinase polymerase amplification to fluorescently detect the pathogen with a sensitivity of 10
2 colony-forming units per millilitre
[53]. Drug-resistant strains to β-lactams were fluorescently detected using a droplet-based microfluidic device and a custom 3D particle counter (
Figure 3). The microfluidic chip comprised separate input channels for bacteria, ampicillin and broth mixture, fluorocillin (a β-lactamase sensor), and oil to encapsulate single bacteria cells into droplets. Antibiotic-resistant clinical isolates could grow inside the droplets, detected by fluorescent microscopy
[54].
Figure 3. Schematic illustration of droplet system coupled to an Integrated Comprehensive Droplet Digital Detection. Flow-focusing microfluidic chip geometry producing encapsulated mycobacteria in droplets. Figure created on Biorender.com, accessed on 26 October 2021.
Investigators are overcoming the challenge of genotyping drug-resistant strains of the pathogen directly from sputum. Researchers detected and genotyped RIF and INH resistance by creating a closed system composed of a microfluidic amplification microarray
[55]. Likewise, the lab-on-a-film platform created by Kukhtin et al. (2020) integrated amplification, hybridisation, washing, and imaging. The authors reported
M. tuberculosis detection in sputum as 43 CFU/mL; however, future work of this method includes sensitivity investigation
[56].