Antimicrobial susceptibility testing with integrated photonic crystal optical tweezers

The abusive use of antimicrobial drugs during the last eighty years has favoured the natural selection of resistant pathogens able to neutralise drugs efficiently. Nowadays, antimicrobial resistance contributes to millions of deaths yearly, threatens the environment and heavily impacts the economy, leading the World Health Organization to rank this burden as one of the top ten health threats. Improving antimicrobial susceptibility testing and developing innovative antimicrobial agents acting through novel mechanisms are essential to contain multi-drug resistant pathogens and limit their spread. Meanwhile, the quest to miniaturise commonly found bulky tools in research laboratories has increased interest in many scientific fields. Thanks to remarkable manufacturing technologies, the development of faster, smaller and cheaper devices for diverse applications has become possible. This is the case also of optical tweezers, which are the tool of choice when non-destructive manipulation of (sub-) micrometre-sized objects is necessary and have proven to be extremely useful for biophysics and life sciences in general. By harnessing the properties of optical resonators at the nanoscale, integrated nanotweezers allow the simultaneous manipulation and acquisition of information about the trapped object through its perturbation on the resonant mode with low optical powers. Using this framework and from the perspective of fighting antimicrobial resistance, we propose an innovative method capable of 1) trapping bacteria, identifying the Gram-type and potentially differentiating several bacterial species, 2) trapping phages (nanoparticle viruses that kill bacteria), and potentially identifying viruses belonging to morphologically different families, 3) performing rapid antimicrobial susceptibility testing with a class of antibiotics targeting the bacterial cell wall synthesis, and 4) monitoring bacteria-phage interaction to detect in real-time phage-induced bacteriolytic events, all of these at the single biological entity level. This is achieved by designing and implementing a robust silicon optofluidic platform free of any surface functionalisation containing hollow resonant photonic crystal nanocavities acting as optical tweezers. The excitation of the nanoresonators is performed via an end-fire setup comprising optimised SU-8 mode adaptors for high fibre-to-waveguide coupling efficiency. By monitoring the transmitted power through the photonic device and thanks to a self-induced back-action mechanism, our device can simultaneously manipulate and acquire information about the trapped object. The silicon transparency in the NIR, the hollow nature of the cavities, and the dynamical light-matter interaction ensure trapping with extremely low optical powers. A free-space optical setup enables the high-resolution imaging of the trapped objects, providing a second investigation channel for qualitative analyses. Finally, the development of a rapid microfluidic circuit and a simple pneumatic control system allow high-rate optical trapping experiments. Hence, the synergy of microfluidics and integrated photonics enables us to participate in the fight against antimicrobial resistance by developing a rapid and effective diagnostic tool handling small volumes.