Fibre-based sensing for bacterial detection with time-resolved single photon spectroscopy

In the dawn of multiple drug resistant (MDR) infections, there is an unmet need for fast, accurate and less invasive diagnostic methods to identify lung infection and inflammation. One approach to tackle this challenge is by combining optical imaging agents with imaging or sensing through optical fibres. Previous approaches use different fibred imaging modalities - spectral, ratiometric, and time resolved. These techniques require a complex optical fibre bundle, and can be limited by distortions from this imaging fibre while also suffering due to tissue autofluorescence and photobleaching.
Recently, complementary metal-oxide semiconductor (CMOS) single photon avalanche diode (SPAD) line sensors are becoming increasingly popular for applications such as time-resolved fluorescence sensing (TRFS). They are highly multiplexed, with good collection efficiency and time resolution (< 500 ps time stamping resolution) which enables the recording of luminescent kinetics down to nanoseconds. The response from excited exogenous fluorophores can be investigated in three dimensions to enhance selectivity and sensitivity: fluorescent intensity, fluorescence emission spectral shape and fluorescence lifetime. We demonstrate that the use of fluorescence emission lifetime in combination with the spectral shape can confirm the presence of bacteria labelled with optical imaging agents, overcoming the known challenge of tissue autofluorescence. Variation in both parameters can also indicate changes in the morphology of bacteria introduced through treatments.
Presented here is an endoscopic approach for optical fibre-based sensing of bacterial infections with clinical molecular probes for direct detection of gram-negative bacteria in size restricted regions such as the alveolar space of the distal lung. While this has been previously demonstrated in fibred imaging modalities, we demonstrate selectivity without reliance on spatial information offering faster interventions. Furthermore, time-resolved spectroscopy allows the use of off-the-shelf optical fibres and makes complex fibre probe development unnecessary, hence enabling deployment of miniaturised disposable fibre probes.