The researchers first developed a photonic technique to immobilize bacteria with light, by designing an optical tweezers on a photonic crystal chip.

These photonic crystals are in the form of silicon "nano-pipes" with a square section of 300 nm on each side. Fitted with two Bragg mirrors, they form optical nanocavities, in the center of which the intensity of the light is very strongly concentrated.

These boxes of light are immersed in a micro-pool containing bacteria animated by Brownian movements. When the laser is injected into the nanocavity, the bacteria are attracted and immobilized above a photonic crystal, under the effect of "gradient forces" that tend to bring them back to the zone of higher light intensity. Once "caught", the bacteria continue to move within the trap and their movements, whose range is limited to less than one hundred nanometers, can be observed.

The consortium's researchers thereby showed that it is possible to distinguish different kinds of bacteria and to determine their gram type. They now want to investigate the feasibility of a rapid test for antibiotic resistance.

With this in mind, they performed a model experiment in which bacteria are stressed by heating, according to different protocols of duration and temperature (45, 51 and 70°C), before being trapped by light.

They observed that, depending on the state of the bacteria, the resonance wavelength of the nanocavity shifts. This value is indeed sensitive to the refractive index of the bacterial membrane which optically interacts with the evanescent field, outside of the photonic crystal.

Since index changes are correlated with the degree of heat stress, viable and non-viable bacteria can be sorted. Bacterial viability under stress can then be probed much faster (typically in less than 4 hours) than with conventional cell culture enumeration methods (24 h).