The human body contains a careful balance of harmful and helpful bacteria, which communicate with each other by secreting molecules. Sensing these secretions can determine the density of bacteria themselves, and probing bacteria interactions in detail will lead to new drug discovery.

However, the molecules related to bacteria intercommunication are very dilute. It often requires days to increase the number of bacteria enough to detect a signal. Hayashi et al. developed a method to detect bacteria and their secretions in minutes by combining optical condensation with fluorescence spectroscopy.

“Conventional analysis methods of microbes and biological nanomaterials take a very long time, and their sensitivity should be improved,” said author Takuya Iida. “If the target biological samples can be remotely assembled into the observation region without any damage, we can provide sensitive and rapid analysis.”

The team used the convection from the photothermal effect to force bacteria to condense in one location. They fixed a submillimeter particle — an imitation bubble — to a glass substrate. A metallic nanofilm covered the bubble and glass, and a laser irradiated the film on top of the bubble to drive photothermal convection.

As a result, bacteria condensed away from the heating area, preventing any thermal damage. The researchers then measured the fluorescence spectrum of molecules in the optically assembled bacteria.

“The combination of optical condensation and spectroscopic techniques enables the analysis of a trace amount of dilute biological substances that are difficult to detect by spectroscopic analysis alone,” said author Shiho Tokonami. “With this research, it will be possible to easily perform rapid and highly sensitive analysis of not only bacteria but also various biological nanomaterials such as nucleic acids or proteins.”

Source: “Quantitative fluorescence spectroscopy of living bacteria by optical condensation with a bubble-mimetic solid-liquid interface,” by Kota Hayashi, Mamoru Tamura, Shiho Tokonami, and Takuya Iida, AIP Advances (2022). The article can be accessed at