Researchers from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of the Chinese Academy of Sciences, together with collaborators, have developed a Raman-activated cell sorting platform that combines rapid cell sorting with sensitive measurements of weak Raman signals and small-volume cell collection.

Called Pit-RACS, the optical-tweezer-assisted platform operates in two modes on the same microfluidic chip, allowing researchers to adjust the sorting strategy according to the strength of the Raman signal.

Their study was published in Small Methods on September 12.

Raman-activated cell sorting can identify living cells by their chemical fingerprints without fluorescent labels. Yet researchers have faced a trade-off: fast, flowing systems struggle to measure weak Raman signals, while systems that allow longer measurements sort cells slowly. Collecting the selected cells in a large volume can also make subsequent experiments difficult.

According to the researchers, cells with strong resonance Raman signals are measured and sorted as they flow through the device. In this mode, they sorted up to 836 ± 1 events per minute with a 10-millisecond spectral acquisition time.

For weaker non-resonance signals, optical tweezers move individual cells into a cell-free buffer stream. This allows for longer measurements without interference from nearby cells. This method sorted approximately 50 cells per minute. Switching modes requires no hardware changes.

The platform can also temporarily store selected cells and release them together in approximately 10 microliters of liquid. In the reported tests, the collected cells were over 97% pure and could proceed to cultivation or genomic analysis without an additional concentration step.

"Raman signals vary greatly between biological targets. We designed the two modes so researchers can choose the measurement time their question requires while using the same sorting platform," said Prof. MA Bo, corresponding author of the study.

To demonstrate industrial strain screening, the researchers sorted a mutant library of Yarrowia lipolytica and identified a strain with 77.4% higher β-carotene yield than the control. In another demonstration, they combined heavy-water labeling with Pit-RACS to enrich phosphate-solubilizing microorganisms from river water. They used the concentrated output directly for metagenomic sequencing, revealing 12 species of phosphate-solubilizing bacteria across nine genera.

"The small collection volume helps connect a cell's measured metabolic phenotype to what we can learn from its genome or cultivation," said Prof. XU Jian, another corresponding author of the study.

The researchers say the platform could support the development of microbial strains and the study of functional microorganisms in environmental samples.

This methodological advance will serve iMAPS (in-situ Metabolic Atlas Projects @ Single-cell, https://www.imaps.info/), an international consortium building a global network of Microbiome Metabolic Observatories to profile, mine and biobank microbes by their in-situ metabolic function at the single-cell resolution level.

Pit-RACS combines two Raman-activated cell sorting modes on one microfluidic chip (Image by QIBEBT)