Identifying bacteria in their natural habitat

Developing antibiotic medicine takes a lot of time. A crucial and slow step in that process is culturing a bacterium of interest for various destructive analyses. To shorten the standard process, researchers are studying how to apply surface-enhanced Raman spectroscopy (SERS).

In SERS, researchers place a bacterium near a metallic surface or nanoparticle. When a laser shines on the surface plasmons in the metal, resonant interactions increase the Raman scattering from the bacterium’s lipids, proteins, and other cellular material. The researchers can then use the boosted scattering signature to quickly identify the bacterium and its molecular structure, potentially without having to culture the sample (see the article by Katrin Kneipp, Physics Today, November 2007, page 40 ).

Researchers have confirmed the viability of the technique for dry samples, which are easy to prepare and measure. But bacterial cells live in water. Now Loza Tadesse and Jennifer Dionne of Stanford University and their colleagues have developed a protocol for liquid SERS measurements that may help facilitate real-time testing of the susceptibility of bacteria to various drug treatments.

Most of the previous efforts to make liquid SERS measurements focused on bacteria in droplets on a solid substrate. But the droplets would dry out too quickly and expose researchers to potentially pathogenic bacteria. Tadesse and her colleagues took a different route: They constructed a 100-µm-tall tower and sealed the liquid bacterial sample, plus a solution of gold nanorods, inside. The plot below shows the significantly larger SERS intensity of Escherichia coli with the gold nanorods (NR) compared with the sample without them.

The bacterial cells remain alive during the liquid SERS measurement, which means that researchers can collect observations in real time. That advantage may be useful for many applications, including testing for a bacterium’s susceptibility to antibiotics and observing the reaction of cancer cells to drug treatment. (L. F. Tadesse et al., Nano Lett., 2020, doi:10.1021/acs.nanolett.0c03189 .)