What do magnetic bacteria do to relax?

Magnetospirillum gryphiswaldense is a special kind of bacterium. Inside its body, it grows magnetic nanocrystals, organized in a linear chain to form a tiny but powerful bar magnet. In a magnetic field – like that of the Earth – the whole bacterium aligns to the field in the same way as a magnetic compass needle. This property helps it to navigate and find food, by reducing to one dimension its movement. For this reason, M. gryphiswaldense and its cousins are known as magnetotactic bacteria. Due to these nano-magnets, a suspension of bacteria is also a strongly magnetic material – applying a small field polarizes the material as the many tiny magnets line up.

Now ICFO researchers María Hernández Ruiz, Dr. Christopher Kiehl and Dr. Vito Giovanni Lucivero, led by ICREA Prof. Morgan W. Mitchell, report an optically pumped magnetometer (a magnetic field sensor) that can precisely measure how the bacteria align to an applied field, and how they lose that alignment (how they relax) when the field is turned off.  Using the precision sensor, the team saw that the relaxation is nearly but not exactly exponential, a result that was predicted by theory but had never been observed. Moreover, the exact form of the relaxation showed that a fraction of the bacteria was relaxing more slowly than others.

The experiment consisted of applying a constant magnetic field to a population of MSR-1 (a much-studied magnetotactic bacterium), causing the bacteria to orient in that direction. Then, a much stronger magnetic field, perpendicular to the original one, was also applied for three seconds. Consequently, the bacteria lost their previous orientation, shifting toward the new field. When the strong field was switched off and only the constant background field remained, the bacteria were left to re-orient themselves, and their magnetic signals spontaneously “relaxed” toward equilibrium.

The OPM, which used a vapor of rubidium atoms to sense the field, measured precisely how these weak magnetic signals changed after the removal of the strong field. The overall relaxation resembles an exponential approach to a steady value. However, the researchers did not observe exactly that, and saw instead small, quantifiable departures from this ideal exponential decay. These subtle differences likely indicate that the individual bacteria within the population have different relaxation rates, rather than these being perfectly homogeneous.

“To the best of our knowledge, this is the first time such magnetic inhomogeneities and relaxation deviations have been directly observed,” says Prof. Morgan Mitchell, supervisor of the study. “For this proof-of-principle experiment, we worked with non-living bacteria, to keep things simple. It will be very interesting to see how living bacteria, which swim in the magnetic field, compare,” adds lead researcher Maria Hernandez Ruiz.

Moreover, the OPM was shown to work efficiently with opaque samples and in the presence of light scattering – two conditions that typically hinder traditional optical techniques.

Together, these results highlight the capability of compact atomic magnetometers to image and quantify magnetic properties in biological samples, positioning them as promising alternatives for such research. Thus, the study marks a key step forward towards understanding the magnetic behavior of magnetotactic bacteria, a fundamental research topic in biophysics that, in the long term, could unlock targeted tumour therapy and other biomedical applications.

Reference:

Hernández Ruiz, M., Kiehl, C., Lucivero, V.G. et al. Magnetotactic bacterial populations studied with a Pound-Drever-Hall atomic magnetometer. EPJ Quantum Technol. 13, 45 (2026).

DOI: https://doi.org/10.1140/epjqt/s40507-026-00487-x

Acknowledgements:

This work has been supported by European Commission projects Field-SEER (ERC 101097313), OPMMEG (101099379) and QUANTIFY (101135931); Spanish Ministry of Science MCIN projects SAPONARIA (PID2021-123813NB-I00), SALVIA (PID2024-158479NB-I00), and MARICHAS (PID2021-126059OA-I00), “NextGenerationEU/PRTR.” (Grant FJC2021-047840-I) and “Severo Ochoa” Center of Excellence CEX2019-000910-S; Generalitat de Catalunya through the CERCA program, DURSI grant No. 2021 SGR 01453 and QSENSE (GOV/51/2022). Fundació Privada Cellex; Fundació Mir-Puig. MHR acknowledges support from Ayuda PRE2021-098880 financiada por MCIN/AEI/ 10.13039/501100011033 y por el FSE+. VGL acknowledges financial support from European Union NextGenerationEU PNRR MUR project MAPPIQS, NQSTI Spoke 9 - CUP E63C22002180006 and from the Italian Ministry of University and Research (MUR) projects “Rita Levi Moltancini”—Bando 2021, AQUSENS and “Budget MIUR - Dipartimenti di Eccellenza 2023 - 2027” (Law 232, 11 December 2016) - Quantum Sensing and Modelling for One-Health (QuaSiModO). The funding bod(ies) had no role in design of the study and collection, analysis, and interpretation of data and in writing the manuscript.