A long-standing challenge in cell biology is to measure the physical properties of living cells as they grow or interact with viruses and drugs. Such knowledge is crucial to our understanding of fundamental processes, such as those involved in cellular shape changes, differentiation and disease. More specifically, how cells regulate their volume and mass during the cell cycle is an important but poorly understood problem. On page 500, Martínez-Martín et al.1 describe a balance that addresses these challenges by enabling tiny changes of cell mass to be measured in just milliseconds.

Instruments known as flow cytometers and Coulter devices have conventionally been used to analyse the volume and size of cells suspended in fluids. Flow cytometers typically take measurements by analysing how cells scatter a laser beam, whereas Coulter devices detect changes in the electrical conductivity of fluids as cells pass through them. But although these methods are useful for studying cell populations, they are not suited to probing single cells at high precision, or to tracking variations in properties over time.

In the past few years, there has been progress in the development of mechanical resonators for measuring the masses of single cells2. Nanomechanical resonators are devices that incorporate a tiny vibrating cantilever. If a cell is attached to the cantilever, or passed through a fluid-filled micrometre-scale channel within it, then the frequency of the vibration shifts in a way that depends on the mass and position of the cell3.

Microchannel resonators have been used to weigh single, suspended cancer cells repeatedly over time in the presence of cancer therapeutics4. This method accurately determined the drug sensitivity and resistance of glioblastoma and primary leukaemia cells. Arrays of microchannel sensors have been used to precisely and rapidly measure the growth rates of many suspended individual cells simultaneously5. The mass and growth rates of single colorectal-carcinoma cells attached to nanomechanical resonators have also been determined6. But although these assays are powerful, they cannot analyse individual adherent cells (those that attach to surfaces in biological settings) in physiologically relevant conditions at the mass and time resolution needed to track fast cellular dynamics.

Martínez-Martín et al. now report an optically excited microresonator that can weigh single or multiple attached cells in physiological conditions, at millisecond time resolution and picogram mass sensitivity (1 pg is 10−12 g). When a single cell is attached at the end of the device's cantilever, the vibrational frequency of the cantilever shifts, as in previously reported devices. But the authors also irradiate the cantilever near its fixed end with a low-power blue laser (Fig. 1), the use of which is compatible with living mammalian cells. The intensity of the laser is varied to generate extremely small cantilever oscillations. An infrared laser is simultaneously focused on the free end of the cantilever, to read out the amplitude and phase signal of the cantilever movement. The mass of the cell is deduced from the vibrational frequency measured before and after cell attachment, and by taking into account the precise location of the added cell mass on the cantilever.

Figure 1: An ultrasensitive balance to weigh living cells.
figure 1

Martínez-Martín et al.1 report a balance based on a micrometre-scale cantilever. The cantilever is vibrated by irradiating it with a blue laser beam, and the vibrations are tracked by detecting an infrared laser beam reflected from the cantilever's free end. The frequency of the vibration depends on the mass and location of a cell attached to the cantilever. By monitoring the vibrations, changes in cell mass can be monitored with millisecond time resolution and picogram mass resolution (1 pg is 10−12 g).

Using the balance, Martínez-Martín and colleagues showed that the mass of living mammalian cells fluctuates by a few per cent over timescales of seconds throughout the cell cycle. Cells that had been fixed — chemically preserved in a way that stops biochemical processes from occurring — did not show mass fluctuations, indicating that these are a feature intrinsic to living cells. The authors linked the fluctuations to basic cellular processes such as ATP synthesis and glycolysis (both of which are involved in cellular energy production), and to water transport across the cell membrane.

The researchers went on to show that cells infected by vaccinia viruses stop growing, but continue to fluctuate in mass until cell death. The authors also developed a method for transmitting viruses from one cell to another by making mechanical contact between cells, thus providing new avenues for studying the mechanisms of virus transfer.

Collectively, Martínez-Martín and co-workers' experiments suggest that fast and subtle mass fluctuations are widespread in living cells and occur throughout the cell cycle. This work represents a major step forward in the development of ultrasensitive tools for monitoring the mass of living cells, and might find applications in many fields, including cell physiology, drug discovery and studies of stem-cell differentiation.

A crucial challenge in the next few years will be to make the technology more user friendly, so that it becomes accessible to a broad range of life-sciences researchers. Parallelization of the method will also be required to investigate multiple cells simultaneously, and to enable high-throughput measurements for basic research and for drug-screening applications.

There is a need for innovative tools to rapidly identify and fight microbial pathogens, because many of these have become resistant to antibiotics. Fluid-filled cantilevers have previously been used3 to weigh single living bacterial cells with sub-femtogram mass resolution (1 fg is 10−15 g), suggesting that this method could be used for detecting very low concentrations of pathogens. Moreover, cantilever technologies have been used to study the interaction of bacterial cell walls with the clinically valuable antibiotics vancomycin and oritavancin7, providing insights into how these drugs work. Cantilevers have also allowed the sensitivity and resistance of bacteria to antibiotics to be measured in minutes, which is much faster than conventional culture methods8. Martínez-Martín and colleagues' cell balance therefore holds great promise for studying the mechanisms of action of antibiotics, and for identifying the most efficient antimicrobial therapies.

Footnote 1