Do bacteria have eyesight? / VAAM - Vereinigung für Allgemeine und Angewandte Mikrobiologie e.V.

Do bacteria have eyesight?

Since the discovery of the microscope, it has been known that some bacteria react to optical stimuli. But how do they do this without eyes?

Some bacteria can perceive the intensity and colour of the surrounding light with the help of light-absorbing proteins. These proteins are similar to the photoreceptors in the human retina. Purple bacteria, for example, can distinguish between the bright midday sun and a shady place. However, this does not automatically give them spatial vision. Over the length of a single bacterial cell, differences in light intensity are simply too small to determine from which direction the light is coming.

However, at least among cyanobacteria, whose metabolism is based on the photosynthetic conversion of solar energy into sugar, there are species that can also "see" the direction of light. For example, the species Synechocystis sp. PCC 6803 uses filamentous cell extensions (pili) to move towards a light source. The bacteria use a physical trick for this: the round cells, which are densely packed with proteins and membranes, have a higher light refractive index than the surrounding aqueous medium. The round cell therefore focusses the incident light on a small area of the cell membrane facing away from the light, similar to an eyeball or a camera lens. At the focal point of the focussed light, photoreceptors or pigments of the photosynthetic apparatus are strongly stimulated. This creates a localised signal that controls the directional movement. Although this form of vision does not allow us to recognise sharp images, it is completely sufficient for spatial orientation.

Schuergers, N. et al. (2016) ‘Cyanobacteria use micro-optics to sense light direction’, eLife 2016;5:e12620. doi: 10.7554/eLife.12620.

Kreft, J. (2012) ‘Lasst das Licht da! Photosensing bei chemotrophenBakterien’, BIOspektrum, 18(1), pp. 12–14. doi: 10.1007/s12268-012-0134-7.