by Mizu Ota and Susan S. Golden
Though we live on the same planet, the world of bacteria is far removed from our own, largely because of the sheer difference in size. Life on the micrometer scale is governed by rules that do not constrain us, and vice versa: aquatic environments are dominated by viscous forces, and external stimuli such as chemical gradients occur on a scale too large to sense over the length of a single bacterial cell. And yet, some strategies used in both worlds carry at least a superficial resemblance. Surprisingly, one of these is the ability, literally, to see.
Our difficulty to "see from the microbial perspective" engenders low expectations for the sophistication of micro-denizens. However, we are now learning that we have severely underestimated the complexity of microbial behavior. Each new finding serves as a humbling reminder that we eukaryotes are not so special. Research by Nils Shuergers et al. goes so far as to suggest that the sense of vision, often touted as a paragon of complexity, may have been present in some bacteria long before multicellular life on Earth. In an unusual (and unrelated) example, some dinoflagellates, ergo eukaryotic microbes, have a remarkable eye-like structure called the ocelloid.
Some species of cyanobacteria, a phylum of photosynthetic bacteria that gave rise to the eukaryotic chloroplast in ancient times, can move towards or away from light of different wavelengths, a property known as phototaxis. One such species is Synechocystis PCC 6803, which uses Type IV pili (T4P) and twitching motility to move. But until now the mechanism by which it senses the light and knows where to go was mysterious.
Is phototaxis governed by a mechanism similar to chemotaxis, where direction of illumination is sensed via a change in concentration over distance? If so, one would be able to "trick" Synechocystis into moving in a certain direction: illuminating an agar disc of bacteria from below to create a graduated light pattern across the disc should result in cells moving laterally towards the brighter end where photons are more plentiful, even though the true light source was below them.
Turns out, you can’t fool cyanobacteria that easily. In the situation described, in which the brightest light does not indicate the location of the source, cells don’t move decisively, but instead twitch randomly around. There’s no doubt they can sense the direction of light very well, though, because illumination from one side of the agar disc results in a unidirectional movement towards the light source. When two side-illuminating light sources are placed at a 90° angles to one another, the cells move not towards one or the other, but in a diagonal between the two.
Cyanobacteria clearly perceive a light source directly, instead of embarking on a biased random walk a la runs and tumbles in chemotaxis. It’s as if they can actually "see" the light — and, incredibly, that may not be far off the mark. There’s an important distinction between sensing a chemical gradient and sensing light direction, in that light acts not only as a particle but as a wave, travelling from its source through the cell that senses it. In passing through, the light path is altered by the refractive properties of the cell it traverses. Specifically, in the case of a spherical cell, light entering from one side will be gathered to a small point at the opposite end, not unlike how the eye focuses incoming light onto the retina. It turns out that cyanobacteria "see" the direction of light by a single cell having lens-like optics—a beautifully simple solution compared to blindly counting photons.
Does the cyanobacterial cell truly act as a lens? Bright-field microscopy showed a spot of light focused at the edge of the Synechocystis cell opposite from the light source, and reflected on the agar surface. To more accurately quantify this lensing effect, the researchers expressed GFP (with a TorA leader sequence , which localized GFP to the periplasm. Illuminating these cells directionally with light of the right wavelength resulted in strong GFP excitation at a focal point at the opposite cell periphery. The fluorescence intensity was almost five times stronger than that at the side facing the light source. For a high-resolution analysis of the lensing effect, the authors captured the light pattern adjacent to the cell on a photosensitive crosslinking polymer substrate. The surface relief was profiled using atomic force microscopy, revealing that the lensing effect is strong and sharp despite the bacterium’s small size, with a focus diameter smaller than the wavelength of the incident light.
For a cyanobacterial cell this light spot would be an effective indicator of light source direction and thus trigger motility. Counterintuitively perhaps, the movement towards the light in Synechocystis is actually a photophobic response, in which the cell moves away from the brighter light spot on the cell periphery, hence towards the source. Whereas presenting a light gradient failed to trick Synechocystis, illuminating a highly focused laser spot on a cell that’s moving toward a general light source makes it head off in the direction opposite of the laser spot, heedless of its previous direction of movement. This happens only when the laser spot is brighter than the lens-formed focus opposite the general light source.
The researchers also considered the possibility that cells perceive light directionality through the decreasing light intensity along the cell body because heavily pigmented thylakoid membranes absorb light as it passes through the cell. It turns out that the path length in question—that is, the size of the cell—is too small for light to diminish significantly along that distance, despite the high pigment concentration. The light-focusing effect is much more intense.
How does Synechocystis translate this optical information into movement? The research group previously showed that directional motility is controlled through dynamic localization of the T4P machinery, including the ATPase PilB1, on the edge of the cell in the direction of movement. The membrane-bound photoreceptor PixJ1 is a strong candidate for the primary receiver of the light signal. Consequent signal transduction may trigger disassembly of the T4P motor apparatuses proximal to the light spot, resulting in movement in the other direction.
Twitching, and other subtle modes of bacterial motility, are less showy than the overt dynamics of flagellar movement. Perhaps certain modes of motility are better suited to specific stimuli. Because flagellar motility does not allow for very direct swimming in a chosen direction, it may be less suited to seeking the light. There are no known examples of flagellated phototactic bacteria, although the flagellated unicellular eukaryote Chlamydomonas does utilize an eyespot for photoreception. The environmental factors that trigger twitching motility are not yet well understood, but we do know that motility based on T4P is an essential part of many complex bacterial behaviors, such as biofilm formation and pathogenicity that are ripe for investigation. Further research on this topic is bound to help us "see" from a bacterium’s point of view.
Maier, B. & Wong, G. C. L. How Bacteria Use Type IV Pili Machinery on Surfaces. Trends Microbiol. 23, 775–788 (2015).
Wadhams, G. H. & Armitage, J. P. Making Sense of it All: Bacterial Chemotaxis. Nat. Rev. Mol. Cell Biol. 5, 1024–1037 (2004).
Schuergers, N. et al. Cyanobacteria use micro-optics to sense light direction. Elife 5, 1–16 (2016).
Wilde, A. & Mullineaux, C. W. Motility in cyanobacteria: Polysaccharide tracks and Type IV pilus motors. Mol. Microbiol. 98, 998–1001 (2015).
Parts of the experimental design are well illustrated in this video which combines various videos that are part of the paper:
Mizu is a graduate student in Prof. Jim Golden's lab in the Division of Biological Sciences of the University of California at San Diego. Susan Golden is a professor at that institution, she holds a Howard Hughes professorship at the University of California at San Diego