Keith S. Taber

The unbelievable truth – do bacteria focus incident light onto their back-sides, so they can tell which way to go?

"Bacteria are just tiny eyeballs" sounds like another science analogy, but is actually something I learned today from BBC Radio 4.

On David Mitchell's "The Unbelievable Truth" panelists read essays on a topic, but populated with false (and preferably funny) statements. The premise is great: the panelists try to sneak in some true facts which sound so unlikely that they are confused with the falsehoods. Panelists get marks for correctly spotting truths in another panelist's  little essay, or for completing their own talk with some of their 'unbelievable' truths not being spotted.

On today's episode I was shocked top learn from Dr Ria Lina that "…bacteria are just tiny eyeballs…".

Because they are not.

Well, not exactly…

Her essay was talking about germs, and included:

"Transmission of disease is determined by how many victims germs can actually see. Viruses have load of tiny little eyes so they are able to see loads and loads of potential victims in all different directions, whereas bacteria are just tiny little eyeballs, and fungi are extremely short sighted poor things, which is why they are only able to infect places like feet."

At the end of the round, David reported that the part about bacteria as eyes was true, although he did not seem very convinced:

David: You have managed to smuggle three truth past the rest of the panel, which are that bacteria are just tiny eyeballs – although to me that sounds a bit like things being put into language that people understand, because they are not like tiny eyeballs, really are they?

Ria: Well the light goes in and it reflects off the back surface which acts like a rudimentary retina…

Right,

…and also you have got to remember that the eye had evolved multiple times in multiple ways, so the squid eye and the human eye even though they both work the same way did not come from the same universal ancestor…

Oh right

…so, bacterial eye is basically what we are seeing now is the beginning of – [sadly interrupted by another panelist]. I'm such a geek…

I presumed there must be some basis for this claim; that Dr Lina (PhD in  viral bioinformatics) must be drawing upon some actual science, but I was not sure what. Whereas the eyeball has a back surface there is no inherent back surface for a bacterium – so this must mean any inside surface.

Although light does reflect off the retina (red eye in camera images is due to the light reflecting from the retina with its rich supply of blood vessels) – the function of the retina is to absorb light and to then signal information forward based on the pattern of light detected. Some species have a reflecting layer (tapetum lucidum) behind the retina to increase efficiency – some light not absorbed by the retina initially gets a second pass though after reflection which allows increased absorption. But this is only useful because there are cells capable of absorbing the light and processing information based on the absorption.

The bacterium is a single cell, so the most sense I could make of this is that when light absorption is useful, a reflecting inner surface could be valuable. This might make sense for cyanobacteria to increase the efficiency of photosynthesis by reflecting light not absorbed on the first pass.

What did we do before the internet?

I did some quick searching on line.

Someone had developed a method of identifying bacterial colonies through light back-scattered, which was useful because the technique using transmitted light was impractical in species that absorb most of the incident light.

Interesting, but the light was not being reflected internally, if I understood the paper.

Someone has developed a technique to increase the light absorbed by photosynthetic algae and cyanobacteria that did use total internal reflection – but if I read correctly this reflection is in the light guide on the way to the cell, not inside it.

So, I was still not buying the eyeballs story. Then I found a report of how "Cyanobacteria use micro-optics to sense light direction",

"Here, we establish that individual Synechocystis cells can directly and accurately perceive the position of a unidirectional light source, and control their motility so as to move towards it. We then show that Synechocystis cells act as microlenses, and that the light intensity gradient across the cell due to this lensing effect is far greater than the effects of shading due to light absorption or reflection. Finally, we use highly-localized laser excitation to show that specific excitation of one side of the cell triggers movement away from the light, indicating that positive phototaxis results from movement away from an image of the light source focused on the opposite side of the cell. Essentially, the cell acts as a microscopic eyeball."

Schuergers et al.,2016

Wow, nature never ceases to amaze.

So, basically the cell itself focuses incident light to be concentrated at the 'back' of the cell (where back is the side opposite the light source), organelles that absorb light can in effect detect this light 'spot', and the bacterium has evolved to move 'forward' towards the light source based on where in the cell this higher light intensity occurs. The system in effect 'knows' which direction to take as forward.

"Here we have shown that Synechocystis cells act as very effective spherical microlenses that focus a sharp image of a light source at the opposite edge of the cell. This implies that positive phototaxis (i.e. movement towards a light source) is actually triggered as a negative response to the focused spot of light at rear periphery of the cell."

Schuergers et al.,2016

This does not seem to involve internal reflection, so perhaps there is another source for the eyeballs claim (possibly with an even more amazing nugget of science) that I have missed and which was the basis of Dr Lina's claim.

Bacteria are not just tiny eyeballs, but…

I still think it is not correct to claim that "bacteria [generally, or even this particular bacteria] are just tiny eyeballs". This is a simplification, and probably not an 'intellectually honest' one that could be considered to be at the 'optimum level of simplification' for communicating a key scientific idea stripped of distracting complications.

(read about the the optimum level of simplification – a key idea in teaching)

Indeed the real wonder of Synechocystis is that it a single cell that acts as an integrated, responsive, coherent system: energy collection unit, eyeball, lens, photo-receptors, controller/processor, and locomotive unit.

Despite this quibble, given the context of the claim (made as part of a comedy show, not a peer reviewed research conference)  I think I am impressed enough to have to revise the 'Tweet' I was going to send Dr Lina calling her out for telling an unbelievable lie on national radio. I should have remembered that it is very difficult to come up with any claim about the living world which is so fantastic that one can be confident there is not an example of a species out there which affirms the claim. When it comes to nature we often need to believe the unbelievable truth.

Work cited:

• Huisung Kim, Iyll-Joon Doh, Jennifer Sturgis, Arun K. Bhunia, J. Paul Robinson, Euiwon Bae (2016) Reflected scatterometry for noninvasive interrogation of bacterial colonies, Journal of  Biomedical Optics. 21(10), 107004, doi: 10.1117/1.JBO.21.10.107004.
• Ooms, M. D., Sieben, V. J., Pierobon, S. C., Jung, E. E., Kalontarov, M., Erickson, D., & Sinton, D. (2012). Evanescent photosynthesis: exciting cyanobacteria in a surface-confined light field. Physical Chemistry Chemical Physics, 14(14), 4817-4823. doi:10.1039/C2CP40271H
• Schuergers, N., Lenn, T., Kampmann, R., Meissner, M. V., Esteves, T., Temerinac-Ott, M., . . . Wilde, A. (2016). Cyanobacteria use micro-optics to sense light direction. Elife, 5, e12620.