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Answer by Tirumalai Kamala:

The phrase group behavior in bacteria can be interpreted more than one way. Is it how does one bacterial species interact as a group or is it how do different bacterial species interact as a group? Though this subject is far from my core expertise, an enduring interest of mine is microbe-multicellular organism interactions. I draw on some little knowledge I have on that subject for this A2A.

Margaret McFall-Ngai and Edward Ruby discovered a most spectacular example of bacterial group behavior in the context of bacterium-animal symbiosis (1). They study the Hawaiian bobtail squid, Euprymna scolopes. This is a small, nocturnal, marine invertebrate. It looks like this (2). Isn’t it a beauty!

The squid hunts for prey in shallow waters in the night. Since it is night and we are talking about water, the squid needs a light source located in its chest to help find its prey. This light source happens to be symbiotic luminescent bacteria. They perform this light-emitting function after inhabiting the specialized light-emitting organ in its mantle cavity. The bacteria emit light downward while the squid controls the light intensity to match that of the moon and the stars such that its shadow is erased (3, 4). This way the squid can both approach its prey unseen and evade its bottom-dwelling predators.

How does this bacterium-squid symbiosis occur? The new-born squid has a juvenile light organ covered by ciliated epithelium. This structure appears to act both to create currents and as a sieve to trap bacteria. But it can’t be just any bacteria. It has to be Vibrio fischerii and only Vibrio fischerii. Inspired by Martinus Beijerinck, Lourens Baas Becking coined an adage on microbiol ecology, “everything is everywhere, but the environment selects” (5, 6). Here, the relevant environment is the squid’s ciliated epithelium and specific attractants secreted by the squid to attract and retain its symbiotic partner.

In response to these bacteria, the epithelium secretes mucus in which the trapped Vibrio fischerii create aggregates. From these surface aggregates, the bacteria migrate through pores on the light organ into crypts deep within the organ. This bacterial migration triggers a dramatic morphogenesis in the squid whereby the entire “bacterial symbiont harvesting apparatus”, i.e. the ciliated epithelium, is shrunk through a process of programmed cell death. Thus starts the spectacular natural dance that is the beginning of the lifelong squid- luminescent Vibrio fischerii partnership. The bacteria live lifelong deep in the squid’s light organ crypts and help it catch its prey by providing a functioning downward projecting light organ. The squid in turn provides its symbionts the nutrition they need (7).

1. development of an animal-bacterial mutualism.
2. Why bacteria matter in animal development and evolution
3. Counterillumination in the Hawaiian bobtail squid, Euprymna scolopes Berry (Mollusca: Cephalopoda)
4. Evidence for light perception in a bioluminescent organ
5. Page on cena.usp.br
6. Page on nature.com
7. Page on nih.gov

Here is the poet http://en.wikipedia.org/wiki/Paul_Muldoon poetic version of this story:

Euprymna-Vibrio Publications

Thank you, Vipassana Vijayarangan, for this, my first A2A.

What are the most interesting examples of group behavior in bacteria?