Imagine you’re a plucky, golf ball-sized squid swimming in the clear blue ocean on a moonlit night. Your round shape means a distinctive shadow casts on the ocean floor thanks to the light of the moon, and predators are looking for prey by night. How do you, a small piece of prey without a protective shell, hide yourself in a sea full of predators?
If you were a Hawaiian bobtail squid, you’d employ a process called counter-illumination to create a natural camouflage in moonlit waters. The light — produced by the mutualistic bacteria Vibrio fischeri within the squid — is cast downward to eliminate the shadow and prevent predators from seeing a silhouette of the squid when looking up.
University of Wisconsin–Madison Professor of Medical Microbiology and Immunology Mark Mandel studies the genetic relationship between the bacteria and their squid hosts. But he didn’t start his scientific career studying bacterial camouflage. When he was working toward his doctorate, Mandel examined genetic regulation in E. coli before deciding to dive further into the relationships microbes develop with other organisms.
“The bobtail squid is special because it has just one bacterium that colonizes in a dedicated organ in the animal, so that allows us to look at the natural processes that occur,” Mandel says. “Our main approach is to mutate the bacteria and look at what changes in the colonization. If it does change, it’s likely the gene we interrupted was important for colonization.”
Bobtail squid in the wild depend on these bacteria for survival. The bacteria in turn receive nutrition and protection from the squid, as well as an opportunity to reproduce.
At dawn, most of the bacteria in the squid’s specialized organ are expelled into the seawater, in a process called venting. This sounds unfortunate for the microbes — but it allows them to be picked up by squid hatchlings and start reproducing inside their new host.
This venting process saturates squids’ nearby waters with these microbes, so the environments in which they live will contain higher concentrations of bacteria than other, squid-free waters.
The few bacteria that remain in the nocturnal squid reproduce during the day so that by nightfall the bacteria are ready to provide light again. The next morning, the cycle starts over.
One key mechanism that Mandel’s lab examines is a population-sensing process called quorum sensing, which many microbes, including Vibrio fischeri, use to determine when the bacteria have reached a certain population.
Once the Vibrio fischeri reach a high-enough density within the squid, they collectively turn on their light all at once, Mandel explains. This population-sensing process was first discovered in Vibrio fischeri in the 1970s, but also occurs in pathogenic microbes like Pseudomonas aeruginosa, which often infect the lungs of people with cystic fibrosis. When these pathogens reach a certain density, they begin attacking the immune system of their hosts.
Most research into the squid-Vibrio mutualistic relationship has focused on a single strain of Vibrio, but Mandel’s most recent work has evaluated several strains of the bacteria to further understand genetic differences and regulation.
In a recent paper, Mandel’s lab examined how the bacteria form a biofilm, which is a mass of bacterial cells that gives protection from toxic substances released by the squid.
“We showed that the biofilm can be regulated in three different ways in different strains,” says Mandel. “One strain doesn’t have the regulator gene and another strain has the regulator but it doesn’t work because of a mutation.”
The biofilm regulator gene RscS controls biofilm formation. In the lab, this activation can be artificially stimulated using a process called overexpression – where the gene is highly active and creates a greater number of proteins than it normally would. Alternatively, the researchers can remove the bacteria’s ability to form a biofilm and see what happens.
“We now understand the necessity of the biofilm in bacteria, because when we mutate them and they don’t form this biofilm, they don’t colonize very well,” Mandel explains.
Another application for Vibrio fischeri involves biomaterial for the military: the luminescent bacteria contain reflectin proteins that allow them to direct light in a specific direction, and the military is looking at using these specialized proteins for use in alternative camouflage materials.
The study of Hawaiian bobtail squid and its bacterial companions involves a great deal of genetic research. Vibrio fischeri’s similarities to how other bacteria colonize their hosts have led to a wide range of other findings.
Thanks in part to Mandel’s lab, we now understand how this unlikely partnership helps these little round squid hide from predators. Without protective shells, these spotted cephalopods could easily struggle out in the vast ocean ecosystem – but they get by with a little help from their bioluminescent friends.
Header photo by Chris Frazee.