Shedding Light on the Vibrio-Squid Symbiosis
For the past 20 years, , has regularly made trips to Australia in search of Australian bobtail squid. Collecting the squid—and shipping them back to the lab—is no easy task. Nishiguchi, a professor of molecular and cell biology at University of California, Merced, has encountered venomous catfish and torpedo eels, and evaded saltwater crocodiles by carefully planning her collection sites and times. , professor and Vice Chair of Medical Microbiology and Immunology at the University of Wisconsin-Madison, goes to Hawaii twice a year to collect Hawaiian bobtail squid. “We usually ship about a dozen or so adults [back to the lab],” he said, noting the squid will yield more than 5,000 hatchlings—each about the size of a fruit fly—over 4-5 months.
Nishiguchi, Mandel and many other scientists collect bobtail squid inhabiting the waters around Hawaii, Australia, the Mediterranean and the Indo-Pacific to study the symbiosis, or mutually beneficial relationship, between the cephalopod and its bacterial symbiont,Vibrio fischeri. Tucked away in the squid’s light organ, the bacteria produce a blue-green glow. For the squid, this bioluminescence is a matter of survival; the bacteria help shield them from predators at night by emitting light when the bacteria reach a critical mass in the squid’s light organ. This light matches the moonlight above, masking the shadow that would otherwise be cast on the ocean floor, and helping to hide the squid from predators below. In return, V. fischeri receives nutrients and shelter.
How V. fischeri gets into the light organ— and how it is maintained there—is a complicated process. Yet, microbiologists find that this symbiosis, which is highly specific and experimentally tractable, serves as a powerful system for uncovering how microbes communicate with their hosts and adapt to their environment.
A Complex Colonization
The partnership between squids and V. fischeri is a multi-step process that begins after the squid first hatches. Squid hatchlings acquire V. fischeri from the surrounding water. “When the squid pumps oxygenated water through its mantle cavity for respiration, environmental bacteria come along with it,” Mandel explained. Exposure to peptidoglycan released in the environment triggers the production of mucus from the ciliated surfaces on both sides of the squid’s body. According to , a faculty associate at Caltech and a staff researcher at Carnegie Science, the squid “shed a huge amount of mucus.” Bacteria stick to the mucus, which contains antimicrobials. These antimicrobials kill off most other bacteria but leave V. fischeri unaffected, which is uniquely resistant to their effects.
Once inside the squid, the bacteria navigate a maze of pores, ducts, an antechamber and a long, narrow bottleneck before finally settling into the crypts of the light organ. McFall-Ngai said that most of the time, only 1 bacterium gets into each crypt. The reason is that the bottleneck is just wide enough for bacteria to single-file through. Once a bacterium colonizes, the bottleneck constricts, shutting out other bacteria. The bacteria begin to multiply in the crypt, and according to McFall-Ngai, the squid are completely colonized by somewhere between 12-15 hours.
Not Just Any Vibrio Can Colonize
Since just 1 bacterium colonizes each of the squid’s 6 crypts, it's important for the squid to harbor only the brightest bacteria. Indeed, bacteria that don’t produce light as their wild-type counterparts. “The squid knows that they’re dark and spits them out,” Nishiguchi said.
She has seen this play out in in which her lab has evolved V. fischeri strains to colonize squid from different habitats. Some of the genes that were affected during experimental evolution experiments were those involved in biofilm formation and light production. “That’s something that the squid is really selecting for,” she said. “They’re culling out ones that they don’t want in there.”
This culling happens every morning at dawn when the bobtail squid expels 90-95% of the bacteria from its light organ. At these low numbers, the bacteria don’t bioluminesce because light production occurs via a density-dependent process called quorum sensing. This isn’t problematic for the squid since they’re largely hidden and inactive during the day. Only after the bacteria reach higher densities, as they do toward nightfall, will they begin to glow.
A Microcosm of Animal-Microbe Interactions
Research into the vibrio-squid symbiosis is useful not just for understanding squid survival, but also for answering fundamental questions about how animals and microbes interact with each other. Animal-microbe interactions are ubiquitous, ranging from symbiotic relationships, microbiomes or pathogenic interactions. In these interactions, each party is influenced by others and the environment they share.
While gut microbiomes are extremely complex in that they harbor , the squid’s light organ only contains a single species. This is an advantage for experimental studies. “It's like being in conversation with just 1 or 2 folks versus a whole party,” said Nishiguchi. This contrasts to a complex microbiome where the microbes are responding to each other and the host at the same time. For the vibrio-squid system, its simplicity allows researchers to manipulate the interaction, and observe the outcomes, by making mutants. “[The vibrio-squid model] is really a great symbiosis because there’s 1 host and 1 microbe, and we can get high resolution when we ask a question,” McFall-Ngai explained.
For Mandel, he sees the vibrio-squid interaction as a model for understanding microbiome development. “One thing we really want to get a sense of is what are the rules of the road for how an animal gets its microbiome,” he said. He added that while we know a lot about the signals that are being exchanged between squid and the bacterium, we don’t quite understand the entire conversation that establishes the bacterium as a symbiont. For example, microbiologists know that biofilm formation is required for colonization, but the role of aggregation within the host is less clear.
For an animal to establish its microbiome, bacteria first interact with the host epithelial surface. McFall-Ngai points out an interesting connection between gut microbes and V. fischeri. “[V. fischeri] associates with the host in a very similar way to how bacteria associate with our gut epithelia,” she said. For instance, the crypts of the gut are similar to the crypts in the light organ, and both gut microbes and V. fischeri secrete outer membrane vesicles that then affect distant regions in the host. “Animals evolved in the oceans that had lots of bacteria, so there’s nothing older in animal evolution than the interactions of bacteria with the apical surfaces of the animal epithelia,” said McFall-Ngai. Therefore, interactions between microbes and surfaces such as the gut, airways, reproductive tract and the squid’s light organ are very similar and evolutionarily conserved—we can learn a lot about the former by studying the latter.
An Early Warning System of Environmental Changes
Beyond helping us understand the microbiome, Nishiguchi said that beneficial microbes, such as V. fischeri, can act as “canaries in a coal mine. [Microbes are] the ones that are going to respond first, and quickly, to any kind of environmental change, such as .” Since microbes can divide rapidly, they adapt on a genetic level more quickly than animals that have longer reproductive timelines.
Scientists can leverage these quick microbial responses to monitor changes in a given environment, such as changes to water quality. One application of this already in play is the system, which uses V. fischeri to detect chemical contaminants in the water supply. The microbes emit light when healthy but exposure to toxic compounds decreases the amount of bioluminescence from the bacteria in a dose-dependent manner.
These are only a few examples of the and how scientists can learn about microbe-animal interactions more broadly from a 1 host-1 symbiont system. “Not every animal has symbiosis, but all animals thus far studied grew up in a microbial-rich soup,” said McFall-Ngai. “[Microbes] have been foundational in shaping the biology of all animals and plants.”
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Author: Jennifer Tsang, Ph.D.
In This Issue
- The Link Between Pets, People and Antimicrobial Resistance
- Shedding Light on the Vibrio-Squid Symbiosis
- Baiting Mice to Beat Lyme Disease
- Skin Deep: How Bd and Bsal Fungi Threaten Amphibian Health
- Food Biosecurity: Flyways, Flocks, CAFOs and Avian Flu
- How FMTs, Coprophagia and the Milk Microbiome Inform Wildlife Conservation With Sally Bornbusch
- Animal Vaccine FAQs: Protecting Pets, Livestock and Wildlife
- New World Screwworm: Rise, Fall and Resurgence
- Is a Future Without Animal Models Possible?