Bioluminescent bacterial partner essential for squid development
Hawaiian bobtail squid. Credit: Margaret McFall-Ngai and Edward Ruby, UH at Mānoa.
The Hawaiian bobtail squid, a small, multi-colored native of coastal waters in Hawai‘i, uses bioluminescence to camouflage itself and evade predators. However, the costume change is only possible through an exclusive symbiotic relationship with a bacterial partner, Vibrio fischeri, which the squid recruits from the ocean environment. A new study led by researchers at the University of Hawai‘i (UH) at Mānoa, revealed that the benefit of the partnership extends far beyond light-production: the bacteria were found to play a vital role in the healthy development of the squid.
“Our recent work revealed that in order to develop properly, the squid host requires a protein provided by its bacterial symbiont,” said Jill (Kuwabara) Smith, lead author of the study, who was a postdoctoral researcher at the Pacific Biosciences Research Center (PBRC) in the UH Mānoa School of Ocean and Earth Science and Technology at the time of this research. “This was very surprising, but given that the work we do with this symbiosis model is always pioneering, just about every new finding is a surprise!”
Many bacteria secrete tiny packets, called outer membrane vesicles, from the surface of their cells. These vesicles can contain proteins, enzymes, genetic material, or infectious agents, if the bacteria are pathogenic. Previous research found that Vibrio fischeri include in these packets a protein referred to as SypC, which is important for initiating the host-microbe connection.
“Once the bacteria and its vesicles are inside the squid host, the new research found that the SypC assumes a new function—it prompts development of the light-organ itself,” Smith shared.
Tracking a rare but important protein
While SypC is a microbial protein and its necessity for squid development seemed unlikely, the research team made a striking discovery: the absence of SypC in bacterial vesicles led to significant developmental disruptions in the squid host. The scientists then chemically attached a fluorescent tag to SypC and used fluorescence confocal microscopy to track how the protein moved throughout the squid. They also monitored gene expression in squid with and without SypC.
Their observations revealed that hemocytes, specialized immune cells that are responsible for removing pathogenic microbes, interact with the bacterial vesicles and transport the bacteria to a distant site where they are essential for induction of light organ development. Further, they found that within squid, the expression of 138 genes changed when SypC was absent.
“In addition to contributing light-production capabilities, Vibrio fischeri are prompting the squid’s development of organs and healthy expression of genes that are involved in a wide range of functions,” said Smith.
From squid to human health
Nearly every organism and environment hosts a collection of microbes—a microbiome—which are an integral component of ecological and human health. But the communication between bacteria and host before, during, and after the meet-up has been mysterious and intriguing.
“Very often an animal’s microbiome is highly complex, so it is difficult to determine what each bacterial species is contributing to the molecular ‘conversation’ with the host animal,” said Margaret McFall-Ngai, senior author on the paper, Professor Emerita at PBRC, and senior staff scientist at Caltech/Carnegie Science in Pasadena. “The host squid that we study naturally interacts with only one bacterial species, so the exchange between partners is much easier to understand.”
Bacteria in human gut tracts also secrete outer membrane vesicles, which are taken up and trafficked by the blood stream to remote tissues, where they affect function of those tissues. However, because there are hundreds of bacterial species in the human gut, and the physical landscape is so large, it is difficult to study. Given the small size of baby Hawaiian bobtail squids (only a couple of millimeters long) it is possible to visualize, by confocal microscopy, the effects of host-microbe interactions with great resolution.
“The goal of our research is to discover those features of symbiosis that are evolutionarily conserved, from less complex animals through humans,” said Smith, who is now a science teacher at ‘Iolani School. “Those elements that are evolutionarily conserved are likely to be very important. Once we discover something, it can provide clues as to how things work in mammalian systems. The squid-vibrio system has guided the biomedical community again and again over the last 35+ years of its study.”
Read also on MSN, Big Island Now, Kaua’i Now, Phys.org, UH News, and Eurekalert.