Billions of bacteria live in our intestines, supplying us with essential vitamins and helping us digest food. Recent research suggests that in some animals, and perhaps in humans as well, such bacterial guests may play an even more fundamental role: they may influence the normal development of some internal organs.
Margaret McFall-Ngai, a developmental biologist at the University of Hawaii, studies the partnership between a squid and the luminous bacteria that live inside its light-emitting organ. Unlike our intestines, which have hundreds of species of resident bacteria, the squid’s organ shelters only one type of microbe. By examining this relatively simple symbiosis, McFall-Ngai hopes to gain insight into more complicated interactions.
The symbiosis in question consists of the one-and-a-half-inch- long Hawaiian bobtail squid, Euprymna scolopes, and the bacteria Vibrio fischeri. By day the squid lives buried in sand; at night it emerges to forage. Millions of the light-producing bacteria nestle inside the squid, and because the light these bacteria emit matches the wavelength of moonlight and starlight, it disguises the squid from nocturnal predators hunting beneath them. In return, the squid supplies the bacteria with nutrients.
When the squid hatch, they lack the bacteria but soon acquire them from seawater. The squid have two appendages covered with cilia that waft the bacteria through pores into the squid’s light organ, where they multiply rapidly. Just hours after the bacteria enter the squid, cells on the tips and surface of the cilia-covered arms begin to die; within four days the structures have entirely disappeared. Within a few weeks the bacteria induce the formation of a lens and reflector that modifies the emitted light. That the bacteria are responsible for these changes is clear: squid raised in water free of V. fischeri, McFall-Ngai has found, never lose the bacteria-collecting arms, and the light organs don’t fully develop.
Why do the arms die? McFall-Ngai thinks that the bacteria trigger a respiratory burst in the squid cells, creating toxic by-products that trigger cell death in the arms, which are near the bacteria. Strangely, the cells closest to the bacteria appear to be protected from the toxins.
McFall-Ngai also found that the squid bathe the bacteria in a toxic enzyme, perhaps to limit their growth. The enzyme, similar to one found in mammalian immune cells, converts hydrogen peroxide and chloride ions into hypochlorous acid, a basic ingredient of bleach. Most bacteria cannot survive this onslaught.
But V. fischeri can. McFall-Ngai thinks it may produce a protective protein that is very similar to one produced by a close relative. That relative is Vibrio cholerae, the infectious agent of cholera. The protein V. cholerae produces inhibits the respiratory burst with which its host attempts to kill it.
These findings suggest a new way to view the evolution of infectious disease. In the conventional view, harmless relationships between bacteria and their hosts probably evolved as the host learned to live with a microbe that initially caused disease. McFall-Ngai thinks evolution may lead to more complex relationships, with periods of cooperation giving way to an antagonistic relationship in which bacterial guests again become agents of disease. The squid and their bacteria may be an example of a temporary evolutionary détente. Cholera, on the other hand, may exemplify the collapse of an uneasy truce: a once peaceful relationship has reverted to a death struggle.
Even more interesting for McFall-Ngai is the question of the role symbiotic bacteria might play in human development. Given the enormous toll in suffering that bacteria have exacted from us, researchers have naturally tended to focus on bacteria as harmful creatures. Researchers like McFall- Ngai are now looking at the beneficial role these microbes play. We haven’t integrated into our thinking that bacteria are essential, she says, and we need to understand how they are involved in maintaining a healthy human body.