Our bodies are teeming with billions of bacteria. While some may cause disease, most are innocuous. A new study sheds light on how we communicate with these friendly bacteria. Eric Libby reports.

Margaret McFall-Ngai researches how benign bacteria affect us at the genetic level. The University of Wisconsin developmental biologist says that there are unique communities of health-promoting bacteria in many tissues including the skin, nasal passages, eyeballs, and our digestive systems.

Killing these bacteria can even leave us prone to diseases. McFall-Nigh notes, for example, that a woman's vagina normally hosts hundreds of types of bacteria. After a treatment of antibiotics, she says, yeast infections are more likely. So there are all these different areas of the human body that require, for their health, interactions with communities of co-evolved microbes.

McFall-Ngai points out that most of our bacterial associates cannot be grown in a lab. Furthermore, determining how any single species of bacteria affects us is difficult. So McFall-Ngai studies a simpler system: an animal which has a special relationship with just one type of bacteria… the Hawaiian bobtail squid and the bacteria Vibrio fischeri.

The tiny bobtail squid has a head about three centimeters long and feeds on shrimp in shallow coastal waters. Its bacterial partner, Vibrio fischeri, lives inside the squid and receives nutrients. In exchange for the food, the bacteria produce light. The squid usually hunts at night and without any camouflage it would cast a shadow.  Instead, the squid uses the bacteria to emit light of the same intensity and color as down-glowing moonlight and starlight and avoids detection from below.

Both the squid and the bacteria can be grown in the lab independently. When they partner up, the bacteria cause the squid cells to change which of their genes are active. Many of these altered genes are also present in mice and humans. Some of them act in the immune response against pathogens.

McFall-Ngai compares these bacteria-induced genetic changes to a language and says it doesn't seem to matter whether the bacteria inducing those changes are disease-causing or health-promoting. Although the same language appears to be used, it's the difference between an argument and a civil conversation.

Understanding the language our bodies use to communicate with bacteria is important to improving medical treatment. Current antibiotics often kill both the bad and the good bacteria, leaving room for opportunistic pathogens to colonize. So, if we want better treatments against our foes, we need to learn how we talk to our friends.