Bacterial snitch: species competes by telling on another.

A bacterial species that typically colonizes people's noses may win out over another bacterium by tattling to the human immune system, a new study suggests.

Both Haemophilus influenzae and Streptococcus pneumoniae can establish residence in the upper respiratory tract either separately or together. The microbes are usually harmless, but in some circumstances, either species can cause ear infections, chronic bronchitis, or pneumonia.

In previous studies, Jeffrey Weiser of the University of Pennsylvania School of Medicine in Philadelphia and his colleagues examined competition between these two microbes. In a series of lab tests, the researchers found that S. pneumoniae always triumphed. The bacterium overpowered H. influenzae by attacking it with hydrogen peroxide and stripping off surface molecules important for H. influenzae's survival.

Elena Lysenko says that she and other researchers in Weiser's lab wonderedwhether the dynamic would change when both bacteria occupied an animal host instead of a petri dish.

To investigate, Lysenko and her colleagues sprayed solutions containing H. influenzae or S. pneumoniae into the nostrils of mice. In some mice, both nostrils were sprayed with the same bacterial species. In others, each nostril got a different species.

When the scientists examined the mice on the 1st, 3rd, and 14th days after the spray treatment, they found the microbes thriving within the nasal cavities of the animals that had received a single species of bacteria. However, in those mice sprayed with both species, only H. influenzae remained after 2 weeks.

The situation was "quite the opposite" from what the researchers had expected, given their petri dish results, says Lysenko.

Searching for an explanation, the scientists examined slices of upper respiratory tissue from the mice. In tissue exposed to both bacteria, Lysenko's team found an extraordinarily large number of neutrophils, immune system cells that fight bacteria. These cells weren't present in the mice that had received just one species.

Further tests in the lab showed that neutrophils exposed to killed H. influenzae attacked S. pneumoniae more aggressively than did unexposed neutrophils. However, killed H. influenzae had no effect on neutrophils' killing of live H. influenzae.

Taken together, these results suggest two scenarios: Either H. influenzae signals the immune system to attack S.pneumoniae or the two species together set off an immune system alarm that isn't tripped by either species individually. Lysenko says that neither scenario explains why the immune onslaught doesn't affect H. influenzae.

Timothy F. Murphy of the State University of New York at Buffalo says that these findings underscore the need for researchers to investigate bacterial competition when studying infections. "A lot of the time, we stick one bacterium into an animal model and forget about interactions [among bacterial species]," he says.

The new results may also have implications for the use of antibacterial drugs and vaccines, says Peter Hermans of the University Medical Center St. Radboud in the Netherlands. He and his colleagues have found that vaccines designed to eliminate one species often increase infections caused by other species. "When one bacterium is removed, the niche is taken over by another," he says.