BioEd Online

Some bacteria that live in our noses call on our immune systems to fight their battles for them.

Many different species of bacteria live in our noses and throats. These 'opportunistic pathogens' are usually no trouble, but can cause infection if a person's immune system is weakened owing to stress or poor health. Still, they need to fight for space and resources with other bacteria living in the same place.

To work out the strategies that common bacteria use in this competition, scientists from the University of Pennsylvania School of Medicine, Philadelphia, pitted Haemophilus influenzae against Streptococcus pneumoniae. Both are leading causes of ear, nose and throat infections, and the latter is a common cause of pneumonia.

They found that S. pneumoniae always came out on top when the two fought it out in a lab dish. But, surprisingly, the results were reversed when the competition took place in mice. They report their results in the journal PLoS Pathology¹.

Nose job

The outcome implied that living in the host gave H. influenzae an advantage. "People tend to study their bacteria as if they exist naturally alone in pure culture," says team member Jeff Weiser. "But they don't, and when you put them together there are often complex interactions."

When they looked closer, the researchers found that the presence of H. influenzae prompts white blood cells called neutrophils to mobilize and move to the area where the bacteria are. The colonizer is resistant to the immune response it stimulates, so can take over more space when its rival is killed off.

This work could have consequences for the way that doctors use antibiotics, the team says. If a treatment removes bacteria that were serving to keep others in check, it could have effects beyond those intended.

Big shots

Weiser suggests their results may explain some side-effects of a pneumonia vaccine given to US children and at-risk patients in Britain.

"The pneumococcal conjugate vaccine has been given to children and has reduced infection, but there has been an increase in ear infections caused by another bacterium," he explains. "There may be secondary effects to be understood."

The overall message that vaccines and antibiotics can interfere with complex interactions between organisms that live in our bodies isn't in itself a surprise.

"People involved in vaccination programmes fully appreciate they are grossly disturbing the balance of organisms resident in the body," notes Mike Barer, a microbiologist at the University of Leicester, UK.

But the interaction of bacteria with the host's immune system is a new twist on this story. Weiser says that more work into the interactions between us and our bacterial flora could lead to greater understanding and perhaps a better way of designing treatments. "Maybe we'll get smarter about using these things in the future."