Mucosal surfaces are home to a large variety of common bacteria that compete with one another for the resources in this crowded habitat. Now, a new report in PLoS Pathogens by Jeffrey Weiser and colleagues reveals that bacteria use an ingenious tactic to outmanoeuvre their competitors — they manipulate the immune response of the host and direct it against their neighbours.

The authors used a mouse model to study the competitive interactions of two bacteria that commonly reside in the human nasopharynx — Streptococcus pneumoniae and Haemophilus influenzae . When pitted against each other in laboratory culture, S. pneumoniae rapidly killed H. influenzae by unleashing a barrage of bactericidal agents. But when these two pathogens were simultaneously introduced into the nasopharynx of mice, the outcome was entirely different — S. pneumoniae was rapidly cleared from the mouse upper respiratory tract, whereas H. influenzae was undepleted. Importantly, the air passages of animals colonized simultaneously with both bacteria were replete with neutrophils — the key inflammatory cells of the innate immune response. And the depletion of neutrophils from co-colonized mice eliminated the survival advantage of H. influenzae, which indicates that the innate immune response of the host is crucial to the competitive strategies of this microorganism.

However, it is not only the chemotactic powers of H. influenzae that determine its competitive success — ex vivo assays showed that components of H. influenzae, but not S. pneumoniae, efficiently activated the microbicidal activity of neutrophils. Also, neutrophils that were primed by H. influenzae did not adversely affect the bacterium itself, and the authors contend that it is this selective response of inflammatory cells to products from one bacterial species, but not another, that provides a mechanism to defeat competitors.

Although as yet unproven, other combinations of co-colonizing bacteria might compete with each other in similar ways. For instance, it has recently been shown that children immunized with pneumococcal vaccine have lower rates of carriage of S. pneumoniae, but are more likely to harbour nasal Staphylococcus aureus and develop staphylocccal ear infections.

These results show that upsetting the delicate balance of the mucosal microflora, for example, by using narrow-spectrum antibiotics and vaccines, might well have adverse consequences for the host, and it is therefore imperative that we understand how microbial communities interact in the living host.