Bacterial–bacterial interactions.

<p>The composition of nasopharyngeal microbiota is constantly subject to interactions between species. Bacterial species can interact with other bacterial species by competition and synergism. Synergism can be characterized by, for instance, the production of components that favors another species, as shown for the production of outer membrane vesicles. These may contain factors that are able to inactivate complement factor C3, thereby allowing another species to escape the immune system. Production of substances by one species, for example hydrogen peroxide (H₂O₂), may eliminate its competitor. The immune system may also be involved in competition, as one bacterium has fewer escape mechanisms to evade the immune system than another and therefore may use co-inhabitants to survive, whereas the reverse phenomenon (i.e., one species may trigger the immune system to combat the other species) may also occur. In addition, since PhC (phosphorylcholine) is shown to be immunogenic and some species may be able to switch off PhC expression whereas others cannot, there might be a selective advantage. Another form of competition involves competition for the same host receptor, as demonstrated for PhC and its receptor platelet activating factor receptor (PAFr). Moreover, one species may use neuraminidase to cut off the sialic acids (SA) that other bacteria may require for attachment to host receptors, therefore inhibiting adherence of the other bacterial species. H₂O₂, hydrogen peroxide; PAFr, platelet activating factor receptor; PhC, phosphorylcholine; NA, neuraminidase; SA, sialic acid (SA); rSA, receptor for sialic acids; Ab, antibodies.