Since many pathogens interact with their host at mucosal surfaces and have to compete with other microflora, scientists at the University of Pennsylvania School of Medicine and Oxford University used a mouse model of nasal infection to investigate whether competition between microbes promoted virulence. They found that Haemophilus influenzae was able to out-compete Streptococcus pneumoniae by recruiting the host’s immune system. S. pneumoniae is normally harmless and ignored by the immune system, but the immune response stimulated by H. influenzae has unintended consequences. S. pneumoniae strains with polysaccharide capsules that confer resistance to the immune attack are able to survive at the expense of non-resistant strains, resulting in a S. pneumoniae population dominated by the resistant phenotype. Unfortunately, the resistant strains are also more dangerous – if they are able to enter the bloodstream they can multiply unchecked and go on to cause pneumonia, septicaemia and meningitis. So in this battle between S. pneumoniae and H. influenzae, with weapons provided by the host, S. pneumoniae prevails at the expense of the host.
The study is published in Current Biology.